Charge and Solvation Effects in Anion Recognition Centers: An Inquiry Exploiting Reactive Arginines

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Abstract
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 characterize the microenvironmental effects in enzymes that mediate the recognition of anionic substrates. A micellar simulation study establishes that octylguanidine (OGn) becomes chemically activated upon incorporation into both cetyltrimethylammonium bromide (CTAB) and Triton X-100 micelles and that the activations correlate with the pKa diminutions induced in its guanidinium 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 diminished pKa's, again due to effects of electrostatic or nonelectrostatic nature as reflected in the results of pH and salt titration experiments. Warwicker's finite difference Poisson−Boltzmann algorithm [Warwicker, J. (1992) J. Mol. Biol. 223, 247−257] is applied to several of the enzymes with available crystal structure coordinates, and indeed, their chemically activated arginines are found to be in an electrostatic microenvironment that can diminish their pKa's, with the magnitudes of these diminutions matching closely the diminutions measured experimentally. Finally, the chemically activated arginines are examined with respect to their atomic atmosphere and are thus found to occur in a local microenvironment that would facilitate their roles as anion anchors. Thus, electrostatic and solvation effects are found to be critical determinants of the arginine role as an anion anchor.