Solubility. In each of the following pairs of amino acids, identify which amino acid would be most soluble in water: (a) Ala, Leu; (b) Tyr, Phe; (c) Ser, Ala; (d) Trp, His.

(a) Ala; (b) Tyr; (c) Ser; (d) His

Explanation:

The general formula for an amino acid is NH₂-CH(R)-COOH

The differences among the different amino acids is caused by the side groups R.

(a) Ala, Leu

Ala: R =                 CH₃-

Leu: R = (CH₃)₂CHCH₂-

Leucine has a larger hydrophobic side group. It should be the less soluble of the pair,

Alanine is the more soluble amino acid.

(b) Tyr, Phe

Tyr: R = HOC₆H₄CH₂-

Phe: R =      C₆H₅CH₂-

Tyrosine has a polar OH group. It can form hydrogen bonds with water.

Tyrosine is the more soluble amino acid.

(c) Ser, Ala

Ser: R = HOCH₂-

Ala: R =       CH₃-

Serine has a polar OH group. It can form hydrogen bonds with water.

Serine is the more soluble amino acid.

(d) Trp, His

Trp: R =       Indole-CH₂-

His: R = Imidazole-CH₂-

Tryptophan has a large aromatic hydrophobic side chain.

Histidine is a basic amino acid. At pH 7 it exists mainly as the anion NH₂CHRCOO⁻, which is hydrophilic.

Histidine is the more soluble amino acid.

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