Syntheses of 1-[(2-hydroxyethoxy)methyl]- and 1-[(1,3-dihydroxy-2-propoxy)methyl]- derivatives of 5-substituted-2,4-difluorobenzene: unnatural acyclo thymidine mimics for evaluation as anticancer and antiviral agents

A group of 1-[(2-hydroxyethoxy)methyl]- (12) and 1-[(1,3-dihydroxy-2-propoxy)methyl]- (13) derivatives of 2,4-difluorobenzene possessing a variety of C-5 substituents (R = Me, H, I, NO2) were designed with the expectation that they may serve as acyclic 5-substituted-2'-deoxyuridine (thymidine) mimics. Compounds 12 and 13 (R = Me, H, I) were inactive as anticancer agents (CC50 = 10(-3) to 10(-4) M range), whereas the 5-nitro compounds (12d, 13d) exhibited weak-to-moderate cytotoxicity (CC50 = 10(-5) to 10(-6) M range) against a variety of cancer cell lines. All compounds prepared (12a-d, 13a-d) were inactive as antiviral agents in a broad-spectrum antiviral screen that also included the human immunodeficiency virus (HIV-1 and HIV-2) and herpes simplex virus (HSV-1 and HSV-2).     

Synthesis and antimicrobial activity of new (4,4,4-trihalo-3-oxo-but-1-enyl)-carbamic acid ethyl esters, (4,4,4-trihalo-3-hydroxy-butyl)-carbamic acid ethyl esters, and 2-oxo-6-trihalomethyl-[1,3]oxazinane-3-carboxylic acid ethyl esters

This work presents a three-step synthesis of a new series of 4-substituted 2-oxo-6-trihalomethyl-[1,3]oxazinane-3-carboxylic acid ethyl esters, from beta-alkoxyvinyl trihalomethyl ketones of general formula X3C-C(O)-CH=C(R)-OR(1), where R = H, Me, Ph, and 4-Me-Ph; R(1) = Me and Et; and X = F and Cl. The Michael addition-substitution of the ethyl carbamate on beta-alkoxyvinyl trihalomethyl ketones furnished the corresponding (4,4,4-trihalo-3-oxo-but-1-enyl)-carbamic acid ethyl esters. These compounds underwent reduction with NaBH4 leading to the respective (4,4,4-trihalo-3-hydroxy-butyl)-carbamic acid ethyl esters. The 3-hydroxy-butyl carbamates were submitted to cyclization reaction with triphosgene to give a series of 4-substituted 2-oxo-6-trihalomethyl-[1,3]oxazinane-3-carboxylic acid ethyl esters. The in vitro antimicrobial activity, of some of the three new series of the title compounds, was assessed against a panel of microorganisms including yeast like fungi, bacteria, and algae, and their minimal inhibitory concentration and minimal fungicidal, bactericidal, and algacidal concentrations were determined. Some of the analyzed carbamates exhibited significant in vitro antimicrobial activity.     

Probing intraligand and charge transfer excited states of fac-[Re(R)(CO)(3)(CO(2)Et-dppz)](+) (R = py, 4-Me(2)N-py; CO(2)Et-dppz = dipyrido[3,2a:2',3'c]phenazine-11-carboxylic ethyl ester) using time-resolved infrared spectroscopy.

The photophysics of fac-[Re(R)(CO)(3)(CO(2)Et-dppz)](+) (R = py (), 4-Me(2)N-py (); CO(2)Et-dppz = dipyrido[3,2a:2',3'c]phenazine-11-carboxylic ethyl ester) was studied with luminescence spectroscopy and time-resolved infrared (TRIR) spectroscopy in the metal carbonyl (2,100-1,800 cm(-1)) and organic ester (1,800-1,600 cm(-1)) regions. For 1, the picosecond TRIR spectra in the metal carbonyl region provided evidence for the formation of an intra-ligand IL (pi-pi) excited state, which partially decays to an equilibrium with the metal-to-ligand charge transfer (MLCT) excited state. For 2 it is evident that both IL (pi-pi) and MLCT excited states are formed within 2 ps of excitation. The magnitude of the nu(CO) shift in the metal carbonyl region following excitation allows the MLCT excited states to be described more precisely as a dpi(Re) -->pi (phenazine) (3)MLCT state for 1 and as a dpi(Re) -->pi (phenanthroline) (3)MLCT state for 2.     

