Ionization properties of titratable groups in ribonuclease T-1 II. Electrostatic analysis

The experimental NMR data for the individual titratable groups in ribonucle ase T-1 presented in the preceding paper were analysed by means of a contin uum dielectric model. The role of two factors, the alteration of hydrogen l oci on the ionizable groups and the conformational flexibility, were analys ed. It was suggested that the position of the titratable hydrogen is essent ial mainly for strongly interacting groups. For groups which are accessible to the solvent and whose ionization is not coupled with the ionization of neighbouring groups, this factor can be neglected. The influence of the con formational flexibility on the electrostatic interactions becomes apparent for the environment of K25. For some strongly interacting groups, non-sigmo idal ionization curves were calculated. On this basis the pH dependence of the NMR chemical shift of the C-13(6)2 resonance of H27, whose ionization i s coupled with E82, was reproduced.