LOW-BARRIER HYDROGEN-BONDING IN MOLECULAR-COMPLEXES ANALOGOUS TO HISTIDINE AND ASPARTATE IN THE CATALYTIC TRIAD OF SERINE PROTEASES

We present spectroscopic evidence for the presence of low-barrier hydrogen bonds (LBHBs) in molecular complexes composed of carboxylic acids and 1-methylimidazole (1-MeIm) dissolved in aprotic organic solvents. A plot of the values of the low-field proton NMR chemical shifts versus the aqueous pK(a) of the carboxylic acid exhibits a positive slope for pK(a) values below 2.1 and a negative slope for higher pK(a) values. The chemical shifts for protons near the maximum in this plot are 18 ppm, similar to that of 18.3 ppm for His(57)-Asp(102) in the protonated catalytic triad of chymotrypsin, The chemical shifts for the proton bonded to C2 of 1-MeIm in these complexes also vary with the pK(a) of the carboxylic acid and reveal a gradual change from neutral, hydrogen-bonded 1-MeIm in complexes of weaker acids to hydrogen-bonded 1-methylimidazolium ion in complexes of stronger acids. The midpoint chemical shift for the C2 proton corresponds to a carboxylic aqueous pK(a) of about 2.1. FTIR spectra of the 1-MeIm-carboxylic acid complexes in CHCl3 indicate that hydrogen bonding is strong and that the complexes are of three types: (a) neutral complexes with the weaker acids (pK(a) greater than or equal to 2.2) in which the antisymmetric carbonyl stretching frequencies are lowered relative to the free acids and the ethyl esters of the same acids; (b) ionic complexes of stronger acids (pK(a) less than or equal to 2.1) in which the carbonyl stretching frequencies are slightly lower than those for the tetrabutylammonium salts of the same acids; (c) ionic complexes of the same acids (pK(a) less than or equal to 2.1) coexisting with type b, in which the carbonyl stretching frequencies are intermediate between those for the tetrabutylammonium salts (bond order 1.5) and those of the same acids or their esters (bond order 2.0). The latter complexes appear to incorporate a low-barrier hydrogen bond and are presented as models for the protonated triad of chymotrypsin and other serine proteases. These enzymes have been postulated to utilize a low-barrier hydrogen bond between His(57) and Asp(102) to facilitate the abstraction of the beta-OH proton from Ser(195) in the course of catalysis [Frey, P. A., Whitt, S. A., and Tobin, J. B. (1994) Science (Washington, D.C.) 264, 1997-1930].