Reactions of the haloalcohols HO(CH2)nCl (n = 2, 3) with platinum(II) - alkynyl and -alkyne complexes. X-ray crystal structure of trans-[Pt(Me) {C(OCH2CH2Cl) (CH2Ph)} (PPh3)2] [BF4]

The chloro complexes trans-[Pt(Me)(Cl)(PPh₃)₂], after treatment with AgBF₄, react with 1-alkynes HC---C---R in the presence of NEt₃ to afford the corresponding acetylide derivatives trans-[Pt(Me) (C---C---R) (PPh₃)₂] (R = p-tolyl (1), Ph (2), C(CH₃)₃ (3)). These complexes, with the exception of the t-butylacetylide complex, react with the chloroalcohols HO(CH₂)_nCl (n = 2, 3) in the presence of 1 equiv. of HBF₄ to afford the alkyl(chloroalkoxy)carbene complexes trans-[Pt(Me) {C[O(CH₂)_nCl](CH₂R) } (PPh₃)₂][BF₄] (R = p-tolyl, N = 2 (4), N = 3 (5); R=Ph, N = 2 (6)). A similar reaction of the bis(acetylide) complex trans-[Pt(C---C---Ph)₂(PMe₂Ph)₂] with 2 equiv. HBF₄ and 3-chloro-1-propanol affords trans-[Pt(C---CPh) {C(OCH₂CH₂CH₂Cl)(CH₂Ph) } (PMe₂Ph)₂][BF₄] (7). T alkyl(chloroalkoxy)-carbene complex trans-[Pt(Me) {C(OCH₂CH₂Cl)(CH₂Ph) } (PPh₃)₂][BF₄] (8) is formed by reaction of trans-[Pt(Me)(Cl)(PPh₃)₂], after treatment with AgBF₄ in HOCH₂CH₂Cl, with phenylacetylene in the presence of 1 equiv. of n-BuLi. The reaction of the dimer [Pt(Cl)(μ-Cl)(PMe₂Ph)]₂ with p-tolylacetylene and 3-chloro-1-propanol yields cis-[PtCl₂{C(OCH₂CH₂CH₂Cl)(CH₂C₆H₄-p-Me}(PMe₂Ph)] (9). The X-ray molecular structure of (8) has been determined. It crystallizes in the orthorhombic system, space group Pna2₁, with a = 11.785(2), B = 29.418(4), C = 15.409(3) Å, V = 4889(1) ų and Z = 4. The carbene ligand is perpendicular to the Pt(II) coordination plane; the PtC(carbene) bond distance is 2.01(1) Å and the short C(carbene)-O bond distance of 1.30(1) Å suggests extensive electronic delocalization within the Pt---C(carbene)---O moietry.     

NMDA-NR2B subtype selectivity of stereoisomeric 2-(1,2,3,4-tetrahydro-1-isoquinolyl)ethanol derivatives

Enantiopure 2-(1,2,3,4-tetrahydro-1-isoquinolyl)ethanol derivatives were tested for their affinity to the ifenprodil binding site of the NMDA receptor, their potency to inhibit [3H]MK801 binding and their NMDA-NR2B subtype selectivity. The (1S,1'S)-configurated series displayed the highest affinity to the ifenprodil binding site. A reasonable potency and NMDA-NR2B subtype selectivity was found for (1S,1'S)-4c (R1=Me, R2=OMe). A high affinity to HERG K+ channels, however, suggests that (1S,1'S)-4c may involve an increased risk of cardiovascular side effects.     

Gold in organic synthesis Part 2. Preparation of benzyl-alkyl and-arylketones via C---C coupling

Reaction of [Au(η²-Ar){CH₂C(O)R}Cl] (Ar=C₆H₄N=N- Ph-₂, R=Me, C₆H₂(OMe)₃-3′,4′,5′; Ar=C₆H₃(N=NC₆H₄Me- 4′)-2, Me-5, R=Me) with PPh₃ and NaClO₄·H₂O (1:2:1) at room temperature, leads to reductive elimination giving [Au(PPh₃)₂]ClO₄ and the corresponding carbon-carbon coupling product ArCH₂C(O)R. A similar process takes place when complexes [Au(η²-Ar){CH₂C(O)R}(PPh₃)Cl] are refluxed in tetrahydrofuran, through elimination of [Au(PPh₃)Cl].     

