VANADATE COMPLEX SPECTROSCOPY AT THE RNASE-A ACTIVE-SITE

Knowledge of the ionicity of the phosphorane intermediate is important to the analysis of the microscopic mechanism of the hydrolysis of the phosphate ester bond by ribonuclease A (RNase A). Five-coordinate uri dine vanadate, an analog of the phosphorane, binds to RNase A as the m onoanion. The absorption spectra of the vanadate is a probe of the ele ctronic structure of the active site. An in vacuo theoretical model of H4VO5- is calculated to have transitions only in the far ultraviolet (UV). However, H2VO5C2H4- has one in the near UV as well as others fur ther into the UV. The transition energy of the monoanion calculated in the field of the protein active site with effective fragment potentia ls shifts modestly to the red. Broad monoanion absorptions are predict ed which would overlap an observed incomplete very broad absorption at tributed to the complex of uridine vanadate with RNase A. The absorpti on bands of neutral ethylene glycol vanadate are predicted to be furth er to the red but also overlap the experimental absorption. (C) 1998 J ohn Wiley & Sons, Inc.