JEE Chemistry Biomolecules Notes Set 04

Carbohydrates: Classification

Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that give these on hydrolysis; the old name 'hydrates of carbon' comes from the general formula \( \ce{C_x(H_2O)_y} \), though some carbohydrates do not fit it. They contain only C, H and O. They are classed by hydrolysis: monosaccharides (glucose, fructose, galactose, ribose) cannot be hydrolysed further; oligosaccharides give 2 to 10 monosaccharide units (disaccharides sucrose, maltose, lactose; the trisaccharide raffinose); polysaccharides (starch, cellulose, glycogen) give many units. A monosaccharide is an aldose if it has an aldehyde group and a ketose if it has a ketone group, and it is named by carbon count, for example aldohexose (glucose), ketohexose (fructose), aldopentose (ribose, arabinose). Sugars are also described as reducing (they give Fehling's, Benedict's and Tollens' tests) or non-reducing (sucrose).

Glucose

1. Glucose, \( \ce{C_6H_{12}O_6} \), is the main sugar of blood and is also called dextrose because it is dextrorotatory. It is made by hydrolysing starch with dilute acid or sucrose with dilute acid (giving glucose and fructose).
2. Open-chain structure: it is a straight chain of six carbons with one aldehyde group and five hydroxyl groups (one primary, four secondary), and it has four chiral carbons. The evidence is: it forms a pentaacetate (five OH), a cyanohydrin, an oxime and a phenylhydrazone (carbonyl), it is reduced to sorbitol, it is oxidised by bromine water to gluconic acid (same chain length) and by nitric acid to saccharic acid, and prolonged heating with HI and red phosphorus gives n-hexane (unbranched chain).
3. Tests and reactions: it reduces Tollens' reagent (silver mirror), Fehling's and Benedict's solutions (red \( \ce{Cu_2O} \)). With excess phenylhydrazine it gives glucosazone; only carbons 1 and 2 react because the osazone is stabilised by an intramolecular hydrogen-bonded (chelate) ring. Glucose and fructose give the same osazone, and glucose and mannose (epimers at C-2) as well. In alkali it rearranges through an enediol to a mixture of glucose, fructose and mannose. Conc. \( \ce{H_2SO_4} \) dehydrates and chars it; yeast enzyme zymase ferments it to ethanol and carbon dioxide.
4. It does not give Schiff's test or the \( \ce{NaHSO_3} \) addition product, and it does not react as a free aldehyde in the ring form. This and the existence of two anomers show that the ring form is a cyclic hemiacetal.

The open-chain formula fails to explain two facts, the lack of the Schiff and bisulphite tests and the existence of two crystalline forms. The ring formula (Haworth) explains both. The C-5 hydroxyl adds to the C-1 aldehyde to give a six-membered pyranose ring (five carbons and one oxygen) with a new chiral centre at C-1, the anomeric carbon. The two forms are the anomers alpha-D-glucose (OH at C-1 down in the Haworth ring, specific rotation +112°) and beta-D-glucose (OH up, +19°). A fresh solution of either changes in rotation until it reaches +52.5°, an equilibrium of about 36 percent alpha and 64 percent beta with a trace of the open form; this is mutarotation. The letter D means that the highest-numbered chiral carbon (C-5) has the same configuration as D-glyceraldehyde, and it is unrelated to the sign of rotation. Glucose with methanol and dry HCl gives the alpha and beta methyl glucosides, which are acetals that no longer show mutarotation or reduce Tollens' reagent.

