4 Sugars and Proteins
After studying this chapter, you should be able to:
- Define carbohydrates (sugars) and explain why the term “carbohydrate” is a misnomer
- Describe the structure and properties of glucose, including the silver mirror reaction and reduction of \(\ce{Cu(OH)2}\)
- Compare the structures of glucose (a polyhydroxy aldehyde) and fructose (a polyhydroxy ketone)
- Distinguish between reducing sugars (maltose) and non-reducing sugars (sucrose), and describe their hydrolysis
- Describe the structure, properties, hydrolysis, and uses of starch and cellulose
- Define amino acids, explain their amphoteric nature, and describe peptide bond formation
- Describe the properties of proteins, including salting out, denaturation, and color reactions
- Explain the catalytic role and characteristics of enzymes
In nature, sugars, proteins, and the fats studied in Chapter 3 are all important organic substances involved in the life activities of animals and plants. Sugars are products of photosynthesis by green plants, which absorb solar energy. Sugars store energy and serve as an important energy source for both animals and plants. Proteins are the fundamental materials that compose various cells. In this chapter, we will study these two classes of substances.
4.1 Section 1: Monosaccharides
Glucose, sucrose, starch, cellulose, and similar substances all belong to the class of sugars. Sugars are also called carbohydrates. This name arose because the first compounds of this class to be discovered were all composed of the three elements C, H, and O, with the ratio of hydrogen atoms to oxygen atoms being exactly \(2:1\). The chemical formula of sugars could be expressed by the general formula \(\ce{C_\)n\((H2O)_\)m\(}\) (where \(n\) and \(m\) may be the same or different). Because of this, they were mistakenly regarded as hydrates of carbon. In fact, the name “carbohydrate” does not reflect their structural characteristics. First, in carbohydrate molecules, H and O do not exist in the form of water molecules. Moreover, many carbohydrates have been discovered in which the ratio of H to O is not \(2:1\) — for example, rhamnose (\(\ce{C6H12O5}\)). Furthermore, many substances that fit the general formula \(\ce{C_\)n\((H2O)_\)m\(}\) are not carbohydrates at all — for example, formaldehyde (\(\ce{CH2O}\)) and acetic acid (\(\ce{C2H4O2}\)). Therefore, although the name “carbohydrate” continues to be used, it has long since lost its original meaning. Structurally, sugars are generally polyhydroxy aldehydes or polyhydroxy ketones, as well as substances that can be hydrolyzed to produce them.
Based on molecular structure, sugars can be classified into monosaccharides, oligosaccharides, and polysaccharides. In this section, we first study monosaccharides.
The most important monosaccharides are glucose and fructose.
Glucose
The chemical formula of glucose is \(\ce{C6H12O6}\). Glucose is the most widely distributed monosaccharide in nature. It is found in grape juice and other sweet fruits. Honey also contains glucose. Glucose is not as sweet as sucrose.
1. Properties and Structure of Glucose
Glucose is a white crystalline substance that is soluble in water. Certain properties of glucose in aqueous solution indicate that its condensed structural formula is:
\[\ce{CH2OH-CHOH-CHOH-CHOH-CHOH-CHO}\]
Glucose is a polyhydroxy aldehyde.
Add \(2\ \text{mL}\) of \(2\%\) silver nitrate solution to a clean test tube. While shaking the test tube, add \(2\%\) dilute ammonia solution dropwise until the precipitate that initially forms just dissolves. The resulting clear solution is the silver ammonia solution (Tollens’ reagent). Add \(1\ \text{mL}\) of \(10\%\) glucose solution to the test tube containing the silver ammonia solution, place the test tube in a water bath, and heat for \(3\)–\(5\ \text{min}\). Observe the silver mirror that forms.
In another test tube, add \(2\ \text{mL}\) of \(10\%\) sodium hydroxide solution and then add \(4\)–\(5\) drops of \(5\%\) copper sulfate solution. Observe the pale blue precipitate of copper(II) hydroxide that forms. Immediately add \(2\ \text{mL}\) of \(10\%\) glucose solution. After heating, observe the red precipitate of copper(I) oxide that forms.
