Showing papers by "Ravindra Pandey published in 1991"

Chlorin and porphyrin derivatives as potential photosensitizers in photodynamic therapy.

Abstract: In order to find a photosensitizer with better optical properties and pharmacokinetics than Photofrin II, a series of new photosensitizers related to methyl pheophorbide-a and chlorin-e6 were synthesized. These compounds absorb at substantially longer wavelengths (lambda max 660 nm) than does Photofrin II (630 nm) and show promise for use in photodynamic therapy. Among the porphyrins, we observed that long carbon chain ether derivatives are better photosensitizers than their ester analogs. These sensitizers were tested for in vivo photosensitizing activity vis-a-vis Photofrin II, using the standard screening system of DBA/2 mice bearing transplanted SMT/F tumors. Most of these photosensitizers were found to have better tumoricidal photosensitizing activity than Photofrin II and demonstrated more rapid attenuation of normal tissue photosensitivity with time after administration vis-a-vis Photofrin II.

Electronic structure of the rocksalt-structure semiconductors ZnO and CdO.

Abstract: ZnO, which normally occurs in the hexagonal wurtzite structure, can be transformed to the cubic rocksalt (NaCl) structure by the application of high pressure; this cubic phase has been reported to be metastable at atmospheric pressure. The band structure of this phase is calculated by the ab initio correlated Hartree-Fock method. Not surprisingly, the band structure of rocksalt ZnO is very similar to that of CdO, which has the same crystal structure; we present a band-structure calculation for CdO, which we believe is more accurate than any in the literature. A hallmark of these band structures is that the valence-band maximum is not at the center of the Brillouin zone, in contrast to the situation in tetrahedrally coordinated II-VI semiconductors. We confirm by direct calculation that this peculiarity of the band structure is a consequence of the hybridization of oxygen 2p-derived orbitals with Zn 3d or Cd 4d states, combined with octahedral point symmetry.

Ab initio band-structure calculations for alkaline-earth oxides and sulfides.

Abstract: The electronic structure of the oxides and sulfides of Mg, Ca, and Sr is computed with use of the self-consistent Hartree-Fock method including correlation. Energy-band structure and density of states are presented and discussed in context with the available experimental and theoretical studies. Our results predict that these materials (except MgS) are direct-band-gap materials.

Influence of heme vinyl- and carboxylate-protein contacts on structure and redox properties of bovine cytochrome b5

Abstract: 'H NMR spectroscopy and optical spectroelectrochemistry on a thin-layer electrode have been utilized to investigate the influence of heme vinyland carboxylateprotein contacts on heme pocket structure and reduction potential of bovine ferricytochrome b,. In spite of the diverse modifications of heme vinyls and carboxylates, IH NMR results indicate that there are no significant perturbations in the heme orientation and essential electronic structure. This allows us to attribute changes in redox properties to the role of each vinylor carboxylate-protein contact in cytochrome b,. While pemptohemin (2-H, 4-vinyl) and isopemptohemin (2-viny1, 4-H) exhibit essentially identical reduction potentials outside the protein in DMF solution, protein reconstituted with pemptohemin shows an (ca. -8 mV vs SHE) closer to that of native protein (2-viny1, 4-vinyl; ca. -2 mV), and protein substituted with isopemptohemin exhibits an . @ (ca. -38 mV) closer to that of deuterohemin (2-H, 4-H; ca. -52 mV). Hence the 4-vinyl group accounts for the dominant electron-withdrawing influence on the porphyrin skeleton of the native protein, with the 2-vinyl group providing a minor effect. These results are consistent with the NMR data which indicate a sterically clamped, largely in-plane (maximal electron withdrawing) 4-vinyl group and a 2-vinyl group that is mobile and largely out of plane (minimal electron withdrawing). The reduction potentials for the cytochrome b, complex of 2,4dimethyldeuterohemin and heptamethyl monopropionate hemin, with the lone propionate of the latter hemin making the protein link as for the 7-propionate of the native protein, are ca. -83 and ca. -58 mV, respectively. Moreover, the reduction potentials vary insignificantly when the pair of carboxylate side chains are both lengthened or shortened by one carbon. Therefore, the influence on reduction potential for the 7-propionate in the native protein is not substantially larger than that for the 6-propionate, and argues against this uniquely oriented 7-propionate of native cytochrome bS providing an important stabilizing interaction for oxidized cytochrome bs.

