INFLUENCE OF THE INDUCTIVE EFFECT ON THE PROTOLYTIC PROPERTIES OF SOME ALIPHATIC AMINO ACIDS

The protolytic properties of a number of aliphatic amino acids, viz. 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 mol/L NaNO are studied by pH monitoring. Using the calculated ionization constants of amino acids and the equation allowing one to quantify the inductive effect in the aliphatic series (the Taft equation), the substituent constant σ* is calculated, with its values showing a positive correlation with the ionization constants of amino acids (pK) (R = 0.9561 and R = 0.8542). The results show that the acidity of the studied amino acids decreases as follows Cys > Ser > Gly > Leu > Ala > Ile > Val. The acid-base properties of amino acids are found to change depending on the inductive effect (the radical constant σ*). Among the studied amino acids, Cys exhibits the highest acidity and is considered to be the weakest base.