TRANSIENT MAGNETIC CIRCULAR DICHROISM

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A new technique will be developed which extends the time resolution of magnetic circular dichroism (MCD) measurements from the millisecond time regime, which standard techniques can resolve, to the nanosecond time regime. MCD will be induced in a laser-photolysed sample by application of a strong magnetic field (100 kgauss) and measured with a recently demonstrated method of measuring circular dichroism (CD) on a nanosecond time scale. The transient MCD technique has a wide range of potential applications since MCD can be induced in any light- absorbing sample by a sufficiently strong magnetic field. This is in contrast to CD, which requires a chiral sample, and linear dichroism, which requires an oriented sample. An effective technique for measuring MCD signals with high time resolution would be an important tool for biophysics since it would provide a link between structural MCD studies and kinetic studies. Since biological processes depend on structural properties of biological molecules and are dynamic in nature, this linkage is clearly of great potential utility. The goal of this research will be to develop and assess the usefulness of the transient MCD technique as a tool for biophysical studies. Initial application of the method will be made to excited states of free-base porphyrins. Their intense, well-studied ground state MCD, suitable excited state lifetimes, and the lack of spectral overlap between ground and excited state absorptions provide excellent conditions for detecting transient MCD. Transient MCD spectra will also be obtained for excited states of copper porphyrins and Ruthenium biporphyrins. In addition, MCD spectra will be obtained for ground state transients generated by phototautomerization of 7-hydroxyflavone, photodissociation of axial ligands from nickel tetraphenylporphyrin in basic solvents, and the photolysis of carbonmonoxymyoglobin. Finally, the transient MCD of cytochrome proteins that have been "pulse- reduced" by solvated electrons obtained by photolysis of ferrocyanide will be studied.