Optically active titanium complexes containing a tridentate linked amido-cyclopentadienyl ligand

Optically active titanium complexes Tieta5:eta1-C5R4SiMe2NC6H10 (OCH2Ph)-2Cl2 (R = H, Me), containing a cyclopentadienyl ligand linked to the chiral trans-2-benzyloxycyclohexylamido group, were synthesized and characterized in both enantiomerically pure forms. A single crystal X-ray structure analysis of (-)-(R, R)-Tieta5:eta1-C5H4SiMe2NC6H10(OCH2Ph)-2Cl2 shows a structure in which the benzyloxy group in the amido sidechain is not interacting with the titanium center. Upon activation with n-butyllithium, these complexes hydrogenate acetophenone N-benzylimine with low enantioselectivity.     

Delivery of floxuridine derivatives to cancer cells by water-soluble organometallic cages

The self-assembly of 2,4,6-tris(pyridin-4-yl)-1,3,5-triazine (tpt) triangular panels with p-cymene (pPr(i)C(6)H(4)Me) ruthenium building blocks and 2,5-dioxydo-1,4-benzoquinonato (dobq) or 5,8-dioxydo-1,4-naphthoquinonato (donq) bridges, in the presence of a pyrenyl-nucleoside derivatives (pyreneR), affords the triangular prismatic host-guest compounds [(pyrene-R)?Ru(6)(pPr(i)C(6)H(4)Me)(6)(tpt)(2)(dobq)(3)](6+) ([(pyrene-R)?1](6+)) and [(pyrene-R)?Ru(6)(pPr(i)C(6)H(4)Me)(6)(tpt)(2)(donq)(3)](6+) ([(pyrene-R)?2](6+)), respectively. The inclusion of six monosubstitutedpyrenyl-nucleosides (pyrene-R1 = 5'-(1-pyrenyl butanoate)-2'-deoxyuridine, pyrene-R2 = 5-fluoro-5'-(1-pyrenyl butanoate)-2'-deoxyuridine, pyrene-R3 = 5'-{N-[1-oxo-4-(1-pyrenyl)butyl]-glycyl}-2'-deoxyuridine, pyrene-R4 = 5-fluoro-5'-{N-[1-oxo-4-(1-pyrenyl)butyl]-glycyl}-2'-deoxyuridine, pyrene-R5 = 5-fluoro-5'-{N-[1-oxo-4-(1-pyrenyl)butyl]-phenylalanyl}-2'-deoxyvuridine, pyrene-R6 = 5-fluoro-5'-{N-[1-oxo-4-(1-pyrenyl)butyl]-phenylalanyl}-2'-deoxyuridine) has been accomplished. The carceplex nature of [(pyrene-R)?1](6+) with the pyrenyl moiety firmly encapsulated in the hydrophobic cavity of the cage with the nucleoside groups pointing outward was confirmed by NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS), while the host-guest nature of [(pyrene-R)?2](6+) was studied in solution by NMR techniques. In contrast to the floxuridine compounds used in the clinic, the host-guest complexes are highly water-soluble. Consequently, the cytotoxicities of these water-soluble compounds have been established using human ovarian A2780 and A2780cisR cancer cells. All the host-guest systems are more cytotoxic than the empty cages alone [1][CF(3)SO(3)](6) (IC(50) = 23 μM) and [2][CF(3)SO(3)](6) (IC(50) = 10 μM), the most active compound [pyrene-R4?1][CF(3)SO(3)](6)being 2 orders of magnitude more cytotoxic (IC(50) = 0.3 μM) on these human ovarian cancer cell lines (A2780 and A2780cisR).     

