Deletion of nifB results in the formation of a variant nitrogenase MoFe protein (DeltanifB MoFe protein) that appears to contain two normal [8Fe-7S] P clusters. This protein can be reactivated to form the holo MoFe protein upon addition of isolated FeMo cofactor. In contrast, deletion of nifH results in a variant protein (DeltanifH MoFe protein) that appears to contain FeS clusters different from the normal P cluster, presumably representing precursors of the normal P cluster. The DeltanifH MoFe protein is not reconstituted to the holo MoFe protein with isolated FeMo cofactor. The EPR and EXAFS spectroscopic properties of FeS clusters in the DeltanifH MoFe protein clearly differ from those of the normal P cluster found in the DeltanifB MoFe protein and suggest the presence of [4Fe-4S]-like clusters. To further characterize the metal cluster structures in the DeltanifH MoFe protein, a variable-temperature, variable-field magnetic circular dichroism (VTVH-MCD) spectroscopic study has been undertaken on both the DeltanifB MoFe protein and the DeltanifH MoFe protein in both the dithionite-reduced and oxidized states. This study clearly shows that each half of the dithionite-reduced DeltanifH MoFe protein contains a [4Fe-4S]+ cluster paired with a diamagnetic [4Fe-4S]-like cluster. Upon oxidation, the VTVH-MCD spectrum of the DeltanifH MoFe protein reveals a paramagnetic, albeit EPR-silent system, suggesting an integer spin state. These results suggest that the DeltanifH MoFe protein contains a pair of neighboring, unusual [4Fe-4S]-like clusters, which are paramagnetic in their oxidized state.

Hydrogen bonds are very important chemical structures that are responsible for many unique and important properties of solvents, such as the solvation power of water. These distinctive features are directly related to the stabilization energy conferred by hydrogen bonds to the solvent. Thus, the characterization of hydrogen bond energetics has been vital for many areas of science. We present a laboratory experiment for physical chemistry in which the hydrogen bond energetics between methyl acetate and water is investigated by Fourier transform infrared spectroscopy (FTIR). The experiment consists of measuring the temperature dependent IR spectra of methyl acetate to determine the changes in the enthalpy and entropy of making/breaking hydrogen bonds. This experiment aims at providing the students with hands-on experience in the following topics: solution and sample cell preparation, IR spectra collection and analysis, and data modeling and thermodynamic calculations. The overall objective of this experiment is to familiarize chemistry students with a methodology used to extract meaningful and up-to-date physical chemistry properties from real experimental data.

The phase-space distribution function corresponding to a ground-state density of a many-electron system proposed earlier is explored as a means for generation of momentum-space properties through density-functional theory. Excellent results are found for the spherically averaged Compton profiles for several atoms and the molecules H2 and N2, as dwell as the directional Compton profiles for N2, thereby providing both a useful scheme for computation of such profiles and confirrnation of the basic theory. The entropy-maximization procedure employed is discussed from the point of vie~of information theory, PACS numbers: 31.15.+ q, 31.20.Lr, 31,90.+ s, 32.80.Cy In connection with a thermodynamic transcription of the density-functional theory of electronic structure, ' there recently has been proposed a phase-space distribution function f(r, p) corresponding to a groundstate electron density p(r). A unique f(r, p) was obtained by the invoking of an entropy-maximization principle, in analogy to the classical case. The resulting distribution function is Maxwellian in nature with a local temperature T(r) and leads to various thermodynamic and fluid-theoretic equations for the electron cloud. " In the present work, this same f(r, p), and a simple generalization of it, are used to predict momentum-space properties of an atom or molecule: the spherically averaged and directional Compton profiles.Consider an N-electron system characterized by the ground-state density p(r). Identify a phase-space distribution function f(r, p) with this density, and assume it to yield the correct kinetic energy density r(r, p) as weil: p(r) ="' d p f(r, p), z d r p(r) =N; (I) t (r, p) = , ' J, d'p p'f (r,-p), f(r, p) = [2mkT(r)] '~'p(r)exp[ -p'/2kT(r)]. (5) Here P(r) = [kT(r)] ', where the local temperature T(r) is defined in analogy with the ideal-gas expression for kinetic energy by -' , p(r) kT(r) = r(r, p). (6) tained' by a maximization of the entropy defined as S = "d r s (r), s (r) = -k J~d 'p f (lnf' -1), (3) subject to the constraints of correct density [Eq. (1)] and correct kinetic energy density [Eq. (2)]. In Eq. (3), k is the Boltzmann constant. Introducing Lagrange multipliers o. (r) and P(r) for the two constraints"respectively, one obtains f(r, p) = exp [ -~( r) -p(r) p'/2], which, on evaluation of the Lagrange multipliers from Eqs. (1) and (2), becomes J d'r t(r, p) =Ek;". ( ) I x&p 8, . p;() If we presume the validity of f(r, p) as a phasespace distribution function, the spherically averaged The most appropriate distribution function is then ob-~m omentum density X(p) is given by X(p) = Jfd r f(r, p) = J d r [p(r)/2n] p(r)exp[ -p(r)p /2]. (7) and the spherically averaged Compton profile, within the impulse approximation, can be obtained fromIq I which is a basic equation for calculation of the averaged Compton profiles of atomic and molecular sysAlthough the term V2p does not contribute to the glotems. bal kinetic energy, it introduces quantum oscillations Now consider the form of the kinetic energy density in the kinet...

NifZ is a member of a series of proteins associated with the maturation of the nitrogenase MoFe protein. An MCD spectroscopic study was undertaken on the Delta nifB Delta nifZ MoFe protein generated in the absence of both NifZ and NifB (deletion of NifB generates an apo-MoFe protein lacking the FeMo cofactor). Results presented here show that, in the absence of NifZ, only one of the two P-clusters of the MoFe protein is matured to the ultimate [8Fe-7S] structure. The other P-cluster site in the protein contains a [4Fe-4S] cluster pair, representing a P-cluster precursor that is electronically identical to the analogous clusters observed in the Delta nifH MoFe protein. These results suggest that the MoFe protein is synthesized in a stepwise fashion where NifZ is specifically required for the formation of the second P-cluster.