Coordination geometries and vibrational properties of cytochromes alpha and alpha 3 in cytochrome oxidase from Soret excitation Raman spectroscopy.

The effects of a symmetry lowering environment on the Raman excitation spectra of a metalloporphyrin have been studied using the generalized crude Born–Oppenheimer vibronic theory. Symmetry considerations reduce the number of matrix elements introduced by the interaction of the porphyrin with its environment. In practice, matrix elements of the model are determined by fitting the Raman excitation spectra. These matrix elements contain information about the interaction of the metalloporphyrin with its environment. This procedure is facilitated if the Raman excitation spectra of the metalloporphyrin in a more symmetrical environment have been measured and analyzed, since only the additional matrix elements describing the differences in the interaction with the environment must be determined. Some of the effects of environmental interactions on the excited states of metalloporphyrins are discussed. Calculations demonstrate that Raman excitation spectra and a new tool—Raman excitation difference spectra, provide a valuable means of examining these environmental effects.

The Raman excitation spectra and absorption spectrum of a metalloporphyrin in an environment of low symmetry

The application of Raman scattering to molecular systems, as discussed in the previous chapters, yield a great deal of information about ground state structural, spectroscopic, and physical properties. In contrast when the incident frequency is in resonance with a real transition of a system, (bringing about the resonance Raman effect) excited electronic state properties may be studied. In addition due to the phenomena of resonance enhancement, specific parts of complicated molecules may be probed without interference from other parts of the molecule or from solvents. These features make the application of resonance Raman scattering to molecules complementary to non-resonance investigations and they make it a very important tool in the study of properties of molecular systems.

The Resonance Raman Effect

A method for the quantitation of light emission is presented which uses the Raman scattering cross section of water as a reference. The method is successfully applied to systems having a known quantum yield, thus providing an independent check of Raman cross‐section measurements. A very weak incoherent emission from the ferrocytochrome c system is quantitated (Φ=3.6×10−6) and interpreted using a simple kinetic model which is consistent with the quantum mechanical treatment. The quantum yield measurements imply that the excited state (S1) lifetime of the ferrocytochrome is on the order of 0.2 ps, while the lifetime of the ferricytochrome is deduced to be much shorter. The implications of these measurements concerning the magnitudes of the excited state (S1, S2) damping factors in ferrocytochrome c are also discussed.

On the quantitation of light emission from cytochrome c in the low quantum yield limit

Resonance Raman spectroscopy of c-type cytochromes

Resonance Raman scattering excitation profile data have been obtained on ferrocytochromes c and b5 in the alpha absorption band region. We observe in cytochrome c that the shape of the excitation profile agrees with the absorption band shape, while in cytochrome b5 it does not. In addition, we observe in cytochrome b5 a linewidth substantially larger than that in cytochrome c. From our data we conclude that the excited state lifetime in cytochrome c is longer than that in cytochrome b5 and that in cytochrome b5 the relaxation of the pi-pi* excited state configuration of the porphyrin ring is different in the x direction than in the y direction. Possible origins of these effects due to coupling to the d-d transitions of the iron atom are discussed.

http://www.pnas.org/content/74/7/2607.full.pdf

F Adar

Papers

Excited state lifetimes in cytochromes measured from Raman scattering data: evidence for iron-porphyrin interactions.