Ultrafast Phase Grating Studies of Heme Proteins:  Observation of the Low-Frequency Modes Directing Functionally Important Protein Motions

Transient phase grating spectroscopy has been used to study the tertiary global protein motions involved with the ligated to deligated conformational transition of carboxymyoglobin (MbCO) from 100 fs to 2 ns. Using counterpropagating beam geometries and monitoring the phase of the acoustics generated by the protein motion, it was possible to obtain picosecond resolution to observe the low-frequency acoustic-like modes of the protein coupled to the bond dissociation process. The asymmetric three-dimensional structure of the protein is expected to direct the reaction forces to specific displacements important to function. This anisotropic force/displacement was directly observed through the polarization analysis of the protein motion and induced material birefringence. These studies were complemented by a study of the absorption anisotropy to provide a probe of relaxation processes local to the heme binding site and epicenter of the reaction forces. The protein photoacoustics demonstrated that the dominant displacement or strain along the reaction coordinate develops on a 2 ps time scale. From the polarization analysis, the most significant strain component is parallel to the heme plane, which is consistent with translation of the F α-helix parallel to the heme plane as part of the allosteric core of the protein motions. The dynamics for the global protein motion and relaxation of the protein in the vicinity of the heme show essentially a 1:1 correspondence in dynamics. From the observed dynamics for the protein strain, amplitude, and two-point spatial correlation of the motion, it is concluded that the dominant coupling coefficient of the reaction forces is to the low-frequency collective modes of the protein. This mechanism is discussed within the context of an efficient mechanism for propagating functionally important protein motions and directing the system along the correct seam in this highly complex potential energy surface.