STARK HOLE BURNING SPECTROSCOPY OF MODEL HEMES

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The research described in this proposal is a continuation of a study of biologically important molecules in the solid state, with respect to their electronic excited state properties, photochemistry and dynamics, using a combination of optical hole-burning and the Stark effect. When an absorbing molecule is dissolved in a matrix, its electronic spectrum is inhomogeneously broadened due to variations in the local environments. If a laser which has a bandwidth much less than the inhomogeneous band width is used to excite these absorbers, it is possible to burn an optical hole in the spectrum. Holes result from photochemistry, transient storage or molecular reorientation. Since the hole widths are often measured in MHz, when coupled with the Stark effect, this becomes a very high resolution probe of molecular properties. The long-term objectives of this research are to extract detailed excited state information (e.g. dipole moments, polarizabilities, geometry) from isolated porphyrin molecules using Stark hole-burning method and develop and extend the techniques so that it can be used as a high resolution probe on biological molecules in more physiologically realistic environments. Specifically we intend to continue our study of simple free base and metalloporphyrins in well defined n-alkane crystal matrices. Then the study will be extended to use randomly oriented porphyrins in glasses. Finally, we will apply Stark hole-burning to more complex biological molecules (e.g. heme) in low temperature glasses and solutions. The methodology involves the growth of single mixed (porphyrin/n-alkane) crystals and making low temperature glass solutions. The sample is placed between electrodes and immersed in liquid N2 or He and an absorption or emission spectrum obtained. Optical holes are burned and scanned with a narrow band laser; the Stark field is applied either DC or pulsed depending on the need. The subsequent electric field effects are then related to molecular excited state properties and dynamics.