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Circular dichroism (CD) and magnetic circular dichroism (MCD) spectra are reported for the 2-Fe ferredoxins from Pseudomonas putida and Spirulina maxima, Chromatium HIPIP, the 4-Fe ferredoxin from Bacillus stearothermophilus, and the 8-Fe ferredoxin from Clostridium pasteurianum. The spectral range spans the near-infrared, visible, and near ultraviolet. In all cases except oxidized 2-Fe ferredoxins, electronic absorption is observed continuously from less than 5000 cm-1 to above 30,000 cm-1. The CD spectra of the two 2-Fe ferredoxins are similar. In contrast, the CD of the 4-Fe and 8-Fe proteins, for a given 4-Fe cluster oxidation level, varies considerable with protein. MCD is less sensitive to protein environment than is CD. In the 2-Fe proteins, MCD at 5 T is appreciably smaller than the CD; in the 4-Fe and 8-Fe proteins, MCD and CD are comparable in magnitude. Both CD and MCD are more highly structured than the corresponding absorption spectra. The CD and MCD spectra reported provide a broader base than heretofore available for the characterization of iron-sulfur proteins containing 2-Fe and 4-Fe clusters and for the evaluation of electronic structural models for these clusters.

Quantitative laser Raman spectroscopy on colored substances is considered, and it is shown that sample positioning and concentration are less critical when 180° illumination is used in place of the more common 90° illumination. In addition, the correction of Raman intensities for sample absorption is easily accomplished for data collected via backscattering (180° geometry), and the appropriate equations are presented. A spinning sample cell and holder are described for 180° illumination. The design utilizes simple and inexpensive sample tubes which are conveniently employed with standard liquids and solids as well as volatile, air-sensitive, and corrosive compounds. Provision for sample heating and cooling is described.

Resonance Raman, optical absorption, circular dichroic, and fluorescence emission spectroscopy of hemerythrins from four species of sipunculids (Phascolopsis gouldii, Phascolosoma agassizii, Themiste dyscritium, and Themiste pyroides) reveals no major differences in their active site or tertiary structures. This precludes any change in iron ligands or coodination geometry and makes it unlikely that the active-site structures of P. gouldii and T. dyscritum hemerythrins could be as disparate as indicated by present crystallographic interpretations (Stenkamp, R. E., Sieker, L. C., and Jensen, L. H. (1976), Proc. Natl. Acad. Sci. U.S.A. 73, 349; Klotz, I. M., Klippenstein, G. L., and Hendrickson, W. A. (1976), Science 192, 335). Resonance Raman enhancement profiles of the stretching modes involving coordinated dioxygen maximize with excitation at approximately 525 nm, and correspond to the circular dichroic (CD) transition at approximately 520 nm. For coordinated azide modes in metazidohemerythrins these profiles maximize with excitation at approximately 505 nm corresponding to the 500-nm CD transition. Hemerythrins also possess another resonance Raman peak at approximately 510 cm-1 which show maximum intensity enhancement at approximately 530 nm and this vibration is most likely associated with a permanent iron ligand.

Resonance Raman spectroscopy has been used as a probe of the structure of ligands at the active site of hemerythrin. Molecularly revealing insights have been obtained with oxyhemerythrin and with metazidohemerythrin. This spectroscopic technique has also facilitated a comparison of oxygen carrier within erythrocytes with that in solution. The electronic state of the bound O2 is the same in the natural environment as in the artificial one.

Excitation of oxyhemerythrin with radia-

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