Based on these experiments, the aldehyde group in glucose is oxidized to a carboxyl group, converting glucose to gluconic acid. These two reactions can be simply represented by the following chemical equations:
\[\ce{CH2OH-(CHOH)4-CHO + 2[Ag(NH3)2]+ + 2OH- ->}\]
\[\ce{CH2OH-(CHOH)4-COOH + 2Ag v + H2O + 4NH3}\]
\[\ce{CH2OH-(CHOH)4-CHO + 2Cu(OH)2 -> CH2OH-(CHOH)4-COOH + Cu2O v + 2H2O}\]
Glucose contains an aldehyde group and, like aldehydes, can be reduced by reducing agents. When hydrogen is added to glucose, it is reduced to a hexahydric alcohol (sorbitol). Glucose also contains alcoholic hydroxyl groups and can undergo esterification reactions with acids.
2. Preparation of Glucose
Industrially, glucose is usually prepared by the hydrolysis of starch, using sulfuric acid or other inorganic acids as catalysts:
\[\ce{(C6H10O5)_$n$ + $n$H2O ->[\text{catalyst}] $n$C6H12O6}\]
3. Uses of Glucose
Glucose is an important nutrient — it is one of the primary energy sources for human life activities. It undergoes oxidation reactions in body tissues, releasing heat to supply the energy people need:
\[\ce{C6H12O6(s) + 6O2(g) -> 6CO2(g) + 6H2O(l)}\]
\[\Delta H = -2803\ \text{kJ}\]
Glucose is used in medicine and in the manufacture of candy. In the mirror-making industry and for silvering thermos flask liners, glucose is commonly used as the reducing agent.
Fructose
The chemical formula of fructose is also \(\ce{C6H12O6}\). Fructose is widely distributed in nature, found in fruits and honey. Fructose is sweeter than sucrose. Pure fructose is a white crystalline substance, though it does not crystallize easily and is usually a viscous liquid. It is readily soluble in water. Experimental evidence shows that the condensed structural formula of fructose is:
\[\ce{CH2OH-CHOH-CHOH-CHOH-CO-CH2OH}\]
Therefore, fructose is a polyhydroxy ketone.
Ribose is a sugar containing five carbon atoms — a pentose. Ribose is an important component of cell nuclei and an indispensable substance for life activities. The important riboses are ribose and deoxyribose. Their condensed structural formulas are:
- Ribose: \(\ce{CH2OH-CHOH-CHOH-CHOH-CHO}\)
- Deoxyribose: \(\ce{CH2OH-CHOH-CHOH-CH2-CHO}\)
Exercises for Section 1
The formula mass of glucose is 180. It contains \(40\%\) carbon and \(6.7\%\) hydrogen, with the remainder being oxygen. Find the chemical formula of glucose.
Write the following chemical equations:
Oxidation of glucose by silver ammonia solution
Reduction of glucose with hydrogen to form a hexahydric alcohol
What is the difference between the molecular structures of glucose and fructose?
People with diabetes have relatively high sugar (glucose) content in their urine. How can you test whether a patient has diabetes?
Silvering thermos flask liners usually uses glucose as the reducing agent. If each thermos flask liner requires \(0.3\ \text{g}\) of silver, how many kilograms of \(98\%\) glucose are needed per day for a factory producing 5000 such thermos flask liners?
4.2 Section 2: Disaccharides
Sugars that produce several monosaccharide molecules upon hydrolysis are called oligosaccharides. Depending on whether two, three, or more monosaccharide molecules are produced, oligosaccharides are further classified as disaccharides, trisaccharides, and so on. The most important among these are the disaccharides. Sucrose and maltose are both disaccharides.
Sucrose
The most familiar disaccharide is sucrose, with the chemical formula \(\ce{C12H22O11}\). Sucrose is a colorless crystalline substance that is soluble in water. Sucrose is an important sweet food and is found in many plants, with sugarcane (containing \(11\%\)–\(17\%\) sugar) and sugar beets (containing \(14\%\)–\(26\%\) sugar) having the highest content.
To each of two clean test tubes, add \(1\ \text{mL}\) of \(10\%\) sucrose solution. To one test tube, add 3 drops of dilute sulfuric acid (1:5). Place both test tubes in a water bath to heat. Then add \(2\ \text{mL}\) of silver ammonia solution to each test tube. Compare the reactions in the two test tubes and observe whether a silver mirror reaction occurs.
Repeat the above experiment using copper sulfate and sodium hydroxide solution in place of silver ammonia solution.