Synthesis, structural investigations on organotin(IV) chlorin-e6 complexes, their effect on sea urchin embryonic development and induced apoptosis

Four new organotin(IV) chlorin derivatives, [chlorin=chlorin-e(6)=21H,23H-porphine-2-propanoic acid, 18-carboxy-20-(carboxymethyl)-8-ethenyl-13-ethyl-2,3-di-hydro-3,7,12,17-tetramethyl-(2S-trans)-], with formula (R(2)Sn)(3)(chlorin)(2).2H(2)O (R=Me, n-Bu) and (R(3)Sn)(3)chlorin.2H(2)O (R=Me, Ph) have been synthesized. The solid state and solution phase structures have been investigated by FT-IR, (119)Sn M?ssbauer, (1)H and (13)C NMR spectroscopy. In the solid state, (R(2)Sn)(3)(chlorin)(2).2H(2)O complexes contain six coordinated Sn(IV), in a skew trapezoidal environment by forming trans-R(2)SnO(4) polymeric units. As far as (R(3)Sn)(3)chlorin.2H(2)O complexes are concerned, Sn(IV) is five coordinated in a polymeric (oligomeric) trigonal bipyramidal environment and eq-R(3)SnO(2) units, in the solid state. In saturated solutions, a polymeric structure comparable to the solid phase, with carboxylate groups of the ligand behaving in monoanionic bidentate fashion bridging Sn(IV) atoms, was detected for the (Me(3)Sn)(3)chlorin.2H(2)O complex, while in more diluted ones a tetrahedral configuration for the trimethyltin(IV) moieties was observed. Cytotoxic activity of the novel organotin(IV) chlorin was investigated in order to assay the effect on sea urchin embryonic development. The results obtained demonstrated that (n-Bu(2)Sn)(3)(chlorin)(2).2H(2)O and (Ph(3)Sn)(3)chlorin.2H(2)O exerted the antimitotic effect on the early stages of sea urchin development. In addition, the cytotoxic effect exerted by (n-Bu(2)Sn)(3)(chlorin)(2).2H(2)O appeared with necrosis of the blastomeres, which were clearly destroyed. After treatment with (Ph(3)Sn)(3)chlorin.2H(2)O, a programmed cell death was triggered, as shown by light microscope observations through morphological assays. The apoptotic events in 2-cell stage embryos revealed: (i) DNA fragmentation, with the TUNEL reaction (terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling); (ii) phosphatidylserine translocation in the membrane, with Annexin-V assay and (iii) cytoplasm blebbing, with the TUNEL reaction. The results demonstrated that the novel compound (Ph(3)Sn)(3)chlorin.2H(2)O was the most toxic derivative, by exerting antimitotic effect very early and by triggering apoptosis in the 2-cell stage of sea urchin embryonic development.     

Steric effects on the substitution of pentaamine-chromium(III) and -rhodium(III) complexes. Anation reaction rate constants as an indicative of the dissociative shift of the mechanism on crowding the [M(RNH2)5H2O] (M = Rh, R = H, Me, Et, Pr; M = Cr, R = H, Me, Pr) complexes

The kinetics of the anation reactions of [M(RNH₂)₅H₂O]³⁺ (M = Rh, R = H, Me, Et, Pr; M = Cr, R = H, Me, Pr) with several ligands (H₃PO₄/H₂PO₄⁻, H₃PO₃/H₂PO₃⁻, CF₃COO⁻, Br⁻, Cl⁻, SCN⁻) have been studied at different temperatures and acidities at I = 1.0 M (LiClO₄. Results obtained for the anation rate constants and thermal activation parameters are compared with the previously published data for R = H, in order to establish the effects of the amine substituents in the reaction mechanism proposed for the substitution reactions of these complexes. The results obtained are interpreted on the basis of a mechanism where the bond formation process is more important in the substitution on M = Cr complexes than in that of the M = Rh complexes, as already pointed out for the published ΔΛ^≠ values for the water exchange on these systems. A simple Langford-Gray classification becomes inadequate to describe these situations where the increase of the steric demand of the amine substituents shifta the I_a-I_d classification to the I_d side, although no dramatic changes in the reaction mechanism are found. It is concluded that a More O'Ferall ‘continuous’ type of approach to the mechanism classification of the substitution reactions is much more useful in this case.     

