title: Conformations of arginine and lysine side chains in association with anions.
abstract: The side chain conformations shown by arginine and lysine in amino-acid and peptide crystal structures and bound to oxyanions in proteins have been analyzed in an attempt to understand the behaviour of these long-chain amino acids in an ionic environment. Except for chi 1, torsions have a preference for the trans conformation. However, for arginine in protein structures, chi 3 and chi 4 appear to be flexible and can be tuned for optimal anion binding. For chi 4, values in the range -80 to 80 degrees are excluded for steric reasons; the remaining region in conformational space is accessible. This orientational variety exhibited by chi 4 has not been hitherto appreciated. Factors that can forbid a chi-angle to be in the trans geometry are the simultaneous binding of the anion by the main- and side-chain atoms, or the sharing of the anion between two different molecules in the crystal structure. Small molecules containing arginine have a distinct tendency to crystallize with two molecules in the asymmetric unit. This may be a general phenomenon for all extended molecules which have hydrogen-bond donors (or acceptors) embedded in a rigid set-up.
authors.affiliation: Physical Chemistry Division, National Chemical Laboratory, Pune, India.
authors.forename: P
authors.initials: P
authors.lastname: Chakrabarti
mesh_terms: D000595:Amino Acid Sequence; D000838:Anions / Q000737:chemistry*; D001120:Arginine / Q000737:chemistry*; D003460:Crystallization; D008239:Lysine / Q000737:chemistry*; D008969:Molecular Sequence Data; D011487:Protein Conformation
chemical_list: D000838:Anions; D001120:Arginine; D008239:Lysine