From these experiments, we can see that sucrose does not undergo the silver mirror reaction, because its molecular structure does not contain an aldehyde group. Sucrose does not exhibit reducing properties — it is a non-reducing sugar. Under the catalytic action of sulfuric acid, sucrose hydrolyzes to produce one molecule of glucose and one molecule of fructose:
\[\ce{C12H22O11 + H2O ->[\text{catalyst}] C6H12O6 + C6H12O6}\]
Therefore, after hydrolysis, sucrose can undergo the silver mirror reaction and can also reduce copper(II) hydroxide.
Maltose
Maltose is a white crystalline substance (commonly seen maltose is an uncrystallized syrup). It is readily soluble in water and has a sweet taste, though not as sweet as sucrose. Maltose can undergo the silver mirror reaction because its molecular structure still contains an aldehyde group. Maltose is a reducing sugar. Under the catalytic action of sulfuric acid, maltose undergoes hydrolysis to produce two molecules of glucose:
\[\ce{C12H22O11 + H2O ->[\text{catalyst}] 2C6H12O6}\]
From the hydrolysis reactions of maltose and sucrose, we can see that disaccharides produce two molecules of monosaccharide upon hydrolysis, while monosaccharides cannot be hydrolyzed into simpler sugars.
Maltose is made from agricultural products rich in starch — such as rice, corn, and sweet potatoes — which undergo hydrolysis under the action of amylase (an enzyme produced by barley malt) at about \(60\,{}^{\circ}\text{C}\):
\[\ce{2(C6H10O5)_$n$ + $n$H2O ->[\text{catalyst}] $n$C12H22O11}\]
Maltose is also used as a sweet food.
Chinese labels in figure: 单糖 = Monosaccharides; 二糖 = Disaccharides; 多糖 = Polysaccharides; 葡萄糖 = Glucose; 果糖 = Fructose; 蔗糖 = Sucrose; 麦芽糖 = Maltose; 淀粉 = Starch; 纤维素 = Cellulose.
Exercises for Section 2
How can you distinguish between maltose and sucrose?
How can a disaccharide be converted to a monosaccharide? How can you experimentally verify that sucrose has been converted to glucose?
What happens when concentrated sulfuric acid is added to sucrose? Why?
How many grams each of fructose and glucose are produced when \(2\ \text{mol}\) of sucrose is hydrolyzed?
4.3 Section 3: Polysaccharides
Polysaccharides are formed by many monosaccharide molecules combining in a specific manner through the removal of water molecules between molecules. Polysaccharides differ in properties from monosaccharides and oligosaccharides — they are generally insoluble in water, have no sweet taste, and have no reducing properties.
Starch and cellulose are the most important polysaccharides. Their general formula is both \(\ce{(C6H10O5)_\)n\(}\). The number of monosaccharide units (\(\ce{C6H10O5}\)) contained in starch and cellulose molecules differs — that is, their \(n\) values are different. Starch and cellulose also differ in structure.
Starch
Starch is a product of photosynthesis by green plants. It is found mainly in the seeds or tubers of plants. Grains contain relatively high amounts of starch. For example, rice contains about \(80\%\) starch, wheat about \(70\%\), and potatoes about \(20\%\).
Starch itself consists mainly of two components: amylose (straight-chain starch, accounting for about \(20\%\)) and amylopectin (branched-chain starch, accounting for about \(80\%\)).
Amylose molecules contain approximately hundreds of glucose units, with formula masses ranging from tens of thousands to over a hundred thousand. Amylopectin molecules contain approximately thousands of glucose units, with formula masses on the order of hundreds of thousands. Starch is a class of compounds with very large formula masses. Such compounds with very large formula masses are called macromolecular compounds (polymers).
1. Properties and Structure of Starch
Starch is a white powdery substance. Amylose can dissolve in hot water. Amylopectin does not dissolve in water but can swell and become moist in water.
Place \(0.5\ \text{g}\) of starch in each of Test Tube 1 and Test Tube 2. Add \(4\ \text{mL}\) of \(20\%\) sulfuric acid solution to Test Tube 1 and \(4\ \text{mL}\) of water to Test Tube 2. Heat both for \(3\)–\(4\ \text{min}\). Neutralize the sulfuric acid in Test Tube 1 with base solution, and pour part of the liquid into Test Tube 3. Add iodine solution to both Test Tube 2 and Test Tube 3 and observe whether a blue color appears. Add silver ammonia solution to Test Tube 1 and heat slightly — observe whether a silver mirror appears on the inner wall of the test tube.
