New Protonation Microequilibrium Treatment in the Case of Some Amino Acid and Peptide Derivatives Containing a Bis(imidazolyl)methyl Group

Abstract

We present a microequilibrium analysis of a series of amino acid and peptide derivatives containing the chelating bis(imidazol-2-yl)methyl group (BIP, Gly-BIMA, His-BIMA, alpha-Glu-BIMA, gamma-Glu-BIMA, and Phe-His-BIMA). NMR measurements were performed in D2O to follow the deprotonation steps. The software PSEQUAD and a specialized program written in MATLAB were used to determine the macroscopic and microscopic constants. The method assumes that the effect of pH on the chemical shift of an NMR-active nucleus can be interpreted by adding the independent effects of the protonation of individual sites. For derivatives containing histidine (His-BIMA and Phe-His-BIMA), the deprotonation steps of the second imidazole and the His-imidazole significantly overlap. In the Glu derivatives (alpha-Glu-BIMA and gamma-Glu-BIMA), the amino and the second imidazole pK values are separate; the deprotonation processes of the first imidazole nitrogen and the side-chain carboxyl group, however, significantly overlap. In gamma-Glu-BIMA, the deprotonation sequence is carboxylate-imidazole1-imidazole2-amino, while in the case of alpha-Glu-BIMA, it changes to imidazole1-carboxylate-imidazole2-amino, according to the microscopic pk values. The main advantage of the method is that it does not require the synthesis and NMR microequilibrium analysis of substances modeling the individual parts of the target ligand, in contrast to the methods used by others. The method presented here demands slightly more mathematical and computational power, which is readily available today.