Millisecond time-scale protein dynamics exists prior to the activation of the bulk solvent matrix

Conformational dynamics of proteins is of fundamental importance in their physiological functions. The exact mechanisms and determinants of protein motions, including the regulatory interplay between protein and solvent motions, are not yet fully understood. In the present work, the thermal activation of phosphorescence quenching was measured in oxygen-saturated aqueous protein solutions to explore protein dynamics in the millisecond range. The sample was brought to cryogenic temperatures in a fast cooling process to avoid the bulk crystallization of ice. The phosphorescence quenching effect was followed by the phosphorescence lifetime of either Zn-protoporphyrin substituting the heme in the β-subunits of human hemoglobin (Zn-HbA) or tryptophan residues of Zn-HbA and human myoglobin (Mb), measured in thermal equilibrium at temperatures varied from 8 to 273 K. The quenching effect was attributed primarily to the activation of collisions with O(2) molecules made possible by the activated millisecond time-scale dynamics of the matrix around the chromophores. We find that, in the studied temperature range, the activation of protein global dynamics facilitating oxygen diffusion takes place at clearly separated lower temperatures and independently from bulk solvent dynamics and that the energy and entropy differences between the studied frozen and thermally activated states are specific for the protein.