Ligand recombination and a hierarchy of solvent slaved dynamics: The origin of kinetic phases in hemeproteins

Ligand recombination studies play a central role both for characterizing different hemeproteins and their conformational states but also for probing fundamental biophysical processes. Consequently, there is great importance to providing a foundation from which one can understand the physical processes that give rise to and modulate the large range of kinetic patterns associated with ligand recombination in myoglobins and hemoglobins. In this work, an overview of cryogenic and solution phase recombination phenomena for COMb is first reviewed and then a new paradigm is presented for analyzing the temperature and viscosity dependent features of kinetic traces in terms of multiple phases that reflect which tier(s) of solvent slaved protein dynamics is (are) operative on the photoproduct population during the time course of the measurement. This approach allows for facile inclusion of both ligand diffusion among accessible cavities and conformational relaxation effects. The concepts are illustrated using kinetic traces and MEM populations derived from the CO recombination process for wild type and mutant myoglobins either in sol–gel matrices bathed in glycerol or in trehalose-derived glassy matrices.