A revised structure for hermonionic acid

Hermonionic acid and its decarboxylated product have been isolated from $\text{Garcinia}$$\text{quaesita}$. ¹³C NMR spectral and chemical evidence indicate that hermonionic acid is 2-0-[2-(3-methylbut-2-enyl)-3-methoxy- 4-hydroxy-5-(3,7-dimethylocta-2,6-dienyl]-4-methoxy-5-(3-methylbut-2-enyl-6-hydroxybenzoic acid. The previously assigned dienone structure for this acid is incorrect.     

Mass spectrometric interrogation of thioester-bound intermediates in the initial stages of epothilone biosynthesis

Direct detection of thioester intermediate mixtures bound to EpoC, a 195 kDa polyketide synthase, has been achieved using limited proteolysis and Fourier-transform mass spectrometry (FTMS). Incubation with various N-acetylcysteamine thioester (S-NAC) substrate mimics produced mass shifts on the EpoC ACP domain consistent with their condensation with an enzyme-bound carbanion produced by the decarboxylation of methylmalonyl-S-EpoC. Reconstitution of EpoA-ACP, EpoB, and EpoC gave a +165.0 Da mass shift consistent with the formation of the methylthiazolyl-methacrylyl product by incorporation of acetyl-CoA, cysteine, and methylmalonyl-CoA. Thioester-templated reaction intermediates and products are typically characterized by quantifying radioactive substrates, either enzyme bound or chemically hydrolyzed. In contrast, the MS-based methodology described here provides semiquantifiable ratios of free enzyme, intermediate, and product occupancy and reveals that certain substrates result in a >50% formation of nonproductive intermediates.     

Synthesis and antimalarial effects of 5,6-dichloro-2-[(4-||4-(diethylamino)-i-methylbutyl] amino||-6-methyl-2-pyrirnidinyl)amino] benzimidazole and related benzimidazoles and lH-imidazo[4,5-b] pyridines

A group of fifty-five 2-[(4-11[(dialkylamino)alkyI]amino11-6-methyl-2-pyrimidinyl)amino]-benzimidazoles (VII) was synthesized in 3-88% yield by the condensation of the requisite 2-[(2-benzimidazolyl)amino]-4-chloro-6-methylpyrimidine (VI) with the appropriate polyamine in ethanol-hydrochloric acid or neat with excess amine containing potassium iodide. The 2-[(2-benzimidazolyl)amino]-6-methyl-4-pyrirnidinol precursors (V), obtained in 11-51% yield by cyclization of 2-(cyanoamino)-4-hydroxy-6-methylpyrimidine with a suitably substituted o-phenylenediamine, were chlorinated with phosphorus oxychloride to give the intermediate 2-[(2-benzimidazolyl)amino]-4-chloro-6-rnethylpyrimidines (VI) (27-99%). Oxidation of 5,6-dichloro-2-[(4-11[4-(diethylamino)-l-methylbutyl] amino 11-6-methyl-2-pyrimidinyl) amino ]benzimidazole ( 29 ) with m-chloroperbenzoic acid gave the distal N⁴'-oxide ( 31 ) (19%). Fusion of 2,3-uiaminopyridine with 2-(cyanoamino)-4-hydroxy-6-methylpyrimidine provided 2-[(4-hydroxy-6-tnethyl-2-pyrimidinyl)amino]-lH-imitlazo[4,5-b]pyrimidine (VIII) (30%), which upon chlori-nation with phosphorus oxychloride (63%) followed by amination with i N, N-diethylethylene-diamine afforded 2-(4-11[2-(diethylamino)ethyl] amino 11-6-methyl-2-pyrimidinyl)-lH-imidazo [4,5-b]pyridine (X) (8%). Thirty-eight of the novel 2-[(4-amino-6-methyl-2-pyrimidinyl)amino]-benzimidazoles possessed “curative” activity against Plasmodium berghei at single subcutaneous doses ranging from 20.640 mg./kg. Orally, thirty-one compounds exhibited suppressive activity against P. berghei comparable with or superior to the reference drugs 1-(p-chlorophenyl)-3-(4-11[2-(diethylarnino)ethyl]amino 11-6-methyl-2-pyrimidinyl)guanidine (I) and quinine hydrochloride, while twelve of them were 5 to 28 times as potent as I and quinine hydrochloride. Eight compounds also displayed strong suppressive activity against P. gallinaceum in chicks. 5,6-Dichloro-2-[(4-112-(diethylamino)ethyl]amino11-6-methyl-2-pyrimidinyl] benzimidazole (18) showed marked activity against a cycloguanil-resistant line of P. berghei, and the most promising member of the series, namely 5,6-dichloro-2-[(4-11[4-(diethylamino)-l-methylbutyl]amino11-6-methyl-2-pyrimidinyl)amino]benzimidazole ( 29 ) (Q = 28), was designated for preclinical toxico-logical studies and clinical trial. Structure-activity relationships are discussed.     

