STRUCTURES OF SOME SUBSTITUTED NITROPHENOLS DETERMINED BY AB-INITIO COMPUTATION - ON THE ORIGIN OF HETEROCLITICITY IN ANTI-NITROPHENOL ANTIBODIES

Nitrophenol and its derivatives are widely used in analytical chemistr y and also as haptens in immunology. In C57BL/6 mice the iodo derivati ve binds the antibody to nitrophenol over ten times more efficiently t han the eliciting hapten nitrophenol. The latter result is unusual sin ce it is expected that the eliciting hapten should bind its antibody b est and all other haptens bind less well. Our X-ray crystallographic a nd molecular modelling studies showed that in the structures of the an tibody in complex with nitrophenol and iodonitrophenol the haptens bin d in a cavity making van der Waals and hydrogen bonding contacts with the antibody. The iodine in the iodonitrophenol-antibody complex point s away from the binding site and does not provide additional contacts with the protein. In order to understand how the iodine substitution i n the nitrophenyl ring contributes to the binding of the hapten to the antibody we have carried out ab initio molecular orbital studies on n itrophenol, iodonitrophenol and their corresponding anions. We have ap proached this problem by analyzing the electron distribution, bond and molecular energies. The wavefunction was determined at the Hartree-Fo ck level using the basis set 3-21G(d,p). Optimized equilibrium geometr ies were characterized by harmonic vibrational analysis. Atomic charge s were obtained from Mulliken's population analysis and by the direct integration of molecular electron densities using Bader's ''Atoms In M olecules''. We find that the substitution with iodine changes the char ge distribution in the nitrophenyl ring, thus enhancing the electrosta tic interaction of the ring with the antibody heavy chain Lysine 59 an d Arginine 50. This explains how the iodine increases the binding affi nity without directly interacting with the protein. (C) 1997 Elsevier Science B.V.