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Partha Basu

Publications and source records attributed to Partha Basu.

24 records · Page 2Linked to original sources
PubMed2002

Synthesis, characterization, electrochemistry, electronic structure, and isomerization of mononuclear oxo-molybdenum(V) complexes: the serine gate hypothesis in the function of DMSO reductases.

Crystal structures of DMSO reductases isolated from two different sources and the crystal structure of related trimethylamine-N-oxide reductase indicate that the angle between the terminal oxo atom on the molybdenum and the serinato oxygen varies significantly. To understand the significance of this angular variation, we have synthesized two isomeric compounds of the heteroscorpionato ligand (L1OH) (cis- and trans-(L1O)Mo(V)OCl(2)), where the phenolic oxygen mimics the serinato oxygen donor. Density functional and semiempirical calculations indicate that the trans isomer is more stable than the cis. The lower stability of the cis isomer can be attributed to two factors. First, a strong antibonding interaction between the phenolic oxygen with molybdenum d(xy) orbital raises the energy of this orbital. Second, the strong trans influence of the terminal oxo group in the trans isomer places the phenol ring, and hence the bulky tertiary butyl group, in a less sterically hindered position. In solution, the cis isomer spontaneously converts to the thermodynamically favorable trans isomer. This geometric transformation follows a first-order process, with an enthalpy of activation of 20 kcal/mol and an entropy of activation of -9 cal/mol K. Computational analysis at the semiempirical level supports a twist mechanism as the most favorable pathway for the geometric transformation. The twist mechanism is further supported by detailed mass spectral data collected in the presence of excess tetraalkylammonium salts. Both the cis and trans isomers exhibit well-defined one-electron couples due to the reduction of molybdenum(V) to molybdenum(IV), with the cis isomer being more difficult to reduce. Both isomers also exhibit oxidative couples because of the oxidation of molybdenum(V) to molybdenum(VI), with the cis isomer being easier to oxidize. This electrochemical behavior is consistent with a higher-energy redox orbital in the cis isomer, which has been observed computationally. Collectively, this investigation demonstrates that by changing the O(t)-Mo-O(p) angle, the reduction potential can be modulated. This geometrically controlled modulation may play a gating role in the electron-transfer process during the regeneration steps in the catalytic cycle.

Binding Sites↗
PubMed2002

Evolution of nitrate reductase: molecular and structural variations on a common function.

The biological transformation of nitrogen oxyanions is widespread in nature and gives rise to a robust biogeochemical cycle. The first step in nitrate reduction is carried out by the enzyme nitrate reductase (NR). Although NR always catalyzes the same chemical reaction (conversion of nitrate into nitrite), its location in the cell, structure, and function are organism-dependent. We use protein sequence data to determine phylogenetic relationships and to examine similarities in structure and function. Three distinct clades of NR are apparent: the eukaryotic assimilatory NR (Euk-NR) clade, the membrane-associated prokaryotic NR (Nar) clade, and a clade that includes both the periplasmic NR (Nap) and prokaryotic assimilatory NR (Nas). The high degree of sequence similarity and a phylogenetic distribution that follows taxonomic classification suggest a monophyletic origin for the Euk-NR early on in the evolution of eukaryotic cells. In contrast, sequence conservation, phylogenetic analysis, and physiology suggest that both Nar and Nap were acquired by horizontal gene transfer. Nap and Nas share a lesser degree of similarity, with Nap a subclade of Nas. Nap from strict anaerobic bacteria such as Desulfovibrio desulfuricans is ancestral to facultative species and may provide an evolutionary link between Nap and Nas. We observed conserved binding sites for molybdenum and pterin cofactors in all four proteins. In pathways involving Euk-NR, Nas, and Nar, for which ammonia is the end product, nitrite is reduced to ammonia by a siroheme nitrite reductase. Nap, however, is coupled to a pentaheme nitrite reductase. In denitrification, whether Nar or Nap is involved, nitrite is reduced to nitric oxide by either a cytochrome cd1 or a copper-containing nitrite reductase. This complexity underscores the importance of nitrate reduction as a key biological process.

Amino Acid Sequence↗
PubMed2002

An analogue system displaying all the important processes of the catalytic cycles involving monooxomolybdenum(VI) and desoxomolybdenum(IV) centers.