From this experiment, we learn that after starch undergoes hydrolysis with acid as a catalyst, it produces reducing monosaccharides that can undergo the silver mirror reaction.
Whether amylose or amylopectin, both undergo hydrolysis under the action of dilute acid, producing a series of intermediate products, ultimately yielding glucose:
\[\ce{(C6H10O5)_$n$ + $n$H2O ->[\text{catalyst}] $n$C6H12O6}\]
Starch also undergoes hydrolysis in the human body. When we chew food, starch begins to hydrolyze under the action of amylase (a protein with catalytic function for starch hydrolysis) in saliva. When we chew rice thoroughly, we can taste sweetness.
In the small intestine, starch continues to hydrolyze under the action of amylase secreted by the pancreas. The glucose produced is absorbed through the intestinal wall into the blood, supplying the nutritional needs of body tissues.
2. Uses of Starch
Starch is an important component of food. It is also an industrial raw material that can be used to make glucose and alcohol, among other products. Under the action of amylase, starch is first converted to maltose and then to glucose. Glucose is then converted to alcohol under the action of zymase (an enzyme in yeast). This is the main process for brewing alcohol from starch-containing materials. The conversion of glucose to alcohol can be briefly represented as:
\[\ce{C6H12O6 ->[\text{catalyst}] 2C2H5OH + 2CO2 ^}\]
Cellulose
Cellulose is the fundamental substance that constitutes cell walls. About half of wood is cellulose. Cotton is one of the purest forms of cellulose in nature, containing about \(92\%\)–\(95\%\) cellulose. Degreased cotton and ash-free filter paper are nearly pure cellulose.
Cellulose molecules contain approximately thousands of glucose units, with formula masses on the order of hundreds of thousands.
1. Properties and Structure of Cellulose
Cellulose is a white, odorless, tasteless substance that is insoluble in water and also insoluble in common organic solvents.
Like starch, cellulose does not exhibit reducing properties. Cellulose can undergo hydrolysis, but with more difficulty than starch. When heated for a long time with dilute acid under a certain pressure, cellulose undergoes hydrolysis, with the final product being glucose.
Place a small amount of cotton or a few pieces of shredded filter paper into a test tube. Add \(3\)–\(4\ \text{mL}\) of \(70\%\) sulfuric acid. Crush the cotton with a glass rod to form a colorless, viscous liquid. Place the test tube in a water bath and heat for about \(15\ \text{min}\). After cooling, pour the contents into a beaker containing \(20\ \text{mL}\) of water and neutralize with sodium hydroxide solution. Take a portion of the neutralized solution and test with copper sulfate and sodium hydroxide solution — a red precipitate of copper(I) oxide can be observed.
The hydrolysis reaction of cellulose can be represented as:
\[\ce{(C6H10O5)_$n$ + $n$H2O ->[\text{catalyst}] $n$C6H12O6}\]
Cellulose molecules are composed of many glucose units. Each glucose unit has three alcoholic hydroxyl groups. Therefore, the cellulose molecule can also be represented as \([\ce{C6H7O2(OH)3}]_n\). Because of the presence of alcoholic hydroxyl groups, cellulose can exhibit some properties of alcohols, such as forming nitrate esters and acetate esters.
2. Uses of Cellulose
Cellulose is commonly used to manufacture cellulose nitrate, cellulose acetate, viscose fiber, and paper, among other products.
(1) Manufacturing Cellulose Nitrate
Cellulose nitrate (commonly called nitrocellulose) is produced by reacting cotton (whose composition is cellulose) with a mixture of concentrated nitric acid and concentrated sulfuric acid under certain conditions.
Add \(5\ \text{mL}\) of concentrated nitric acid (density \(1.4\ \text{g/cm}^3\)) and \(10\ \text{mL}\) of concentrated sulfuric acid (density \(1.84\ \text{g/cm}^3\)) successively to a beaker to prepare the mixed acid. Immerse a small piece of cotton in the mixture for \(8\)–\(10\ \text{min}\). Then remove it, wash thoroughly, and dry. Ignite the dried cellulose nitrate and a small piece of cotton simultaneously, and compare their combustion behavior.