Fructose, Sucrose, Starch, Cellulose

Fructose (fruit sugar, laevulose) is a ketohexose, laevorotatory (-92°) and the sweetest sugar; it has three chiral carbons (so 8 stereoisomers) and exists as a five-membered furanose ring in combined form. Although it has no aldehyde group it reduces Tollens' and Fehling's reagents because alkali converts it to glucose and mannose through an enediol. Sucrose (cane sugar), \( \ce{C_{12}H_{22}O_{11}} \), is alpha-D-glucopyranosyl-beta-D-fructofuranoside; the glycosidic link joins the anomeric carbons of both units (C-1 of glucose and C-2 of fructose), so it has no free aldehyde or ketone group, does not reduce Fehling's solution and does not show mutarotation or form an osazone. Sucrose is dextrorotatory (+66.5°); hydrolysis by dilute acid or invertase gives equal amounts of glucose (+52.5°) and fructose (-92°), the mixture is laevorotatory (about -20°), and this change of sign is called inversion, the product being invert sugar. Maltose (two alpha-glucose units, 1,4 link) and lactose (galactose with glucose) are reducing disaccharides. Raffinose is a trisaccharide.

Starch is the food reserve of plants: about 20 percent amylose, a water-soluble unbranched chain of alpha-glucose with 1,4 links, and 80 percent amylopectin, a water-insoluble branched polymer with 1,4 links and 1,6 branch points. Starch gives a blue colour with iodine and is hydrolysed by diastase (amylase) to maltose and finally to glucose. Glycogen, the animal reserve, is stored in the liver and muscle and is like amylopectin but more highly branched. Cellulose is a linear polymer of beta-D-glucose with beta-1,4 links, is the structural material of plant cell walls and is the most abundant organic compound; humans cannot digest it because we lack the enzyme, but it is roughage. Cellulose treated with nitric and sulphuric acids gives cellulose nitrate (gun cotton, explosive), with acetic anhydride gives cellulose acetate (safety film, acetate rayon), and with NaOH and \( \ce{CS_2} \) gives viscose, from which rayon and cellophane are made. Carbohydrates are tested generally with Molisch's reagent (alpha-naphthol and conc. sulphuric acid, a violet ring). Glucose supplies about 4 kcal per gram.

Amino Acids and Peptides

Alpha-amino acids have the amino group and the carboxyl group on the same carbon, \( \ce{H_2N-CHR-COOH} \). They are crystalline, high-melting, water-soluble solids because they exist as zwitterions, \( \ce{^+H_3N-CHR-COO^-} \), which are amphoteric. At the isoelectric point (a particular pH) the zwitterion has no net charge, does not move in an electric field and has its minimum solubility. All natural amino acids except glycine (R = H) are chiral and have the L-configuration. They are classified as neutral, acidic (aspartic and glutamic acids) or basic (lysine, arginine, histidine), and as essential (valine, leucine, isoleucine, lysine, threonine, methionine, phenylalanine, tryptophan, with arginine and histidine in children) which the body cannot make, or non-essential. Cysteine carries an -SH group, and two cysteines form a disulphide cross-link; tyrosine and phenylalanine have aromatic rings. Amino acids give a purple colour with ninhydrin and release nitrogen with nitrous acid.

A peptide bond is the amide link formed between the carboxyl group of one amino acid and the amino group of the next with loss of water; two residues make a dipeptide, three a tripeptide, and a chain of more than about 10 units is a polypeptide. A chain of n residues has n minus 1 peptide bonds. Three different amino acids can be arranged in 3! = 6 different tripeptides if each is used once.

Proteins and Enzymes

1. Primary structure is the sequence of amino acids. Secondary structure is the regular local folding held by hydrogen bonds: the alpha-helix and the beta-pleated sheet. Tertiary structure is the overall three-dimensional folding held by hydrogen bonds, disulphide bridges, ionic and hydrophobic interactions. Quaternary structure is the arrangement of several chains, as in haemoglobin (four subunits).
2. Fibrous proteins (keratin, collagen, myosin, silk fibroin) are insoluble in water and form structures; globular proteins (insulin, albumin, haemoglobin, enzymes) are soluble. Conjugated proteins have a non-protein prosthetic group (haemoglobin has haem, a chromoprotein; glycoproteins, phosphoproteins).
3. Denaturation is the permanent loss of secondary and tertiary structure by heat, acid, alkali or heavy metal ions such as \( \ce{Ag^+} \) and \( \ce{Hg^{2+}} \), as in the coagulation of egg white; the primary structure is untouched.
4. Tests: the Biuret test (violet with alkaline \( \ce{CuSO_4} \)) detects peptide bonds, the xanthoproteic test (yellow with conc. \( \ce{HNO_3} \)) detects aromatic rings, Millon's test detects tyrosine (phenol), and ninhydrin detects free amino groups.

