ULTRAFAST PHASE GRATING STUDIES OF HEME-PROTEINS - OBSERVATION OF THELOW-FREQUENCY MODES DIRECTING FUNCTIONALLY IMPORTANT PROTEIN MOTIONS

Transient phase grating spectroscopy has been used to study the tertia ry global protein motions involved with the ligated to deligated confo rmational transition of carboxymyoglobin (MbCO) from 100 fs to 2 ns. U sing counterpropagating beam geometries and monitoring the phase of th e acoustics generated by the protein motion, it was possible to obtain picosecond resolution to observe the low-frequency acoustic-like mode s of the protein coupled to the bond dissociation process. The asymmet ric three-dimensional structure of the protein is expected to direct t he reaction forces to specific displacements important to function. Th is anisotropic force/displacement was directly observed through the po larization analysis of the protein motion and induced material birefri ngence. These studies were complemented by a study of the absorption a nisotropy to provide a probe of relaxation processes local to the heme binding site and epicenter of the reaction forces. The protein photoa coustics demonstrated that the dominant displacement or strain along t he reaction coordinate develops on a 2 ps time scale. From the polariz ation analysis, the most significant strain component is parallel to t he heme plane, which is consistent with translation of the F alpha-hel ix parallel to the heme plane as part of the allosteric core of the pr otein motions. The dynamics for the global protein motion and relaxati on of the protein in the vicinity of the heme show essentially a 1:1 c orrespondence 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 f orces is to the low-frequency collective modes of the protein. This me chanism is discussed within the context of an efficient mechanism for propagating functionally important protein motions and directing the s ystem along the correct seam in this highly complex potential energy s urface.