(a) Why glucose pentaacetate does not react with hydroxylamine

Hydroxylamine $(H_2N-OH)$ reacts with a free carbonyl group to form an oxime.

In glucose the aldehyde group is not actually free — the molecule exists almost entirely in the cyclic hemiacetal form, in which C-1 carries an $-OH$ group. A small amount of the open-chain form is nevertheless in equilibrium with it, which is why ordinary glucose does react.

When glucose is acetylated, all five $-OH$ groups, including the one at C-1, are converted into $-OCOCH_3$ groups. The ring is now locked shut and can no longer open, so no free $-CHO$ group can ever be produced.

With no carbonyl group available, the pentaacetate cannot form an oxime.

This observation was one of the key pieces of evidence for the cyclic structure of glucose.

(b) Why amino acids behave like salts

An amino acid contains both an acidic $-COOH$ group and a basic $-NH_2$ group. In the solid state and in aqueous solution the proton transfers from the acid to the base within the same molecule, giving a zwitter ion :

$$H_2N-\underset{|}{\overset{R}{C}}H-COOH \rightleftharpoons \overset{+}{H_3}N-\underset{|}{\overset{R}{C}}H-COO^-$$

This dipolar ion carries both a positive and a negative charge, so the crystal is held together by strong ionic (electrostatic) forces exactly as in a salt.

The salt-like consequences follow :

• High melting points (they usually decompose rather than melt)
• Soluble in water but largely insoluble in organic solvents
• Amphoteric — they react with acids through $-COO^-$ and with bases through $-NH_3^+$

(c) Why water-soluble vitamins must be taken regularly

The water-soluble vitamins (the B-complex group and vitamin C) dissolve in the aqueous body fluids. Any excess passes into the blood, is filtered out by the kidneys and is readily excreted in the urine.

Because they cannot be stored in the body, a fresh supply is needed every day. A gap in the diet soon produces deficiency symptoms — scurvy from lack of vitamin C, beri-beri from lack of $B_1$.

Contrast : the fat-soluble vitamins A, D, E and K dissolve in fat and are stored in the liver and adipose tissue, so they need not be supplied daily.

(d) Why the two DNA strands are complementary

The two strands are held together by hydrogen bonds between the nitrogenous bases, and the geometry of those bonds allows only two specific pairings :

$$\text{Adenine} = \text{Thymine} \quad \text{(two hydrogen bonds)}$$

$$\text{Guanine} \equiv \text{Cytosine} \quad \text{(three hydrogen bonds)}$$

A large purine must always pair with a small pyrimidine for the double helix to keep a uniform width, and only these combinations have their hydrogen-bond donors and acceptors correctly placed.

Consequently, the sequence of one strand completely fixes the sequence of the other. This complementarity is what makes self-replication possible : each strand serves as a template for building its partner.

(e) Nucleoside versus nucleotide

$$\text{Base} + \text{Sugar} \rightarrow \text{Nucleoside} \xrightarrow{H_3PO_4} \text{Nucleotide}$$

(f) Two types of secondary structure of proteins

1. $\alpha$-Helix — the polypeptide chain coils into a right-handed spiral, stabilised by hydrogen bonds between the $>C=O$ of one amino acid and the $-NH$ of the fourth residue along the chain. Example : keratin in hair and wool.
2. $\beta$-Pleated sheet — the chains lie stretched out side by side and are held together by hydrogen bonds between neighbouring chains, giving a folded sheet. Example : fibroin in silk.

(g) Role of enzymes in biological systems

Enzymes are biological catalysts, almost all of which are globular proteins.

Their functions :

• They increase the rate of biochemical reactions enormously — often by a factor of $10^{6}$ to $10^{12}$ — by providing an alternative path of lower activation energy.
• They are highly specific, each enzyme catalysing essentially one reaction or one type of reaction. This is described by the lock and key model, in which the substrate fits the enzyme's active site precisely.
• They work under the mild conditions of the body — around $37\ ^\circ C$ and near-neutral pH — where the same reactions would otherwise need high temperature or strong acid.
• Being catalysts, they are not consumed and are recovered unchanged.

Examples : maltase hydrolyses maltose to glucose; urease hydrolyses urea to ammonia and carbon dioxide.

(Any five of the seven parts were required.)