TRANSFERABILITY OF THE -COOH...-OOC DYAD GEOMETRY FROM THE GAS-PHASE TO CRYSTALS AND PROTEINS

Simple models of the -COO- ... HOOC- moiety were studied by ab initio and semiempirical MNDO/PM3 methods combined with retrieval from the Ca mbridge Crystallographic Database and Protein Data Bank in order to ch eck transferability of the non-bonded geometry. We found that the gas- phase O ... O distance (252 pm) elongates in crystals and proteins to 256 +/- 14 and 262 +/- 27 pm, respectively. C-O ... O-C dihedral angle s are much more variable, however, energetically excluded values appea r only exceptionally in crystals and less frequently in proteins. Most probably due to packing effects -COO- ... HOOC- conformations, prefer red in the gas phase, are poorly populated in crystals