TIME-RESOLVED MAGNETIC CIRCULAR DICHROISM

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DESCRIPTION: Development and use of time resolved magnetic circular dichroism spectroscopy (TRMCD) is proposed. The application of a magnetic field induces chirality and the observed signal is generally localized to the moiety who's absorption is being probed. This technique is used for heme centers. Extension of the technique to the near UV is to be developed, and preliminary data on the static MCD of tryptophan is encouraging (will require an extra electromagnet to compensate for the Faraday rotation of the cell windows). The central idea is to photolyze a ligand and then study the kinetics. The resolution of the TRMCD apparatus is about 50-100 ns in the heme center absorption (partly complicated by signals that result from the protein diffusional reorientation time, about 100 ns- this complication has been analyzed and is probably correctable). Studies on cytochrome oxidase, cytochrome c3, and cytochrome c' are ongoing. A particularly important finding of the past period was the finding of a transient ligation at the cytochrome a3 site of cytochrome oxidase, resulting in a model for part of the gating mechanism coupling energy release by oxygen reduction to proton pumping (the ligand shuttle model). An unknown ligand in the protein binds to the protein iron center and this has been best studied with this technique. Most interesting future studies center on hemoglobin and the R to T allosteric transition, if the trp residues can be isolated and measured. Some hope has been demonstrated by static MCD on trp in solution. Mutants should isolate which trp residues yield signal. Recent TRCD results, just submitted to Biochemistry, are exciting. Outstanding studies except there were two problems cited in last review: the problem of protein orientational relaxation, which is on the 100 ns time scale, and the issue of productivity.