The mononuclear monooxomolybdenum(VI) complex hydrotris(3,5-dimethyl-1-pyrazolyl)borato(bis-4-ethoxyphenolato)oxomoylybdenum(VI) cation, [LMoVIO(p-OC6H4-OC2H5)2]+, where L- = hydrotris(3,5-dimethyl-1-pyrazolyl)borate, has been synthesized by chemical and electrochemical oxidation from the corresponding neutral oxomolybdenum(V) species, LMoVO(p-OC6H4-OC2H5)2. The molybdenum(VI) species has been characterized by NMR, IR, and resonance Raman spectroscopies, mass spectrometry, and electronic spectroscopy. Acetonitrile solutions of cationic [LMoVIO(p-OC6H4-OC2H5)2]+ react with tertiary phosphines (PR3) to generate phosphineoxide-bound adducts, [LMoIV(OPR3)(p-OC6H4-OC2H5)2]+, which subsequently generate the cationic desoxo species, [LMoIV(p-OC6H4-OC2H5)2]+ and OPR3. In the presence of water and an oxidizing agent the desoxo species generates the monooxomolybdenum(V), LMoVO(p-OC6H4-OC2H5)2, and completes the catalytic cycle. The oxygen atom transfer reaction has been probed by isotope-labeling experiments, vibrational spectroscopies, and mass spectrometry. This study describes an analogue complex that can exhibit all important processes of the catalytic cycle involving monooxomolybdenum(VI) and desoxomolybdenum(IV) centers.

Binding Sites↗
PubMed1997

Oxomolybdenum(V)/Iron(III) Porphyrinate Complexes: Effect of Axial Ligand Plane Orientation on Complex Stability, Reduction Potential, and NMR and EPR Spectra.

The compounds {5,10,15-tri-p-tolyl-20-[[2,3-[((hydrotris(3,5-dimethylpyrazolyl)borato)oxomolybdenio)dioxy]phenyl]porphyrinato}bis(2-methylimidazole)iron(III) chloride, Fe(2,3-Mo-TTP)(2MeImH)(2)Cl (1), and {5,10,15-tri-p-tolyl-20-[3,4-[((hydrotris(3,5-dimethylpyrazolyl)borato)oxomolybdenio)dioxy]phenyl]porphyrinato}bis(2-methylimidazole)iron(III) chloride, Fe(3,4-Mo-TTP)(2MeImH)(2)Cl) (2), have been prepared in order to assess the effect of axial ligand plane orientation upon the stability, reduction potential, and NMR and EPR spectra of these novel (porphryinato)iron(III)-Mo(V) systems that possess two S = (1)/(2) metal centers. The proton NMR spectra of 1 and 2 are characteristic of perpendicular orientation of the planes of the axial 2MeImH ligands. These results contrast with those previously reported (Basu, P.; Shokirev, N. V.; Enemark, J. H.; Walker, F. A. J. Am. Chem. Soc. 1995, 117, 9042-9055) for the analogous compounds with NMeIm as the axial base (3, 4) whose (1)H NMR spectra are characteristic of one or both axial ligands in parallel planes. The equilibrium constants (beta(2)) for binding the bulky 2MeImH ligands of 1 and 2 are more than an order of magnitude smaller than those for NMeIm binding to 3 and 4. Three distinct pseudo-Nernstian one-electron couples are observed for 1 and 2 in DMF that can be assigned to the Fe(III/II), Mo(V/IV), and Fe(II/I) reductions, with the Fe(III/II) couple being most positive. The Fe(III/II) and Mo(V/IV) potentials are similar to those for 3 and 4 and only slightly perturbed from those of the individual isolated components. The EPR spectrum of 1 shows features due to Mo(V) and low-spin Fe(III) that are perturbed by weak exchange coupling (2.6 GHz, 0.078 cm(-)(1)) between the two metal centers which are separated by approximately 7.9 Å. The "large g(max)" feature characteristic of the 2MeImH adducts of Fe(III) tetraphenylporphyrinates is shifted toward the Mo(V) signal to 2.85; the anisotropy of the Mo(V) signal is lost, and no molybdenum hyperfine can be detected. The EPR spectrum of 2, which has a metal-metal separation of approximately 9.4 Å, shows an unperturbed "large g(max)" value of 3.41 for the Fe(III) center. The Mo(V) part of the spectrum is slightly perturbed from that of the precursor catecholate complex but is essentially identical to that of 4, which exhibits a rhombic Fe(III) signal.

Journal Article↗
PubMed1996

Multifrequency ESEEM Spectroscopy of Sulfite Oxidase in Phosphate Buffer: Direct Evidence for Coordinated Phosphate.

The molybdenum(V) coordination environment of sulfite oxidase has been investigated by multifrequency ESEEM spectroscopy in approximately 70 mM phosphate buffer at pH = 6.5 in both H(2)O and D(2)O. The FT-ESEEM spectra in H(2)O typically consist of three lines. One of these lines is always close to twice the Larmor frequency of the P atom (2nu(P)) and is assigned to one or more coordinated phosphates, providing the first direct unambiguous detection of such coordination. Extensive simulations of this phosphate signal at the various operational frequencies indicated that the coordinated phosphate group(s) probably does (do) not adopt a fixed orientation, and as a result, a description of the Mo.P hyperfine interaction required the introduction of a distribution of such orientations, with Mo.P distance(s) of 3.2-3.3 Å. The other two lines in the FT-ESEEM spectra in H(2)O, located at nu(H) and 2nu(H), were assigned to matrix protons. In D(2)O buffer two additional lines, assigned to matrix deuterons, were also seen.

Journal Article↗