Influence of the Length of the Alanine Spacer on the Acidic–Basic Properties of the Ac–Lys–(Ala)n–Lys–NH2 Peptides (n = 0, 1, 2, …, 5)

By using the potentiometric titration method, we have determined the pK(a) values of the two terminal lysine groups in six alanine-based peptides differing in the length of the alanine chain: Ac-Lys-Lys-NH(2) (KK), Ac-Lys-Ala-Lys-NH(2) (KAK), Ac-Lys-Ala-Ala-Lys-NH(2) (KAK2), Ac-Lys-Ala-Ala-Ala-Lys-NH(2) (KAK3), Ac-Lys-Ala-Ala-Ala-Ala-Lys-NH(2) (KAK4), and Ac-Lys-Ala-Ala-Ala-Ala-Ala-Lys-NH(2) (KAK5) in aqueous solution. For each compound, the model of two stepwise acid-base equilibria was fitted to the potentiometric-titration data. As expected, the pK(a) values of the lysine groups increase with increasing length of the alanine spacer, which means that the influence of the electrostatic field between one charged group on the other decreases with increasing length of the alanine spacer. However, for KAK3, the pK(a1) value (8.20) is unusually small and pK(a2) (11.41) is remarkably greater than pK(a1), suggesting that the two groups are close to each other and, in turn, that a chain-reversal conformation is present for this peptide. Starting with KAK3, the differences between pK(a1) and pK(a2) decrease; however, for the longest peptide (KAK5), the values of pK(a1) and pK(a2) still differ by about 1 unit, i.e., by more than the value of log(10) (4) = 0.60 that is a limiting value for the pK(a) difference of dicarboxylic acids with increasing methylene-spacer length. Consequently, some interactions between the two charged groups are present and, in turn, a bent shape occurs even for the longest of the peptides studied.