In principle, the absolute configuration (AC) of a chiral molecule can be deduced from its optical rotation (OR) and/or its electronic circular dichroism (ECD). In practice, this requires reliable methodologies for predicting OR and ECD. The recent application of ab initio time-dependent density functional theory (TDDFT) to the calculation of transparent spectral region OR and ECD has greatly enhanced the reliability with which these phenomena can be predicted. TDDFT calculations of OR and ECD are being increasingly utilized in determining ACs. Nevertheless, such calculations are not perfect, and as a result, ACs determined are not 100% reliable. In this paper, we examine the reliability of the TDDFT methods in the case of chiral alkenes. Sodium d line specific rotations, [alpha]D, are predicted for 26 conformationally rigid alkenes of known AC, ranging in size from 5 to 20 C atoms, and with [alpha]D values in the range of 0-500. The mean absolute deviation of predicted [alpha]D values from experimental values is 28.7. With one exception, beta-pinene, the signs of [alpha]D are correctly predicted. Errors in calculated [alpha]D values are approximately random. Our results define a "zone of indeterminacy" within which calculated [alpha]D values cannot be used to determine ACs with >95% confidence. TDDFT ECD spectra are predicted for eight of the alkenes and compared to experimental spectra. Agreement ranges from modestly good to poor, leading to the conclusion that TDDFT calculations of ECD spectra are not yet of sufficient accuracy to routinely provide highly reliable ACs. TDDFT OR calculations for two conformationally flexible alkenes, 3-tert-butylcyclohexene and trans-4-carene, are also reported. For the former, predicted rotations are incorrect in sign over the range 589-365 nm. It is possible that the AC of this molecule has been incorrectly assigned.

The absolute configurations (ACs) of the iridoid natural products, plumericin (1) and isoplumericin (2), have been re-investigated using vibrational circular dichroism (VCD) spectroscopy, electronic circular dichroism (ECD) spectroscopy, and optical rotatory dispersion (ORD). Comparison of DFT calculations of the VCD spectra of 1 and 2 to the experimental VCD spectra of the natural products, (+)-1 and (+)-2, leads unambiguously to the AC (1R,5S,8S,9S,10S)-(+) for both 1 and 2. In contrast, comparison of time-dependent DFT (TDDFT) calculations of the ECD spectra of 1 and 2 to the experimental spectra of (+)-1 and (+)-2 does not permit definitive assignment of their ACs. On the other hand, TDDFT calculations of the ORD of (1R,5S,8S,9S,10S)-1 and -2 over the range of 365-589 nm are in excellent agreement with the experimental data of (+)-1 and (+)-2, confirming the ACs derived from the VCD spectra. Thus, the ACs initially proposed by Albers-Schönberg and Schmid are shown to be correct, and the opposite ACs recently derived from the ECD spectra of 1 and 2 by Elsässer et al. are shown to be incorrect. As a result, the ACs of other iridoid natural products obtained by chemical correlation with 1 and 2 are not in need of revision.

The near-infrared magnetic circular dichroism (MCD) of Rhodospirillum rubrum, Chromatium vinosum, and Rhodopseudomonas palustris cytochromes c' are reported. The spectra of the reduced protein are very similar to those of deoxymyoglobin. The spectra of the oxidized proteins in the pD range 1-13 can be analyzed on the basis of four species A, B, C, and D. The existence of nine species, reported in a recent electron paramagnetic resonance study, is not substantiated. The MCD spectra support the assignment of B as high spin and C and D as low spin. The MCD of species A is close to that of high-spin proteins and does not support the recently proposed assignment of a mixed high- and intermediate-spin ground state for this species. The energies of the near-IR electronic transitions of all four oxidized species point to axial ligation via oxygen, assuming histidine to be the opposite axial ligand. Unfortunately, insufficient model compounds with ligation by carboxyl or hydroxyl moieties exist to enable more precise assignments.

Circular dichroism (CD) The enzyme nitrogenase (N2ase) has been isolated and purified from various nitrogen-fixing organisms and is currently the subject of intensive investigations (1-4). Active N~ase systems have been shown to consist of two essential metalloproteins-the MoFe protein and ;(20)(21)(22)(23)(24)(25)(26)(27)(28) and the Fe protein (containing -4 Fe and t4 S2-)-which together, in the presence of a suitable electron donor, catalyze ATP-dependent reduction of N2 to NH3. Existing evidence suggests that electrons derived from the primary reductant are transferred via the Fe protein to the MoFe protein, which is believed to provide the site for N2 binding (1-5).Despite study by various spectroscopic and other techniques, important aspects of the structure and catalytic role of the metal centers in N2ase remain to be elucidated (1-4). Recent systematic studies of the circular dichroism (CD) and magnetic circular dichroism (MCD) of simple iron-sulfur proteins (6, 7) have shown that CD and MCD can be useful in characterizingThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. 2585 iron-sulfur cluster type, oxidation level, and protein environment and that more information is afforded by these probes than by unpolarized absorption spectroscopy. Because the Fe and MoFe proteins evidently contain iron-sulfur clusters-albeit unconventional in many respects-we have undertaken to explore the value of CD and MCD in the study of N2ase. Electronic spectroscopy has so far found relatively limited application in the study of this enzyme (1-4). Electronic absorption spectra of the two components are almost featureless.CD spectra have been obtained in the polypeptide.absorption region (wavelengths <300 nm) (8, 9), but CD was reported to be absent at longer wavelengths in both N2ase components from Azotobacter chroococcum (10) and was also undetectable in Fe protein from Klebsiella pneumoniae (8). Very weak visible CD has been reported in the Klebsiella MoFe protein; however, the spectrum was not given (8). No MCD work has been published.We report here spectra demonstrating that CD and MCD are observable in both N2ase components across the near-infrared-visible-near-ultraviolet spectral region

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