Studies on the 1,1-diphenyl-2-picrylhydrazyl radical scavenging mechanism for a 2-pyrone compound

The radical scavenging mechanisms for the 2-pyrone compound, 4-hydroxy-3,6-dimethyl-2H-pyrane-2-one (1), and the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical (4) in several solvent systems were evaluated by the quantitative change in compounds detected at 270 nm and subsequent HPLC analyses. The HPLC profile for each condition suggested that the reaction proceeded by a different mechanism in each solvent system. In organic solvents (CHCl3, iso-propanol, and EtOH), 1-[4-(3,4-dihydro-3,6-dimethyl-2,4-dioxo-2H-pyran-3-yl) phenyl]-1-phenyl-2-picrylhydrazine (2) was produced as an adduct of the DPPH radical and 1. On the other hand, the reaction in a buffer solution (an acetate buffer at pH 5.5) gave several degradation products with 1[4-(2,3-dihydro-2,5-dimethyl-3-oxo-fur-2-yl) phenyl]-1-phenyl-2-picrylhydrazine (5), this being structurally elucidated by spectroscopic analyses. The decrease of the DPPH radical in each reaction system suggests that compound 1 could scavenge about 1.5-1.8 equivalents of the radical in organic solvents and about 3.5-3.9 in the buffer solution.     

The formation of 2-hydroxy-4-hydroxymethyl-3-(indol-3-yl)-cyclopent-2-enone derivatives from ascorbigens

A facile preparation is described of 3-(indol-3-yl)-2-hydroxy-4-hydroxymethylcyclopent-2-enone and its N-derivatives in 15-40% yields by the degradation of ascorbigen or its N-derivatives in a warm solution of L-ascorbic acid through a sequential domino reaction. The same cyclopentenone derivatives were obtained in 30-40% yields by the condensation of (N-alkylindol-3-yl)glycolic acids with ascorbic acid. 2,6-Dihydroxy-1-(indol-3-yl)hexa-1,4-diene-3-one and 2-hydroxy-4-hydroxymethyl-5-(indol-3-yl)cyclopent-2-enone were identified as intermediates in this reaction.     

Mechanism of inactivation of gamma-aminobutyrate aminotransferase by 4-amino-5-fluoropentanoic acid. First example of an enamine mechanism for a gamma-amino acid with a partition ratio of 0

The mechanism of inactivation of pig brain gamma-aminobutyric acid aminotransferase (GABA-T) by (S)-4-amino-5-fluoropentanoic acid (1, R = CH2CH2COOH, X = F) previously proposed [Silverman, R. B., & Levy, M. A. (1981) Biochemistry 20, 1197-1203] is revised. apo-GABA-T is reconstituted with [4-3H]pyridoxal 5'-phosphate and inactivated with 1 (R = CH2CH2COOH, X = F). Treatment of inactivated enzyme with base followed by acid denaturation leads to the complete release of radioactivity as 6-[2-hydroxy-3-methyl-6-(phosphonoxymethyl)-4-pyridinyl]-4-oxo-5-+ ++hexenoic acid (4, R = CH2CH2COOH). Alkaline phosphatase treatment of this compound produces dephosphorylated 4 (R = CH2CH2COOH). These results support a mechanism that was suggested by Metzler and co-workers [Likos, J. J., Ueno, H., Feldhaus, R. W., & Metzler, D. E. (1982) Biochemistry 21, 4377-4386] for the inactivation of glutamate decarboxylase by serine O-sulfate (Scheme I, pathway b, R = COOH, X = OSO3-).     

A functional model for pMMO (particulate methane monooxygenase): Hydroxylation of alkanes with H2O2 catalyzed by beta-diketiminatocopper(II) complexes

The reaction of copper(II) complexes supported by a series of beta-diketiminate ligands ((R1,R2)L, [(Dipp)N-C(R(2))-C(R(1))-C(R(2))-N(Dipp)](-), Dipp=2,6-diisopropylphenyl; see ) and H(2)O(2) has been examined spectroscopically at a low temperature. The beta-diketiminatocopper(II) complexes with R(2)=H (no substituent on the beta-carbon) provided a copper-oxygen intermediate that exhibited the same spectroscopic features as those of the bis(mu-oxo)dicopper(III) complex generated by the reaction of corresponding beta-diketiminatocopper(I) complex and O(2). On the other hand, the beta-diketiminatocopper(II) complexes with methyl substituent on the beta-carbon (R(2)=Me) did not produce such an intermediate in the same reaction. The beta-diketiminatocopper(II) complexes carrying an electron-withdrawing substituent on the alpha-carbon (R(1)=NO(2) or CN) but no beta-substituent (R(2)=H) exhibited a high catalytic activity in the oxygenation reaction of alkanes with H(2)O(2). Mechanism of the catalytic oxygenation reaction as well as the substituent effects of the ligands on the copper(II)-H(2)O(2) reactivity is discussed.     

