The goal of this research is to dramatically improve the attainable time-resolution of transient circular dichroism (CD) and transient electron spin resonance (ESR) experiments. While the two techniques may seem quite different, the transient grating methods for the two experiments are similar; they differ in only the polarizations of the excitation pulses which create the grating. While the experiments proposed here are for a picosecond YAG system, the techniques are applicable to higher time-resolution experiments. In myoglobin and hemoglobin, the bonding of O2 or CO to the heme group produces significant conformational changes in the surrounding protein structure. Since the heme is not choral, changes in the CD following photodissociation of a ligand can only result from changes in the total protein surrounding the heme group. A transient grating technique will be used to separate the signal from the large background which would otherwise obscure the results. In addition to the above experiments, CD can be applied to create an all-optical ESR technique. The electron spins of Mn2+ ions in solution can be aligned with circularly polarized light even in the absence of an applied magnetic field. The proper selection of excitation polarizations produces a spatial modulation in the electron spin orientation and therefore a spatial modulation in the absorption coefficients of left and right circularly polarized light. It is expected that this ESR technique will be used to probe the dynamics of biological systems.
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