Solvent-temperature perturbations of ionizable groups as a tool for the investigation of the active site of enzymes.

Co-solvent and temperature effects on the pK of histidine (imidazolium) residue 46 of trypsin, as well as of weak electrolytes (buffers), which have been reported in two preceding papers, can be satisfactorily explained in terms of enthalpy-entropy compensation patterns. Such patterns have been generated for various mixed solvents between 20 degrees and minus 20 degrees and minus 50 degrees. Under these conditions compensation temperature, T-c, is strongly dependent on the nature of the ionizable group studied: 240 plus or minus 10 K for neutral acids and 310 plus or minus 5 K for cationic acids. This work focuses on the possibilities offered and on the problems raised by the use of this methodology as a tool in the investigation of the active site of enzymes. Furthermore, it is shown in the case of histidine residue 46 of trypsin that the co-solvent effect vanishes at the compensation temperature, a result of great practical significance if applicable to any ionizable group at the active site of enzymes.