Proton Transfer in Peptide Systems

Abstract

This thesis details investigations into the proton transfer reactions of peptide systems and how this knowledge can be used to understand the chemical behaviour of peptides.

The acidity of the α-proton in peptides has an essential role in numerous biochemical reactions. The carbon acidity of cyclic dipeptides, diketopiperazines (DKPs), has been studied. Hydrogen deuterium exchange reactions monitored by 1H NMR spectroscopy were used to measure second order rate constants for deprotonation by deuteroxide, kDO (M^–1 s^–1) at 25 °C and I = 1.00 (KCl). Values recorded were between 1.12 × 10^–1 – 1.02 × 10^–3 M^–1 s^–1 and in the order of Prolyl > Glycyl > Alanyl > Tyrosyl. Prolyl residues had higher than expected kinetic acidities – this was attributed to a stereoelectronic effect; the other residues followed expected trends. Estimates for the pKas were interpolated from a Brønsted relationship as pKa = 18.8 – 23.7. It is posited that the stereoelectronic effect could play a role in enzyme catalysis by lowering the barrier to enolate formation.

The carbon acidity of triketopiperazines (TKPs), DKPs with an additional carbonyl, was also studied. Values of kDO for the α-protons were between 5.09 × 10^5 – 8.99 × 10^6 M^–1 s^–1. This is ~ 10^6-fold larger than that of the DKPs and was attributed to the early development of aromaticity in the transition state for the TKPs. A Brønsted relationship for carbon acids with aromatic enolates gave pKa estimates between -3.35 – 4.39.

Native Chemical Ligation (NCL) is one of the two principle techniques used in the chemical synthesis of proteins and ligates an N-terminal cysteine thiolate with a C- terminal thioester that upon rearrangement forms the native amide bond. We have measured the pKas of thiols in a series of cysteine derivatives using UV-Vis spectrophotometry at 25 °C and I = 0.3 (NaCl). Our results have been used to develop a detailed understanding of the mechanism of kinetically controlled NCLs.