Cellulose nitrate burns much more rapidly than cotton. The reaction to form cellulose nitrate is represented as:
\[[\ce{C6H7O2(OH)3}]_n + 3n\ce{HONO2} \xrightarrow{\ce{H2SO4}(\text{conc.})} [\ce{C6H7O2(ONO2)3}]_n + 3n\ce{H2O}\]
Cellulose generally cannot be completely esterified to form the trinitrate (containing \(14.14\%\ \ce{N}\)). Nitrate esters with different degrees of esterification differ in properties. Industrially, cellulose nitrate with lower nitrogen content is called collodion cotton (containing \(10.5\%\)–\(12\%\ \ce{N}\)), and that with higher nitrogen content is called guncotton (containing \(12.5\%\)–\(13.8\%\ \ce{N}\)).
Guncotton looks similar to cotton in appearance. It ignites and burns rapidly upon contact with fire, and explodes in sealed containers. It can be used as smokeless gunpowder.
Collodion cotton also burns easily but does not explode. It is used to manufacture lacquer. A solution of collodion cotton in an ethanol–ether mixture, commonly called collodion, is used for sealing bottle openings and similar purposes.
(2) Manufacturing Cellulose Acetate
Cellulose acetate (commonly called acetate fiber) is produced by reacting cotton with a mixture of acetic acid and acetic anhydride (\(\ce{(CH3CO)2O}\)) under certain conditions.
Cellulose acetate is mainly used to manufacture movie film base. Its advantage is that it does not catch fire easily.
(3) Manufacturing Viscose Fiber
Cellulose is treated successively with concentrated sodium hydroxide solution and carbon disulfide, and the resulting product is dissolved in dilute sodium hydroxide solution to form a viscose solution. When this viscose solution is forced through fine holes into a dilute acid solution, cellulose is regenerated as viscose fiber. Long viscose fibers are commonly called rayon (artificial silk), and short fibers are called staple fiber (artificial cotton) — both can be used for textile production. If the viscose solution is forced through a narrow slit into dilute acid, a transparent thin film is produced, commonly called cellophane.
(4) Papermaking
Papermaking is one of the great inventions of ancient China and a magnificent contribution of the Chinese people to human civilization. In 1957, paper from the early Western Han period (made from hemp plant fibers) was discovered in a tomb at Baqiao, Xi’an, Shaanxi Province. This demonstrates that paper made from plant fibers was already in use in China as early as the 2nd century BCE. Modern papermaking uses plant fibers — wood fibers and herbaceous plant fibers such as reeds, rice straw, wheat straw, and bagasse — as primary raw materials. To convert plant fibers into pulp, either mechanical methods (grinding) or chemical treatment methods can be used. Chemical treatment uses chemicals such as calcium bisulfite or sodium hydroxide to dissolve and remove the non-cellulose components from the raw material, separating out the cellulose to obtain chemical pulp. After pulping, the material is bleached, beaten, formed into thin sheets, and dried to produce paper.
Exercises for Section 3
During the hydrolysis of starch to produce glucose, what method can be used to verify that the starch has been completely hydrolyzed?
A factory uses \(2\ \text{t}\) of dried sweet potatoes containing \(54\%\) starch to produce ethanol. If \(85\%\) of the starch is converted to ethanol during fermentation, and the ethanol produced contains \(5\%\) water, how many tonnes of this ethanol can be obtained?
Unripe apple flesh turns blue when exposed to iodine, while the juice of ripe apples can reduce silver ammonia solution. How do you explain these two phenomena?
To produce \(1\ \text{t}\) of cellulose trinitrate, how much refined cotton linters and how much \(60\%\) concentrated nitric acid are needed?
Give examples to illustrate the interconversion relationships among monosaccharides, disaccharides, and polysaccharides.
4.4 Section 4: Amino Acids
Amino Acids
Compounds formed by replacing a hydrogen atom on the hydrocarbon group of a carboxylic acid molecule with an amino group are called amino acids. The following are the names and structural formulas of several amino acids:
Glycine (aminoacetic acid):
\[\ce{H2N-CH2-COOH}\]
Alanine (\(\alpha\)-aminopropionic acid):
\[\ce{CH3-CH(NH2)-COOH}\]
Phenylalanine (\(\alpha\)-amino-\(\beta\)-phenylpropionic acid):
\[\ce{C6H5CH2-CH(NH2)-COOH}\]
Glutamic acid (\(\alpha\)-aminoglutaric acid):
\[\ce{HOOCCH2CH2-CH(NH2)-COOH}\]
The amino acids listed above are all \(\alpha\)-amino acids — that is, compounds formed by replacing the \(\alpha\)-hydrogen atom (the hydrogen atom on the carbon atom nearest to the carboxyl group; hydrogen atoms on the next-nearest carbon atom are designated \(\beta\)-hydrogen atoms, and so on) with an amino group.
