Raman K Afshar

University of California, Santa Cruz, Santa Cruz, California, United States

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Publications (8)44.08 Total impact

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    ABSTRACT: An Fe(III) nitro complex [(bpb)Fe(NO2)(py)] (2) of the tetradentate ligand 1,2-bis(pyridine-2-carboxamido)benzene (H2bpb, H is the dissociable amide proton) has been synthesized via addition of NaNO2 to [(bpb)Fe(py)2](ClO4) (1) in MeCN or DMF. This structurally characterized Fe(III) nitro complex exhibits its nuNO2 at 1384 cm(-1). The reaction of 1 with 2 equiv of Et4NX (X = Cl-, Br-) affords the high-spin complexes (Et4N)[(bpb)Fe(Cl2)] (3) and (Et4N)[(bpb)Fe(Br)2] (4), respectively. The structure of 4 has been determined. The addition of an equimolar amount of Et4NCl, Et4NBr, or Et4NCN to a solution of 2 affords the mixed-ligand complexes (Et4N)[(bpb)Fe(NO2)(Cl)] (5), (Et4N)[(bpb)Fe(NO2)(Br)] (6), and (Et4N)[(bpb)Fe(NO2)(CN)] (7), respectively. These complexes are all low spin with isotropic g values of 2.15. Under anaerobic conditions, the reactions of 5-7 with Ph3P in MeCN afford the five-coordinate {Fe-NO}7 nitrosyl [(bpb)Fe(NO)] (and Ph3PO) via secondary oxygen-atom (O-atom) transfer. The O-atom transfer to Ph3P by 5-7 becomes catalytic in the presence of dioxygen with transfer rates in the range of 1.70-13.59 x 10-3 min(-1). The O-atom transfer rates and turnover numbers (5 > 6 > 7) are reflective of the strength of the axial donors (Cl- > Br- > CN-). The catalytic efficiencies of complexes 5-7 are limited due to formation of the thermodynamic end products [(bpb)Fe(X)2]- (where X = Cl- for 5, Br- for 6, and CN- for 7).
    No preview · Article · Dec 2006 · Inorganic Chemistry
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    ABSTRACT: A high-spin iron(III) chloro complex [(PaPPy3)Fe(Cl)](ClO4) (1, where PaPPy3-=N,N-bis(2-pyridylmethyl)amine-N-propyl-2-pyridine-2-carboxamide) has been synthesized via the “template effect” and structurally characterized. The template reaction leads to the formation of a new carbon–nitrogen bond from coupling of a primary alkyl chloride with a secondary amine.
    No preview · Article · Dec 2006 · Inorganic Chemistry Communications
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    ABSTRACT: Four cobalt(III) complexes containing the polypyridine pentadentate ligands N,N-bis(2-pyridylmethyl)amine-N′-ethyl-2-pyridine-2-carboxamide (PaPy3H), N,N-bis(2-pyridylmethyl)amine-N′-[1-(2-pyridylethyl)acetamide (MePcPy3H), and N,N-bis(2-pyridylmethyl)amine-N′-(2-pyridylmethyl)acetamide (PcPy3H), have been synthesized. All three ligands bind the Co(III) center in the same fashion with the exception of loss of conjugation between the carboxamide moiety and the pyridine ring in the latter two. The structures of [(PaPy3)Co(OH)][(PaPy3)Co(H2O)](ClO4)3·3H2O (1), [(PaPy3)Co(NO2)](ClO4)·2MeCN (2), [(MePcPy3)Co(MeCN)](ClO4)2·0.5MeCN (3), and [(PcPy3)Co(Cl)](ClO4)·2MeCN (4) have been determined. These ligands with strong-field carboxamido N donor stabilize the +3 oxidation state of the Co center as demonstrated by the facile oxidation of the corresponding Co(II) complexes (prepared in situ) by H2O2, [Fe(Cp)2](BF4), or nitric oxide (NO). The Co–Namido bond distances of 1–4 lie in the narrow range of 1.853–1.898Å. 1H NMR spectra of these complexes confirm the low-spin d6 ground states of the metal centers.
    No preview · Article · Sep 2006 · Inorganica Chimica Acta
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    ABSTRACT: An Fe(II) carbonyl complex [(PaPy3)Fe(CO)](ClO4) (1) of the pentadentate ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide (PaPy3H, H is the dissociable amide proton) has been synthesized and structurally characterized. This Fe(II) carbonyl exhibits its nu(CO) at 1972 cm(-1), and its 1H NMR spectrum in degassed CD3CN confirms its S = 0 ground state. The bound CO in 1 is not photolabile. Reaction of 1 with an equimolar amount of NO results in the formation of the {Fe-NO}7 nitrosyl [(PaPy3)Fe(NO)](ClO4) (2), while excess NO affords the iron(III) nitro complex [(PaPy3)Fe(NO2)](ClO4) (5). In the presence of [Fe(Cp)2]+ and excess NO, 1 forms the {Fe-NO}6 nitrosyl [(PaPy3)Fe(NO)](ClO4)2 (3). Complex 1 also reacts with dioxygen to afford the iron(III) mu-oxo species [{(PaPy3)Fe}2O](ClO4)2 (4). Comparison of the metric and spectral parameters of 1 with those of the previously reported {Fe-NO}6,7 nitrosyls 3 and 2 provides insight into the electronic distributions in the Fe(II)-CO, Fe(II)-NO, and Fe(II)-NO+ bonds in the isostructural series of complexes 1-3 derived from a non-heme polypyridine ligand with one carboxamide group.
