Carboxyl group hydrogen bonding in X-ray protein structures analysed using neutron studies on amino acids

A method is proposed to make a distinction between ionized and neutral carboxyl groups in X-ray protein structures. This is based on an analysis of the relative hydrogen bonding populations and bond-length bond-valence correlations in high-precision neutron studies of amino acids and small peptides. With the help of this method, four amino acid residues containing carboxyl groups in the refined structure of triclinic hen egg-white lysozyme have been analysed. Two of these, Glu-35 and Asp-52, are involved in lysozyme function, while the other two, Glu-7 and Asp-101, form a protein-protein inter-molecular contact in the triclinic structure.