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.

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.