Amino acids are crystalline substances with relatively high melting points, generally in the range of \(200\)–\(300\,{}^{\circ}\text{C}\). Amino acids are generally soluble in water but insoluble in diethyl ether.
Since amino acid molecules contain both carboxyl groups and amino groups, they are both acidic and basic (amphoteric). Therefore, amino acids can form salts with both acids and bases. For example:
\[\ce{H2N-CH2-COOH + HCl -> [^+H3N-CH2-COOH]Cl-}\]
\[\ce{H2N-CH2-COOH + NaOH -> H2N-CH2-COONa + H2O}\]
The first salt is acidic (because the molecule still contains a carboxyl group), and the second salt is basic (because the molecule still contains an amino group).
When proteins are hydrolyzed under the action of acid, base, or enzymes, they pass through the polypeptide stage and ultimately yield various \(\alpha\)-amino acids. Therefore, amino acids are the “building blocks” of proteins.
Polypeptides
When the carboxyl group of one amino acid molecule and the amino group of another amino acid molecule undergo a condensation reaction with the elimination of a water molecule, the product is called a peptide. The amide bond structure \(\ce{-CO-NH-}\) in the product is called a peptide bond. The chemical equation for two amino acid molecules reacting to form a peptide is:
\[\ce{H2N-CHR-COOH + H2N-CHR'-COOH -> H2N-CHR-CO-NH-CHR'-COOH + H2O}\]
A compound containing one peptide bond, formed from two amino acid molecules with elimination of water, is called a dipeptide. A compound containing multiple peptide bonds, formed from several amino acid molecules with elimination of water, is called a polypeptide. Polypeptides are usually chain-shaped. Proteins yield polypeptides upon hydrolysis; polypeptides upon further hydrolysis ultimately yield \(\alpha\)-amino acids. There is no strict dividing line between polypeptides and proteins — generally, those with a formula mass less than \(10{,}000\) are called polypeptides.
4.5 Section 5: Proteins
Properties of Proteins
Proteins are widely distributed in living organisms and are the fundamental materials that compose cells. The main components of animal muscle, skin, blood, milk, hair, fur, horns, and hooves are all proteins. Plant organs also all contain proteins — for example, wheat seeds contain about \(18\%\) protein. All enzymes are proteins. Filterable viruses contain proteins, though their composition is more complex.
Proteins are macromolecular compounds constructed from \(\alpha\)-amino acids linked through peptide bonds. Their formula masses vary greatly — some are in the tens of thousands, some in the hundreds of thousands, and occasionally some reach tens of millions. The formula masses of nucleoproteins are even larger — for example, the nucleoprotein of tobacco mosaic virus has a formula mass exceeding twenty million, and that of adenovirus is even larger.
Nucleoproteins usually consist of proteins and nucleic acids. There are two types of nucleic acids: those containing ribose are ribonucleic acid (RNA), and those containing deoxyribose are deoxyribonucleic acid (DNA). DNA has a double-helix structure (see color plate), held together by hydrogen bonds. It is a substance closely related to biological heredity.
Starch, cellulose, and proteins are all naturally occurring macromolecular compounds (natural polymers).
The structure of proteins is extremely complex. Various \(\alpha\)-amino acids are arranged in a specific sequence within polypeptide chains, and polypeptide chains are linked to each other by hydrogen bonds, forming spatial (three-dimensional) structures. Proteins with different structures have different physiological functions — even if the chemical composition remains unchanged, a change in the spatial structure can alter physiological function.
Besides carbon, hydrogen, and oxygen, all proteins contain nitrogen and small amounts of sulfur, with nitrogen content of \(15\%\)–\(18\%\) and sulfur content of \(0.3\%\)–\(2.5\%\).
Below we briefly introduce some properties of proteins:
1. Amphoteric Nature
Proteins are macromolecular compounds constructed from \(\alpha\)-amino acids through peptide bonds. In protein molecules, both amino groups and carboxyl groups are present. Therefore, like amino acids, proteins are amphoteric substances.