PVT properties of concentrated aqueous electrolytes: V. Densities and apparent molal volumes of the four major sea salts from dilute solution to saturation and from 0 to 100°C

The densities of the major sea salts (NaCl, Na₂SO₄, MgCl₂, and MgSO₄) have been measured from 25 to 95°C and to saturation. These results have been combined with literature data and fitted to equations of the form $$\Delta d = Am{\text{ }} + {\text{ }}Bm^{3/2} {\text{ }} + {\text{ }}Cm^2 {\text{ }} + {\text{ }}Dm^{5/2} $$ where Δd=d?d_o (d_o is the density of water) and A, B, and C, etc., are polynomial functions of temperature. The standard deviations of the fits were better than ±50×10^?6 g-cm^?3 for all the salts from 0 to 95°C and to saturation. The apparent molal volumes V_? of the salts have been fitted to the equations of Pitzer. The infinite dilution values of V_? were in good agreement with literature data, provided the results were not overfit. The large deviations of V_? for MgSO₄ from additivity as a function of concentration were attributed to the formation of MgSO₄ ion pairs.     

Structure of a de novo designed protein model of radical enzymes

The use of side chains as catalytic cofactors for protein mediated redox chemistry raises significant mechanistic issues as to how these amino acids are activated toward radical chemistry in a controlled manner. De novo protein design has been used to examine the structural basis for the creation and maintenance of a tryptophanyl radical in a three-helix bundle protein maquette. Here we report the detailed structural analysis of the protein by multidimensional NMR methods. An interesting feature of the structure is an apparent pi-cation interaction involving the sole tryptophan and a lysine side chain. Hybrid density functional calculations support the notion that this interaction raises the reduction potential of the W degrees /WH redox pair and helps explain the redox characteristics of the protein. This model protein system therefore provides a powerful model for exploring the structural basis for controlled radical chemistry in protein.     

The thermal decomposition of ammonium metavanadate

On heating, ammonium metavanadate (AMV) decomposes in several atmospheredependent stages. An important decomposition intermediate, ammonium hexavanadate (AHV), may also be prepared by wet-chemical methods and the kinetic parameters for the thermal decomposition of AMV and of the AHV preparation have been obtained. The kinetic study has been supplemented by surface-area measurements and by electron microscopic examination of the surfaces of reactant, intermediate and product crystallites. On the basis of the type of decomposition curve, the measured activation energies, and the effects of oxygen and water vapour on the decomposition rate, it has been concluded that in vacuum and in inert atmospheres the evolution of ammonia is the rate-determining step, while in oxidizing atmospheres evolution of water is rate determining. Comparison of the kinetic parameters with thermodynamic data for the decomposi. tion has led to suggestions as to the nature of the activated complexes involved.     

