CHARGE AND SOLVATION EFFECTS IN ANION RECOGNITION CENTERS - AN INQUIRY EXPLOITING REACTIVE ARGININES

Following a long-standing suggestion of Riordan et al. [Riordan, J. F. , McElvany, K. D., and Borders, C. L., Jr. (1977) Science 195, 884-885 ], we sought to exploit chemically activated arginines as probes to ch aracterize the microenvironnental effects in enzymes that mediate the recognition of anionic substrates. A micellar simulation study establi shes that octylguanidine (OGn) becomes chemically activated upon incor poration into both cetyltrimethylammonium bromide (CTAB) and Triton X- 100 micelles and that the activations correlate with the pK(a) diminut ions induced in its guanidiniun,group by the effects of electrostatic or nonelectrostatic nature as reflected in the results of pH and salt titration experiments. Next, a protein modification study establishes that the modifiable arginines in a number of enzymes also have diminis hed pK(a)'s, again due to effects of electrostatic or nonelectrostatic nature as reflected in the results of pH and salt titration experimen ts. Warwicker's finite difference Poisson-Boltzmann algorithm [Warwick er, J. (1992) J. Mol. Biol. 223, 247-257] is applied to several of the enzymes with available crystal structure coordinates, and indeed, the ir chemically activated arginines are found to be in an electrostatic microenvironment that can diminish their pK(a)'s, with the magnitudes of these diminutions matching closely the diminutions measured experim entally. Finally, the chemically activated arginines are examined with respect to their atomic atmosphere and are thus found to occur in a l ocal microenvironment that would facilitate their roles as anion ancho rs. Thus, electrostatic and solvation effects are found to be critical determinants of the arginine role as an anion anchor.