2. Salting Out
To a test tube containing egg white solution, slowly add saturated ammonium sulfate or sodium sulfate solution. Observe the precipitate that forms. Add a small amount of the liquid with precipitate to a test tube containing clean water and observe whether the precipitate dissolves.
Some proteins, such as egg white, can dissolve in water to form solutions. The diameter of protein molecules reaches the size of colloidal particles, so protein solutions possess the properties of colloids.
A small amount of salt (such as ammonium sulfate or sodium sulfate) can promote the dissolution of proteins. However, if a concentrated inorganic salt solution is added to a protein solution, it decreases the solubility of the protein, causing it to precipitate out of solution. This process is called salting out. Protein precipitated in this way can still redissolve in water without affecting the original properties of the protein. Therefore, salting out is a reversible process. This property can be exploited by using repeated salting out to separate and purify proteins.
3. Denaturation
To each of two test tubes, add \(3\ \text{mL}\) of egg white solution. Heat one test tube, and add a small amount of lead acetate solution to the other. Observe what happens. Transfer the coagulated protein and the precipitate into two separate test tubes containing clean water and observe whether they dissolve.
Under the action of heat, acids, bases, heavy metal salts, ultraviolet light, and other agents, proteins undergo changes in properties and coagulate. This coagulation is irreversible — the proteins cannot be restored to their original form. This change is called denaturation. After denaturation, proteins lose their original solubility and their physiological activity. High-temperature sterilization works by using heat to coagulate proteins, thereby killing cells. Heavy metal salts (such as copper salts, lead salts, mercury salts, etc.) can coagulate proteins, which is why they are toxic to humans.
4. Color Reactions
To a test tube containing \(2\ \text{mL}\) of egg white solution, add a few drops of concentrated nitric acid and heat gently. Observe the color of the precipitate that forms.
Proteins can undergo color reactions with many reagents. For example, some proteins turn yellow when treated with concentrated nitric acid. Proteins that exhibit this reaction generally have benzene rings in their molecules — for example, their peptide chains contain phenylalanine units. When concentrated nitric acid accidentally splashes on skin, causing it to turn yellow, this is because the concentrated nitric acid reacts with skin proteins in a color reaction.
In addition, when proteins are burned, they produce an odor characteristic of burnt feathers.
Protein is an indispensable nutrient for humans and animals. The proteins we consume in food are hydrolyzed under the action of pepsin in gastric juice and trypsin in pancreatic juice, producing amino acids. After absorption by the body, amino acids recombine to form the various proteins the body requires.
Life processes are closely related to proteins — without proteins, there is no life. In 1965, Chinese scientists were the first in the world to synthesize a biologically active protein — crystalline bovine insulin — by artificial methods. This was an important contribution to research on proteins and life.
Proteins are important not only biologically but also have wide industrial applications. Animal wool and silk are composed of proteins and are important textile raw materials. When animal hides are treated with tanning agents, the proteins they contain become insoluble in water and resistant to decay, allowing them to be processed into soft, tough leather.
Animal glue is mainly composed of proteins and is obtained by boiling bones and skins. Colorless, transparent animal glue is called gelatin and is a raw material for making photographic films and photographic paper.
Casein, the protein in milk, is not only used as food but can also be combined with formaldehyde to synthesize casein plastic.
Enzymes
Enzymes are a class of proteins that possess the characteristics of proteins — for example, they denature when heated slightly or when exposed to strong acids, strong bases, or heavy metal salts. Enzymes also have their own distinctive characteristics: they are catalysts produced by living organisms, catalyzing many organic chemical reactions and complex reactions in living systems — such as oxidation–reduction and hydrolysis reactions. Enzymes retain their activity even after being separated from living organisms. The fermentation of starch and cellulose to glucose under the action of microorganisms, and the fermentation of glucose to alcohol, both involve the action of various enzymes produced by microorganisms.
In the physiological activities of humans, animals, and plants, enzymes play important roles. For example, starch in food is hydrolyzed by amylase in saliva and pancreatic juice; fats in food are hydrolyzed by lipase in pancreatic juice and intestinal fluid; proteins in food are hydrolyzed by protease (pepsin and trypsin) in gastric and pancreatic juices.