Hydron transfer catalyzed by triosephosphate isomerase. Products of isomerization of dihydroxyacetone phosphate in D2O

The product distributions for the reactions of dihydroxyacetone phosphate (DHAP) in D(2)O at pD 7.9 catalyzed by triosephosphate isomerase (TIM) from chicken and rabbit muscle were determined by (1)H NMR spectroscopy using glyceraldehyde 3-phosphate dehydrogenase to trap the first-formed products of the thermodynamically unfavorable isomerization reaction, (R)-glyceraldehyde 3-phosphate (GAP) and [2(R)-(2)H]-GAP (d-GAP). Three products were observed from the reactions catalyzed by TIM: GAP from isomerization with intramolecular transfer of hydrogen (18% of the enzymatic products), d-GAP from isomerization with incorporation of deuterium from D(2)O into C-2 of GAP (43% of the enzymatic products), and [1(R)-(2)H]-DHAP (d-DHAP) from incorporation of deuterium from D(2)O into C-1 of DHAP (40% of the enzymatic products). The ratios of the yields of the deuterium-labeled products d-DHAP and d-GAP from partitioning of the intermediate of the TIM-catalyzed reactions of GAP and DHAP in D(2)O are 1.48 and 0.93, respectively. This provides evidence that the reaction of these two substrates does not proceed through a single, common, reaction intermediate but, rather, through distinct intermediates that differ in the bonding and arrangement of catalytic residues at the enediolate O-1 and O-2 oxyanions formed on deprotonation of GAP and DHAP, respectively.     

Oxidation of diiron and triiron sulfurdithiolato complexes: mimics for the active site of [FeFe]-hydrogenase

The oxidation of the hexacarbonyl(1,3-dithiolato-S,S')diiron complexes 4a-4c with varying amounts of dimethyldioxirane (DMD) was systematically studied. The chemoselectivity of the oxidation products depended upon the substituent R (R=H, Me, 1/2 (CH2)(5)). For R=H, four oxidation products, 6a-6d, have been obtained. In the case of R=Me, three products, 7a-7c, were formed, and for R=1/2 (CH2)(5), only complex 8 was observed. These observations are due to steric and electronic effects caused by the substituent R. Additionally, oxidation of the triiron complex 5 with DMD was performed to yield the products 9a and 9b. X-Ray diffraction analyses were performed for 6a-6d, 7a, and 7c, as well as for 9a and 9b. The electronic properties were determined by density-functional theory (DFT) calculations.     

Regulation of tyrosinase by tetrahydropteridines and H2O2

Recently two alternative mechanisms have been put forward for the inhibition of tyrosinase by 6R-l-erythro 5,6,7,8-tetrahydrobiopterin (6BH(4)). Initially allosteric uncompetitive inhibition was demonstrated due to 1:1 binding of 10(-6)M 6BH(4) to a specific domain 28 amino acids away from the Cu(A) active site of the enzyme. Alternatively it was then shown that 10(-3)M 6BH(4) inhibit the reaction by the reduction of the product dopaquinone back to l-dopa. In the study presented herein we have used two structural analogues of 6BH(4) (i.e., 6,7-(R,S)-dimethyl tetrahydrobiopterin and 6-(R,S)-tetrahydromonapterin) confirming classical uncompetitive inhibition due to specific binding of the pyrimidine ring of the pterin moiety to the regulatory domain on tyrosinase. Under these conditions there was no reduction of l-dopaquinone back to l-dopa by both cofactor analogues. Inhibition of tyrosinase by 6BH(4) occurs in the concentration range of 10(-6)M after preactivation with l-tyrosine and this mechanism uncouples the enzyme reaction producing H(2)O(2) from O(2). Moreover, a direct oxidation of 6BH(4) to 7,8-dihydrobiopterin by tyrosinase in the absence of the substrate l-tyrosine was demonstrated. The enzyme was activated by low concentrations of H(2)O(2) (<0.3 x 10(-3)M), but deactivated at concentrations in the range 0.5-5.0 x 10(-3)M. In summary, our results confirm a major role for 6BH(4) in the regulation of human pigmentation.     

