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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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Navallles, Barols, and Nguyen Reply: Our Letter [1] was devoted to an x-ray diffraction study of a well aligned sample of the newly discovered twist grain boundary smectic-C (TGBC) phase. We reported the observation of a q-fold modulated ring of scattering in the plane q»q, perpendicular to the helical axis with q =16, 18, or 20. %e interpreted this observation as the discovery of the commensurate TGB structure predicted by Renn and Lubensky [2]. Similar experiments on TGB~samples by Srajer et al. [3] and ourselves on other homologs of the same series (10and ] ]F2BTFOIM7) always showed uniform rings of scattering which we interpreted as incommensurate states.Two origins can be invoked to explain the observed commensurability.1. The finite thickness of the cell (-25 pm) contains a limited number n of helical pitches (from 12 to 25 on increasing temperature) and a limited number ns of smectic slabs (of order 16 to 20 times n). The structure is thus commensurate by construction with a rational commensurability ratio nb/n of order qo, the natural irrational value. This is well described by Galerne and we agree totally on this point.2. The possible existence of long range interaction of the smectic slabs favoring a commensurate lock-in of the structure. Unlike the previous one, this commensurability is intrinsic and would survive infinite sample thickness.The origin of such an interaction is indeed unclear to us: We pointed out in our Letter that the ferroelectric polarization of the smectic slabs of the TGBC material might play a role. But this possibility has not been demonstrated yet. Let us emphasize in this case that the finite thickness effect is always present and may compete with intrinsic commensurability.Galerne's Comment [4] suggests that the first effect alone can explain our observations. The best reply would obviously be a new experiment with free boundaries which would keep showing the commensurability. This is unfortunately not possible since rigid boundary conditions are required to align the sample. Increasing the thickness also destroys the quality of the alignment.Let us see, however, if the data of our Letter can be consistent with the interpretation of the Comment.We recall first that scattering patterns exhibiting well defined rings of spots were recorded at all temperatures within the TGBC domain: The 32 data points of Fig. 3 of Ref.[1] come from scattering patterns very similar to Fig. 1(b) [1]. Basically the contrast did not change very much with temperature and uniform rings (i.e. , incommensuratelike) were never observed. Galerne's interpretation produces successions of commensurate and incommensurate patterns unless several values of n and nb coexist in the sample at a given temperature. In this latter case, commensurate zones are always present and the contrast does not deviate very much from an average value calculated over all possible values of n and nb. Such a distribution of values is actually quite acceptable and consistent with experimental observations under a polarizing micros...

We show in this paper that requantized classical molecular dynamics simulations (rCMDSs) are capable of predicting various refined spectral-shape parameters of absorption lines of CO2 broadened by N2 with high precision. Combining CMDSs and a requantization procedure, we computed the auto-correlation function of the CO2 dipole moment responsible for the absorption transition. Its Fourier-Laplace transform directly yields the spectrum. Calculations were made for two temperatures, 200 and 296 K, at 1 atm and for a large range of Doppler widths, from the near-Doppler to the collision-dominant regimes. For each temperature and each line, the spectra calculated for various Doppler widths were simultaneously fit with the Hartmann-Tran (HT) profile. This refined profile, which takes into account the effects of the speed dependent collisional line broadening, the Dicke narrowing, and the collisional line mixing, was recommended as a reference model to be used in high-resolution spectroscopy (instead of the simplified Voigt model). The HT parameters retrieved from the rCMDS-calculated spectra were then directly compared with those deduced from high-precision measurements [J. S. Wilzewski et al., J. Quant. Spectrosc. Radiat. Transfer 206, 296–305 (2018)]. The results show a very good agreement, even for those parameters whose influence on the spectra is very small. Good agreement is also obtained between measured and predicted temperature dependences of these parameters. This demonstrates that rCMDS is an excellent tool, highly competitive with respect to high quality measurements for precise line-shape studies.

In this paper we present a self-consistent-field calculation for energy levels of He-like ions immersed in dense and hot plasmas. Exchange and correlation energies between bound electrons are shown to be much less perturbed than the single-particle energy. Concurrently, a fully quantummechanical impact calculation has been made to obtain the matrix elements of the complex electron line-broadening operator I,. In the special case of the principal series, it is shown that the line shifts given by the two approaches are in good agreement for typical electron temperatures and densities of laser plasmas.

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In order to evaluate the simultaneous influence of a neighboring ion located at an arbitrary distance R and of free electrons on atomic bound states in high-density plasmas (N,~1 0" cm ') we have suggested a transient molecule model that consists of a two-ion center surrounded by bound and free electrons. The Dirichlet boundary condition is laid down in such a way to make the total electrostatic potential constant over the entire molecular envelope and also to obtain the limit of two independent ion spheres or that of one united ion sphere when R~~or R~0, respectively.A closed form has been obtained for the interaction potential between bound and free electrons at the high-temperature limit. Energy levels and wave functions have been evaluated by diagonalizing the total Hamiltonian in the subspace of the lowest 40 molecular levels. A possible appearance of new spectral components and, in particular, a drastic reduction of the Stark eject are clearly shown.