    No preview · Article · Jun 2006 · Inorganic Chemistry
  • Raman K Afshar · Apurba K Patra · Pradip K Mascharak
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    ABSTRACT: Modification of Cys25 at the active site of the cysteine protease papain by S-nitrosylation inhibits its hydrolytic ability. Previous studies have demonstrated that NO donors N-nitrosoanilines inhibit papain activity via formation of S-NO bond formation at the active site while NO donors such as S-nitroso-N-acetyl-penicillamine (SNAP), N-nitrosoaniline derivatives, and S-nitroso-glutathione (GSNO) inhibit the enzyme via S-thiolation by thiyl radicals generated from the S-nitrosothiols. In this study, we report papain inactivation by a photosensitive {Mn-NO}(6) nitrosyl [(PaPy(3))Mn(NO)](ClO(4)) (1) where PaPy(3)(-) is the anion of the designed ligand N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide. This nitrosyl releases NO upon exposure to visible light of low intensity (50W tungsten lamp). With N(alpha)-benzoyl-l-arginine-p-nitroanilide (l-BApNA) as the substrate, the dissociation constant for the breakdown of the enzyme-inactivator complex (K(I)) and the overall inactivation rate constant (k(i)) were calculated to be 2.46mM and 64.8min(-1), respectively. The papainS-NO adduct has been identified using electrospray mass spectrometry (ESI-MS). The results demonstrate that controlled inactivation of papain can be achieved with the {Mn-NO}(6) nitrosyl 1 and light. The reaction is clean and the extent of inactivation is directly proportional to the exposure time.
    No preview · Article · Aug 2005 · Journal of Inorganic Biochemistry
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    ABSTRACT: Two new iron nitrosyls derived from two designed pentadentate ligands N,N-bis(2-pyridylmethyl)-amine-N'-(2-pyridylmethyl)acetamide and N,N-bis(2-pyridylmethyl)-amine-N'-[1-(2-pyridinyl)ethyl]acetamide (PcPy(3)H and MePcPy(3)H, respectively, where H is the dissociable amide proton) have been structurally characterized. These complexes are similar to a previously reported (Fe-NO)6 complex, [(PaPy(3))Fe(NO)](ClO(4))(2) (1) that releases NO under mild conditions. The present nitrosyls, namely [(PcPy(3))Fe(NO)](ClO(4))(2) (2) and [(MePcPy(3))Fe(NO)](ClO(4))(2) (3), belong to the same (Fe-NO)6 family and exhibit (a) clean (1)H NMR spectra in CD(3)CN indicating S = 0 ground state, (b) almost linear Fe-N-O angles (177.3(5) degrees and 177.6(4) degrees for 2 and 3, respectively), and (c) N-O stretching frequencies (nu(NO)) in the range 1900-1925 cm(-)(1). The binding of NO at the non-heme iron centers of 1-3 is completely reversible and all three nitrosyls rapidly release NO when exposed to light (50 W tungsten bulb). In addition to acting as photoactive NO-donors, these complexes also nitrosylate thiols such as N-acetylpenicillamine, 3-mercaptopropionic acid, and N-acetyl-cysteine-methyl-ester in yields that range from 30 to 90% in the absence of light. The addition of alkyl or aryl thiolate (RS(-)) to the (Fe-NO)6 complexes in the absence of dioxygen results in the reduction of the iron metal center to afford the corresponding (Fe-NO)7 species.
    No preview · Article · Oct 2004 · Inorganic Chemistry
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    ABSTRACT: The non-heme iron(III) complex [(PaPy3)Fe(NO 2)](ClO4) (1) transfers oxygen in the temperature range 45-65 °C to PPh3 to afford OPPh3 in MeCN. The product, [(PaPy3)Fe(NO)](ClO4), a {Fe-NO}7 species, reverts rapidly back to 1 in the presence of dioxygen, thus making the system catalytic (see scheme). Formation of an oxo-bridged species terminates the oxo-transfer process (PaPy3H = N-[N,N-bis(2-pyridylmethyl)aminoethyl]-2-pyridinecarboxamide).
    No preview · Article · Sep 2003 · Angewandte Chemie International Edition
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    ABSTRACT: Reversible NO binding in acetonitrile is observed in the non-heme FeIII complex [Fe(PaPy3)(NO)](ClO4)2 (see scheme). The NO moiety is photolabile, and can dissociate from this complex under very mild conditions. The carboxamido nitrogen donor trans to NO appears to playa crucial role in the observed photolability. (PaPy3 = N-[N,N-bis(2-pyridylmethyl)aminoethyl]-2-pyridinecarboxamide).
    No preview · Article · Jul 2002 · Angewandte Chemie International Edition