Reactions of site-differentiated [Fe4S4]2+, 1+ clusters with sulfonium cations: reactivity analogues of biotin synthase and other members of the S-adenosylmethionine enzyme family

The first examples of reduced 3:1 site-differentiated Fe(4)S(4) clusters have been synthesized as [Fe(4)S(4)(LS(3))(SR')](3-) (R=Et, Ph) by chemical reduction of previously reported [Fe(4)S(4)(LS(3))(SR')](2-) clusters, and isolated as NBu(4)(+) salts. The reduced clusters were characterized by electrochemistry and EPR, 1H NMR, and M?ssbauer spectroscopies. The reaction of oxidized clusters with the sulfonium ions [PhMeSCH(2)R](+) (R=COPh, p-C(6)H(4)CN) in acetonitrile results in electrophilic attack on coordinated thiolate and production of PhSMe and R'SCH(2)R when the reaction occurs at the unique cluster site. The reactions of reduced clusters with these substrates were examined in relation to the reductive cleavage of the cofactor S-adenosylmethionine, the first step in the catalytic cycle of biotin synthase. Product analysis indicated a approximately 4:1 ratio of reductive cleavage to electrophilic attack. The cleavage products are PhSMe, R'SCH(2)R, and RCH(3) for both clusters, and also PhMeS=CHR and RCH(2)CH(2)R from secondary reactions when the sulfonium cation is [PhMeSCH(2)COPh](+) and [PhMeSCH(2)-p-C(6)H(4)CN](+), respectively. Reaction schemes for reductive cleavage based on product distributions are presented. These results parallel those previously reported for homoleptic [Fe(4)S(4)(SR')(4)](2-,3-) clusters and demonstrate that site-differentiated clusters sustain a high percentage of reductive cleavage, a necessary result in the context of biotin synthase activity preceding an investigation of the mode of binding of sulfonium substrates and inhibitors at the unique iron site. [LS(3)=1,3,5-tris[(4,6-dimethyl-3-mercaptophenyl)thio]-2,4,6-tris(p-tolylthio)benzene(3-)].     

Effect of hydrophobic structure on the catalysis of nitric oxide release from zwitterionic diazeniumdiolates in surfactant and liposome media.

The effect of phospholipid liposomes and surfactant micelles on the rate of nitric oxide release from zwitterionic diazeniumdiolates, R1R2N[N(O)NO]-, with significant hydrophobic structure, has been explored. The acid-catalyzed dissociation of NO has been examined in phosphate-buffered solutions of sodium dodecylsulfate (SDS) micelles and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-[phospho-(1-glycerol)] sodium salt (DPPG) phospholipid liposomes. The reaction behavior of dibenzylamine-, monobenzylamine-, and dibutylamine-derived substrates [1]: R1 = C6H5CH2, R2 = C6H5CH2 NH2+(CH2)2, 2: R1 = C6H5CH2, R2 = NH3+(CH2)2, and 3: R1 = n-butyl, R2 = n-butyl-NH2+(CH2)6] has been compared with that of SPER/NO, 4: R1 = H2N(CH2)3, R2 = H2N(CH2) 3NH2+(CH2)4]. Catalysis of NO release is observed in both micellar and liposome media. Hydrophobic interactions contribute to micellar binding for 1-3 and appear to be the main factor facilitating catalysis by charge neutral DPPC liposomes. Binding constants for the association of 1 and 3 with SDS micelles were 3-fold larger than those previously obtained with comparable zwitterionic substrates lacking their hydrophobic structure. Anionic DPPG liposomes were much more effective in catalyzing NO release than either DPPC liposomes or SDS micelles. DPPG liposomes (at 10 mM total lipid) induced a 30-fold increase in the NO dissociation rate of SPER/NO compared to 12- and 14-fold increases in that of 1 and 3.