The value of pk1 increases for Alanine when it's part of an oligopeptide compared to free Alanine due to decreased electrostatic interaction between the amino group and carboxylic group which lowers the strength of acidic group i.e. carboxylic group.
The amino acids are amphoteric in nature and have two or three pK values, depending on their side chains. pKa1 is the α-carboxyl group, pKa2 is the α-ammonium ion, pKa3 is the side chain group (if applicable).
Electrostatic interaction between the carboxylate anion and the protonated amino group of the Alanine zwitterion favorably affects the ionization of the carboxyl group. This favorable interaction decreases as the length of the poly-Ala increases, resulting in an increase in pK1.
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