Role of hydrogen bonding and water clusters in deamidation of peptide in glycerol-water solutions

The study is part of investigations on relationships between water content, structure, and rates of chemical reactions in amorphous systems. This paper reports Asn deamidation of a hexapeptide in an amorphous liquid matrix, glycerol with water concentration of 1 to 30 wt%, at 50 °C. Using an amorphous liquid system allows focusing on the chemical and structural features of water effects, by minimizing the "molecular mobility" aspect. High-performance liquid chromatography (HPLC) is used to quantify both the loss of the parent compound and the accumulation of the cyclic succinimide intermediate and the hydrolysis products, Asp and iso-Asp. The rate constants for succinimide formation (k₁) and succinimide hydrolysis (k₂ and k₃) are determined by fitting the HPLC data to specific kinetic models. The apparent pH of the solutions is confirmed to be independent of water content by using two orthogonal approaches. The experimental studies are complemented by molecular dynamics (MD) simulations of the hydrogen-bonding network around the Asn. This work reveals two water-content regions with distinct effects on deamidation. The first region shows a nearly constant k₁ for water concentrations up to 8 wt%, whereas a significant increase in k₁ with increased water content is observed in the second region above 12 wt% water. The water content threshold for the deamidation rate coincides with the spectroscopically determined thresholds for hydrogen bonding and water clustering in glycerol/water mixtures, as reported previously by a range of techniques including Raman spectroscopy. The study highlights relevance of hydrogen bonding and water clustering pattern for chemical processes including deamidation, and provides a basis for follow-up studies on the role of amorphous structure in deamidation in amorphous freeze-dried peptide and protein formulations.