Embedded-cluster study of the lithium trapped-hole center in magnesium oxide.

Abstract: The lithium trapped-hole center in magnesium oxide is studied by using the simulation program package icecap (ionic crystal with electronic clusters, automatic program). The defective crystal is simulated as a molecular cluster embedded in a shell-model lattice. The molecular cluster, which contains the trapped-hole center, is treated quantum mechanically by using the unrestricted Hartree-Fock approximation. Correlation correction is also included in the calculations. Different types and sizes of defect cluster are used to determine the equilibrium configurations of the defective lattice. The role of the correlation correction is found to be significant. The results show very consistently that the ${\mathrm{Li}}^{+}$ ion and the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ion (the ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ion with the trapped hole) move toward each other and the axial ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ion moves away from the ${\mathrm{Li}}^{+}$ ion.

Comparative Ab Initio study of electronic and ionic properties of lithium nitride (Li3N), lithium phosphide (Li3P), and lithium arsenide (Li3As)

Abstract: Ab initio Hartree–Fock band structure calculations are presented for the first time for lithium phosphide (Li3P) and lithium arsenide (Li3As) in the hexagonal P6/mmm crystal structure. Results are compared to those for lithium nitride (Li3N). The new calculations for Li3N agree with previous Hartree–Fock calculations, except for the valence band structure where results of previous pseudopotential calculations are confirmed. Geometry optimization for Li3P yields a lattice parameter a of 4.45 A and a c value of 4.80 A. These values differ markedly from experimental results reported to be 4.271 and 7.590 A, respectively. A similar discrepancy is found for lithium arsenide: a = 4.60 A and c = 4.96 A have to be compared to the reported experimental values of 4.397 A for a and 7.824 A for c. Force constants are derived for in-plane and interplane vibrations. The band structures for Li3P and Li3As are found to be very similar to the one calculated for Li3N. Using Li3P as an example, it is shown how the band structure of the insulator can be derived from the band structures of the two metallic constitutent Li2P and Li monolayers. The metal–insulator transition occurs if the inter-plane distance falls below 4.25 A. Contrary to expectations raised earlier, it is found that the 3d electrons in arsenic are strongly localized, evidenced by a very narrow d band width of 0.1 eV. They cannot be used to explain the difference in conductivity between the phosphide and the arsenide. A Mulliken population analysis gives charge distributions close to the ideal ionic structure (Li+)3X3−, X = N, P, and As. Overall it is found that hexagonal lithium phosphide and lithium arsenide arsenide are more similar to lithium nitride and less anisotropic than suggested previously. This discrepancy could be due to the use of polycrystalline samples in earlier experiments.

Syntheses of cationic porphyrins and chlorins

Abstract: Treatment of vinyl-chlorins 2,4,5 and -porphyrins 8 with N,N-dimethylmethyleneammonium iodide affords the corresponding vinyl-extended Mannich adducts 3,6,7,9; with deuteroporphyrin IX dimethyl ester 10 the product is the bis-Mannich base 11; quaternization of the products is readily accomplished with methyl iodide to afford a general method for synthesis of water-soluble cationic sensitizers for use in photodynamic therapy.

Ring syntheses of the four N-methylprotoporphyrin isomers and of related N-methyl and N,N′-dimethylporphyrins

Abstract: Total syntheses of the four isomeric N-methylprotoporphyrin IX dimethyl esters have been achieved by the MacDonald and tripyrrene/a,c-biladiene routes using appropriate N-methylpyrroles and N-methyldipyrromethanes as intermediates. The syntheses of related N-methyl and N,N′-dimethylporphyrins have also been achieved by the MacDonald route.

Ring Syntheses of the Four N-Methylprotoporphyrin Isomers and of Related N-Methyl and N,N′-Dimethylporphyrins.

Abstract: Total syntheses of the four isomeric N-methylprotoporphyrin IX dimethyl esters have been achieved by the MacDonald and tripyrrene/a,c-biladiene routes using appropriate N-methylpyrroles and N-methyldipyrromethanes as intermediates. The syntheses of related N-methyl and N,N′-dimethylporphyrins have also been achieved by the MacDonald route.