Pour \(10\ \text{mL}\) of freshly prepared \(1\%\) starch solution into a test tube and add about \(1\ \text{mL}\) of saliva. Mix them uniformly and keep the mixture at room temperature. Take a test tube, add 5 drops of the mixture, and test with iodine solution. Simultaneously perform a comparison test with starch solution that has not had saliva added.
When analyzing this experiment, compare it with the experiment in which starch is hydrolyzed using a strong acid catalyst. We can see that hydrolyzing starch with amylase, compared to using a strong acid, has two advantages: first, mild conditions — no heating is required; second, the reaction is fast and highly efficient. Enzyme catalysis has another characteristic: specificity — for example, amylase catalyzes only the hydrolysis of starch.
Over a thousand different enzymes are now known. Most enzymes used in large quantities in industry are produced through microbial fermentation, and many enzymes have been crystallized. Enzymes have found wide application: amylase is used in the food, fermentation, textile, and pharmaceutical industries; protease is used in medicine and the leather industry; lipase is used for fat hydrolysis and wool degreasing. Enzymes are also used in the chemical industry to produce various organic solvents and reagents, such as citric acid, acetone, and butanol.
Exercises for Sections 4–5
What are amino acids? What special properties do they have?
A certain protein contains \(0.64\%\) sulfur, and its molecules contain only two sulfur atoms. Calculate the formula mass of this protein.
Three test tubes contain solutions of protein, starch, and soap, respectively. How can you distinguish them?
Based on the properties of proteins, answer the following questions:
Why can boiling be used to sterilize medical instruments?
Why can copper sulfate or mercuric chloride solutions kill bacteria?
Why are heavy metal salts of copper, mercury, and lead toxic to humans and livestock?
If someone accidentally ingests heavy metal salts, why can drinking large amounts of milk, egg white, or soy milk serve as an antidote?
Why can the burning test be used to distinguish wool fabrics from cotton fabrics?
4.6 Chapter Summary
I. Sugars
Monosaccharides are polyhydroxy aldehydes or polyhydroxy ketones, such as glucose and fructose. Monosaccharides cannot be hydrolyzed into simpler sugars. Glucose has reducing properties and is an important nutrient.
Oligosaccharides produce two or several monosaccharide molecules upon hydrolysis. The most important oligosaccharides are disaccharides, such as sucrose and maltose. Sucrose does not have reducing properties, but its hydrolysis products do. Sucrose is an important sweet food.
Polysaccharides are macromolecular compounds formed by many monosaccharide molecules combining in a specific manner through the removal of water molecules, such as starch and cellulose. Neither starch nor cellulose has reducing properties. Polysaccharides can be hydrolyzed to monosaccharides, and their hydrolysis products have reducing properties. Starch and cellulose have important uses in daily life and in industrial production.
II. Proteins
Amino acids: Proteins yield \(\alpha\)-amino acids upon hydrolysis. Amino acids exhibit both acidic and basic properties (they are amphoteric).
Proteins are macromolecular compounds constructed from \(\alpha\)-amino acids linked through peptide bonds. A small amount of salt can promote protein dissolution; a large amount of salt can cause proteins to precipitate from solution. Proteins exhibit both acidic and basic properties. Proteins denature when heated or treated with heavy metal salts. Proteins are the material basis of life.
Enzymes are proteins that catalyze many reactions in organisms and in organic chemistry — they are biological catalysts. Enzyme catalysis is characterized by specificity, high efficiency, and mild reaction conditions. Enzyme-catalyzed reactions are already widely applied in production.
Review Problems
Are the following statements correct? Explain your reasoning.
Fats, carbohydrates, and proteins are all main components of human food and are all natural macromolecular compounds.
Carbohydrates are hydrates of carbon.
Egg white solution is a solution, but it has the properties of a sol (colloidal solution).
A disaccharide is a sugar containing two carbon atoms.
Are sucrose and maltose structural isomers? Are starch and cellulose structural isomers? Explain your reasoning.
During the hydrolysis of starch to produce glucose, how can you determine whether: (1) the starch has not yet begun to hydrolyze, (2) the starch is currently undergoing hydrolysis, or (3) the hydrolysis of starch is complete?
What are the differences in composition among guncotton, cellulose acetate, rayon, and filter paper?
Translator’s note: The original gives \(669.9\ \text{kCal}\). Converting: \(669.9 \times 4.184 \approx 2803\ \text{kJ}\).↩︎