INTERMEDIATE STRUCTURE OF CARBON ATOM FROM NONBONDED [[pi]]-[[pi]] INTERACTION TO [[sigma]]-COVALENT BOND. T. Ishida^l)*, Y. In¹⁾, H. Nagata^l), C. Hayashi^l), M. Doi^l) and A. Wakahara^{2) 1)}Department of Physical Chemistry, Osaka University of Pharmaceutical Sciences, 2-10-65 Kawai, Matsubara, Osaka 580, Japan, ²⁾Fujisawa Pharmaceutical Co., Ltd., 2-1-6 Kashima, Yodogawa-ku, Osaka 532, Japan

Two kinds of Cu(II) complex crystals (1 and 2) of pyridoxal-5-phosphate (PLP)-pyridoxamine-5-phosphate (PMP) Schiff base were obtained during the in vitro Cu(II)-mediated transamination reaction of L-alanine by PLP.

Concerning the intermolecular carbon...carbon short contact, the distance less than 3.4 Å, which corresponds to the sum of van der Waals radii for aromatic carbon atom, is generally thought to be very unstable because of the strict van der Waals repulsion. However, the X-ray crystal analysis of 1 showed unusual C...C short contacts of ~2.3 Å between the -CH-N-CH- fragments of the centrosymmetrically-related Schiff bases, which correspond to an intermediate distance of the intermolecular [[pi]]-[[pi]] short contact and the usual [[alpha]]-covalent bond. On the other hand, the X-ray crystal analysis of 2 showed the structure in which one of these two C...C short contact pairs observed in 1 was covalently bonded.

Judging from these X-ray results and other spectroscopic data, the crystal structure of 1 could correspond to the first presentation of the carbon Sp² sp³ transition state at the atomic level, and the appearance of its structure is primarily due to the delicate/elaborate combination of (i) the Cu(II)-mediated deprotonation (production of carbanion) of azomethine carbons [-CH-N-CH-], (ii) the structural stability of ligand molecule reinforced by the pyramidal coordination of Cu(II) ion and (iii) the sparing solubility of 1 in the acidic reaction solution, resulting in the inhibition of transition to 2 and the precipitation of 1. The chemical and structural features of short contact carbon atom in 1 have been discussed by reference to the structure of 2. The in vitro Cu(II)-dependent formation of complexes 1 and 2 from PLP and amino acid have been also considered.