Synthesis and antimalarial effects of N,N-dialkyl-6-(substituted phenyl)-1,2,4,5-tetrazin-3-amines

The synthesis of a series of N,N-dialkyl-6-(substituted phenyl)-1,2,4,5-tetrazin-3-amines (IV) by two routes is described. The first route (Scheme I) involved the oxidative cyclization of formazans (II) to 3-bromo-6-(substituted phenyl)-1,2,4,5-tetrazines (III), followed by treatment with amines. The second (Scheme II) utilized the treatment of 3-(methylthio)-6-(substituted phenyl)-1,2,4,5-tetrazines (VII) with amines to provide the desired products. The intermediate 3-(methylthio)-6-(substituted phenyl)-1,2,4,5-tetrazines (VII) were obtained by thiobenzoylation of hydrazinecarbohydrazonothioic acid methyl ester with [[(substituted phenyl)thioxomethyl]thio]-acetic acids (V) to afford the 1,2-dihydro-3-(methylthio)-6-(substituted phenyl)-1,2,4,5-tetrazines (VI). Oxidation with bromine in acetic acid provided the desired intermediates. The target 6-(substituted phenyl)-1,2,4,5-tetrazin-3-amines (IV) displayed modest antimalarial activity.     

De novo design of a redox-active minimal rubredoxin mimic

Metal-binding sites in metalloproteins frequently occur at the interfaces of elements of secondary structure, which has enabled the retrostructural analysis of natural proteins and the de novo design of helical bundles that bind metal ion cofactors. However, the design of metalloproteins containing beta-structure is less well developed, despite the frequent occurrence of beta-conformations in natural metalloproteins. Here, we describe the design and construction of a beta-protein, RM1, that forms a stable, redox-active 4-Cys thiolate Fe(II/III) site analogous to the active site of rubredoxin. The protein folds into a beta-structure in the presence and absence of metal ions and binds Fe(II/III) to form a redox-active site that is stable to repeated cycles of oxidation and reduction, even in an aerobic environment.     

Novel siloxane polymers as polymer electrolytes for high energy density lithium batteries

A variety of disubstituted (double-comb) polysiloxane polymers have been prepared containing linear, branched, and cyclic oligoethyleneoxide units, –(OCH₂CH₂)_n–, in the side chains and as part of the siloxane backbone. Copolymers, using mixtures of linear ethylene oxide side chains, were also synthesized. These polymers were doped with LiN(SO₂CF₃)₂ (LiTFSI, 1) and conductivities of the polymer-salt complexes were determined as a function of temperature and doping level. The maximum conductivity of these polymers at 25 ° C was 2.99 ×10^–4, for a copolymer containing equimolar amounts of side chains with n = 5 and 6.     

One site fits both: A model for the ternary complex of folate + NADPH in R67 dihydrofolate reductase, a D2 symmetric enzyme

R67 dihydrofolate reductase (DHFR) is a novel enzyme that confers resistance to the antibiotic trimethoprim. The crystal structure of R67 DHFR displays a toroidal structure with a central active-site pore. This homotetrameric protein exhibits 222 symmetry, with only a few residues from each chain contributing to the active site, so related sites must be used to bind both substrate (dihydrofolate) and cofactor (NADPH) in the productive R67 DHFR?NADPH?dihydrofolate complex. Whereas the site of folate binding has been partially resolved crystallographically, an interesting question remains: how can the highly symmetrical active site also bind and orient NADPH for catalysis? To model this ternary complex, we employed DOCK and SLIDE, two methods for docking flexible ligands into proteins using quite different algorithms. The bound pteridine ring of folate (Fol I) from the crystal structure of R67 DHFR was used as the basis for docking the nicotinamide-ribose-P_i (NMN) moiety of NADPH. NMN was positioned by both DOCK and SLIDE on the opposite side of the pore from Fol I, where it interacts with Fol I at the pore's center. Numerous residues serve dual roles in binding. For example, Gln 67 from both the B and D subunits has several contacts with the pteridine ring, while the same residue from the A and C subunits has several contacts with the nicotinamide ring. The residues involved in dual roles are generally amphipathic, allowing them to make both hydrophobic and hydrophilic contacts with the ligands. The result is a `hot spot' binding surface allowing the same residues to co-optimize the binding of two ligands, and orient them for catalysis.