We have measured the dispersion of the gas phase depolarization ratio of Rayleigh scattered light for 12 linear and symmetric top molecules. Combining these data with known refractive index data we obtain the frequency dependence of the polarizability anisotropy. For all molecules studied we find that the polarizability anisotropy increases more rapidly with increasing frequency than the bulk polarizability. We have correlated this behavior with the oscillator strength and direction of the first electronic transition. We have also compared our zero frequency extrapolated anisotropies with the anisotropies determined from static fields with the Kerr effect and the Stark effect in microwave spectroscopy.

We present spectroscopic data that show that a nitrile hydratase from Rhodococcus rhodochrous J1 is the first reported example of a native protein that contains a non-corrin Co 3+ ion with a mixed S and N(O) ligand field. Nitrile hydratases catalyze the addition of water to nitriles, yielding amides as the exclusive product, 3 and are used as industrial catalysts for the production of acrylamide. 4 The most thoroughly characterized nitrile hydratase is from Rhodococcus sp. R312 5 and is a (R ) 2 tetramer that contains two low-spin non-heme ferric ions of unknown function. These metal ions exist in a tetragonally distorted octahedral ligand field of three histidine imidazoles, two cysteine thiolates, and a hydroxide. 6-8 Two cobaltcontaining nitrile hydratases have been identified in R. rhodochrous J1. 9 We purified one of those enzymes 10,11 a multimer of R heterodimers totaling approximately 500 000 Da and containing non-corrin Co 3+ . 10 We measured the cobalt 12 and the protein 13 concentrations of samples of purified enzyme and found one cobalt ion per (R ). The same experiment yielded an unusually high 280 (2.7 (mg/mL) -1 cm -1 ), consistent with an earlier report. 10 EPR spectra of concentrated samples (0.3 mM cobalt) showed no signals attributable to the protein from 4 to 77 K, consistent with the presence of Co 3+ . When treated with sodium dithionite and methyl viologen, the samples developed an EPR spectrum characteristic of low-spin Co 2+ (Figure 1; g 1,2,3 ) 2.378, 2.206, 1.998; A Co 1,2,3 ) 58, 11, 97 G). 14 The cobalt K-edge X-ray absorbance spectrum of this nitrile hydratase (in the presumed Co 3+ form) is very similar to the Fe 3+ K-edge spectrum of the Rhodococcus sp. R312 enzyme 15,16 (Figure 2a,b). Using the method of Roe et al., 17 the area (in units of eV (% edge height)) of the lowest energy pre-edge peak in the cobalt spectrum (assigned to a 1s f 3d transition) is 6.3, slightly larger than areas we obtained for six-coordinate Co-(S 2 CNEt 2 ) 3 (3.6), Co(acac) 3 (3.9), and [Co(en) 3 ]Cl 3 (4.6), but much smaller than that found for the four-coordinate Co(im) 2 -Cl 2 (16.8). The size of the pre-edge peak is consistent with six-or possibly five-coordinate cobalt in nitrile hydratase, with distortions from octahedral symmetry that increase the peak area by approximately 50% compared to those of symmetrical sixcoordinate models. The pre-edge peak for the six-coordinate 7,8,16 iron in Rhodococcus sp. R312 nitrile hydratase is also approximately 50% larger than those of symmetrical six-coordinate Fe 3+ model complexes. 16 The best fits of the first sphere Fourier-filtered EXAFS are shown in Figure 2c and assume two sulfur scatterers at 2.20 Å and three ( ν 2 ) 1.3) 18 or four ( ν 2 ) 1.5) nitrogen scatterers at 1.95 Å. Any other integer values of n S or n N in two-shell fits gave ν 2 g 3.0 and were rejected on the basis of the criterion that ν 2 for a correct model is expected to be within one unit of the minimum ν 2 obtained. 19 The value of ν 2 increased by only 0.2-0.3 for each nitrogen ...

Measurements of reorientational relaxation times of simple aromatic compounds have been made by both depolarized Rayleigh light scattering and 13C NMR spin-lattice relaxation. Combination of the reorientation times determined by these techniques makes it possible to extract the reorientation times about the different molecular axes. The viscosity dependence of reorientation times about the individual molecular axes has been measured for benzene, mesitylene, toluene, and nitrobenzene. The viscosity dependence, which is highly anisotropic, is discussed in terms of a ``slip'' model of reorientational motion.

Measurements of orientational relaxation times of chloroform and nitrobenzene in a variety of solvents have been made by depolarized light scattering. At constant concentration a plot of reorientational relaxation time of nitrobenzene versus solution viscosity was found to fit a straight line with nonzero intercept. The reorientational relaxation time of both solutes increased with increasing solute concentration (at constant viscosity). For nitrobenzene and chloroform, plots of the reorientational relaxation time versus solute concentration were linear. This concentration dependence is attributed predominantly to pair correlations. The ``static'' correlation parameter f and the ``dynamic'' correlation parameter g are determined for both chloroform and nitrobenzene. The carbon-13 spin-lattice relaxation time (T1) for both solutes in solution has also been determined. The reorientational relaxation time of chloroform determined by NMR agrees well with the reorientational relaxation time determined by depolarized light scattering extrapolated to zero solute concentration.