CARBOXYL GROUP HYDROGEN-BONDING IN X-RAY PROTEIN STRUCTURES ANALYZED 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 anal ysis 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 res idues containing carboxyl groups in the refined structure of triclinic hen egg-white lysozyme have been analysed. Two of these, Glu-35 and A sp-52, are involved in lysozyme function, while the other two, Glu-7 a nd Asp-101, form a protein-protein inter-molecular contact in the tric linic structure.