Influence of the Inductive Effect on the Protolytic Properties of Some Aliphatic Amino Acids

Abstract

The protolytic properties of a number of aliphatic amino acids were studied using pH measurements: serine (Ser), cysteine (Cys), glycine (Gly), alanine (Ala), valine (Val), leucine (Leu), and isoleucine (Ile) in aqueous solutions at T = 298.2 K and I = 0.1 M NaNO₃. Using the calculated ionization constants of amino acids and the equation that allows quantitative evaluation of the inductive effect in the aliphatic series (Taft equation), the substituent constant σ* was calculated, whose values showed positive correlation with the ionization constants of amino acids (pK_i) (R² = 0.9561 and 0.8542). The results allowed us to establish that the acidity of the amino acids decreases in the following order: Cys > Ser > Gly > Leu > Ala > Ile > Val. It was established that the acid-base properties of amino acids change regularly depending on the inductive effect (radical constant σ*). Among the amino acids studied, Cys exhibits the highest acidity and is considered the weakest base.