Effect of sodium poly(styrene sulfonate) on thermoreversible gelation of gelatin

The effect of an added polyanion, sodium poly(styrene sulfonate) (NaPSS), on the thermoreversible gelation and remelting of gelatin gels has been investigated by polarimetry and rheology. The presence of NaPSS can either enhance or reduce collagenlike helix formation, depending on the polymer concentration relative to that of gelatin and the gelation temperature. At temperatures < 20°C, the helical content is reduced by increasing the amount of added NaPSS, demonstrating the disruption of helical structure of gelatin by the polyanion. Synchronous measurements of optical rotation and modulus at 25°C, in both gelation and remelting, indicate that the optical rotation at the gel point for the pure gelatin is lowered on addition of NaPSS. At low frequency, the storage modulus of gelatin is increased by the addition of a small amount of NaPSS relative to that of gelatin, but decreased with excess NaPSS. The mechanical properties of gelatin with and without NaPSS will be discussed in light of the competition between network junction formation by strands of triple helices among gelatin chains and temporary ionic crosslinking between gelatin and the polyanion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2287–2295, 1999     

The Antimalarial Natural Product Symplostatin 4 Is a Nanomolar Inhibitor of the Food Vacuole Falcipains

1. Download : Download high-res image (486KB)
2. Download : Download full-size image

Highlights? A convergent chemical synthesis of symplostatin 4 (Sym4) and Sym4 derivatives is reported ? Sym4 irreversibly inhibits malarial falcipains in infected red blood cells ? Sym4's unusual methyl-methoxypyrrolinone group is required for nanomolar inhibition of falcipains     

Synthesis and coordination chemistry of tri-substituted benzamidrazones

A series of N(1),N(1),N(3)-tri-substituted benzamidrazones of the general formula [PhC(NHR)=NNMe(2)] (R = Me, n-Pr, i-Pr, n-Bu, Bn, Ph; 1a-f) was synthesized via condensation of 1,1-dimethylhydrazine with the corresponding imidoyl chloride, [PhC(Cl)=NR]. Multinuclear NMR data, and zero-point energy DFT calculations conducted with the B3LYP functional and 6-31G+(d,p) basis set, suggest that these compounds exist as a single tautomer in solution; possessing a weak intramolecular hydrogen bond and a structure dominated by the localised resonance structure ArC(NHR)=N-NMe(2). An X-ray crystallographic study upon PhC(NHPh)=NNMe(2) (1f) demonstrated that this compound adopts an identical tautomer in the solid state. Reactions of [PhC(NHMe)=NNMe(2)] (1a) with [LMCl(2)](2) (M = Ru, L = cymene; M = Rh, Ir, L = Cp*) results in the stoichiometric formation of products of the formula [LM{PhC(=NMe)NHNMe(2)}Cl](+)Cl(-) (2a-c) in which the amidrazone chelates the metal in a κ(2)-N(1),N(3)-coordination mode. Formation of this five-membered chelate occurs with a concomitant tautomerisation of the amidrazone ligand to an alternative tautomer, i.e. [PhC(=NMe)NHNMe(2)], the latter tautomer is expected to be readily energetically accessible based upon the aforementioned DFT calculations. This series of salts may be deprotonated with lithium hexamethyldisilazide to form the corresponding charge neutral complexes [LM{PhC(NMe)=NNMe(2)}] (3a-c). In contrast, the reaction of N(1),N(1),N(3)-tri-substituted benzamidrazones with [(cymene)RuCl(2)](2) in the presence of NaOAc yielded a mixture of cyclometallation (C-H activation) and amidrazone chelation/deprotonation (N-H activation) products. Reaction of 1a yielded an inseparable mixture of products, whilst the reaction of 1c resulted in formation of the cyclometallated product [LM{C(6)H(5)C(=N(i)Pr)NHNMe(2)}] (L = cymene, M = Ru; 4a) in a modest 62% yield. This latter complex could be isolated as a crystalline orange solid, full characterisation including single crystal X-ray diffraction demonstrated that the amidrazone coordinates in a κ(2)-N(2),C-coordination mode.     

Dialkylaluminum N,O-dimethylhydroxylamine complex as a reagent to mask reactive carbonyl groups in situ from nucleophiles

Aluminum complexes of N,O-dimethylhydroxylamine are effective reagents to mask carbonyl groups in situ from nucleophilic addition by organolithiums, Grignard reagents, and borohydrides. The utility of this process by selectively adding nucleophiles into carbonyl groups on a variety of structures as well as distinguishing between carbonyl groups on a sensitive natural product is demonstrated. (1)H NMR analysis supports the in situ masking of the more reactive carbonyl group.     

Dinickel catalyzed carbonylation reactions using metal carbonyl reagents as CO sources

Dinickel complexes promote stoichiometric Pauson–Khand reactions of enynes and CO (g). However, catalytic turnover is precluded by the strong binding of CO to the Ni₂ active site. This article describes the use of M(CO)_x reagents as stoichiometric CO precursors in Ni₂-catalyzed carbonylation reactions. As part of these studies, well-defined heterotrinuclear Ni₂Fe, Ni₂Co, and Ni₂Mn carbonyl clusters are synthesized using the [NDI]Ni₂ platform as a template (NDI = naphthyridine–diimine).     

Modulation of humoral immune response to oral BCG vaccination by Mycobacterium bovis BCG Moreau Rio de Janeiro (RDJ) in healthy adults

Background  

Oral administration of BCG was the route initially used by Calmette and Guérin, but was replaced by intradermal administration in virtually all countries after the Lubeck accident. However, Brazil continued to administer oral BCG Moreau RDJ, which was maintained until the mid-1970s when it was substituted by the intradermal route. Although BCG vaccination has been used in humans since 1921, little is known of the induced immune response. The aim of this study was to analyse immunological responses after oral vaccination with M. bovis BCG Moreau RDJ.     

Amphiphilic maleic acid-containing alternating copolymers—1. Dissociation behavior and compositions

The dissociation of the two adjacent carboxylic acids in maleic acid-containing copolymers is expected to differ from those of poly(acrylic acid) and poly(methacrylic acid) where the acids are separated by two carbons on the backbone. In this work, we have employed potentiometric titration and NMR spectroscopy to characterize the dissociation behavior and chemical compositions of several water-soluble maleic acid-containing copolymers. A distinct two-step process corresponding to the dissociation of the two adjacent carboxylic acids is observed in aqueous CaCl₂ (0.02 N) solution for copolymers of maleic acid and isobutylene, diisobutylene, and styrene. Such a two-step ionization process is less recognizable, however, for the copolymers of maleic acid and linear alkenes ranging from n-hexene to n-octadecene. Nevertheless, the compositions of all copolymers, including the extent of neutralization and the ratio of the comonomer moieties, are estimated from the titration curves. The chemical composition derived from potentiometry and NMR spectroscopy for all copolymers are approximately 1 : 1 (maleic acid : comonomer). With the exception of the hydrophobically modified copolymer of isobutylene-maleic acid, no obvious conformational transition was observed over the whole range of ionization for these hydrophobic maleic acid-containing copolymers. This is related to the aggregated state of these copolymers in aqueous media. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3571–3583, 2004