Conformations of arginine and lysine side chains in association with anions

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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 χ1, torsions have a preference for the trans conformation. However, for arginine in protein structures, χ3 and χ4 appear to be flexible and can be tuned for optimal anion binding. For χ4, values in the range –80 to 80° are excluded for steric reasons; the remaining region in conformational space is accessible. This orientational variety exhibited by χ4 has not been hitherto appreciated. Factors that can forbid a χ-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.