Enzymes are biological catalysts, almost all globular proteins, produced in living cells. They are highly specific, have a defined active site, work at body temperature and an optimum pH, lower the activation energy and are poisoned by inhibitors. Examples: amylase (ptyalin in saliva) and diastase convert starch to maltose, maltase gives glucose, invertase hydrolyses sucrose, zymase of yeast ferments glucose, pepsin and trypsin digest proteins (the stomach pH is about 2), lipase hydrolyses fats, and carboxypeptidase removes the C-terminal residue. Haemoglobin carries oxygen in blood on the iron of haem, and carbon monoxide binds more strongly to give carboxyhaemoglobin. Insulin is a protein hormone that lowers blood sugar and promotes glycogen formation. Antibodies are proteins (immunoglobulins) that neutralise antigens.

Fats, Oils and Lipids

Lipids are biomolecules insoluble in water and soluble in organic solvents, such as fats, oils, waxes, phospholipids and steroids. Fats and oils are triglycerides, the triesters of glycerol with long-chain fatty acids; the ester group is their characteristic group. Fats are solid with mostly saturated acids (palmitic, stearic); oils are liquid with unsaturated acids (oleic, linoleic, linolenic). Alkaline hydrolysis (saponification) gives glycerol and soap, and the enzyme lipase gives glycerol and fatty acids. Hydrogenation with nickel hardens oil into vegetable ghee. Waxes are esters of long-chain fatty acids with long-chain monohydric alcohols. Phospholipids are esters of glycerol with two fatty acids and one phosphate group; their polar heads and non-polar tails form the bilayer of cell membranes with the polar heads outward. Fats give about 9 kcal per gram, more than carbohydrates and proteins (about 4), and are the main energy reserve.

Vitamins and Hormones

Vitamins A, D, E and K are fat soluble and are stored in the body; B and C are water soluble and must be supplied regularly. Hormones are chemical messengers made by endocrine glands: insulin (pancreas, a polypeptide that lowers blood glucose), glucagon (raises blood glucose), thyroxine (thyroid), adrenaline, testosterone and estradiol (sex hormones, steroids), cortisone (adrenal corticoid), parathormone (raises blood calcium) and calcitonin (lowers it). Glutathione is a tripeptide present in most cells.

Nucleic Acids

Nucleic acids are polymers of nucleotides. A nucleoside is a pentose sugar joined at C-1 to a heterocyclic base, and a nucleotide is a nucleoside with a phosphate esterified at C-5. DNA has 2-deoxyribose; RNA has ribose. The purines are adenine and guanine; the pyrimidines are cytosine, thymine (DNA only) and uracil (RNA only). Nucleotides are linked by phosphodiester bonds between the C-5 of one sugar and the C-3 of the next, and the order inside a nucleotide is base, sugar, phosphate.

DNA (Watson and Crick, 1953) is a double helix of two antiparallel strands held by hydrogen bonds between the base pairs A=T (two bonds) and G≡C (three), so that by Chargaff's rule A = T and G = C. It stores hereditary information and replicates (multiplication) by using each strand as a template. A gene is the segment of DNA that codes for one polypeptide. RNA is single-stranded and of three kinds: messenger RNA (carries the code), ribosomal RNA and transfer RNA (carries an amino acid and has the anticodon). In transcription DNA is copied into m-RNA, and in translation the ribosome reads the m-RNA in triplets (codons, three nucleotides for one amino acid; the third base is less specific) to build the protein. A change in the base sequence is a mutation. ATP, adenosine triphosphate, is the energy carrier of the cell.