Answer:
(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.
