Carlos Rivas

Extrusion of selenium and tellurium atoms from selenophene and tellurophene by reaction with trinuclear iron, ruthenium, and osmium clusters: crystal structures of [Os6(μ-H)(μ3-Se)(μ4-C4H3)(CO)20] and of [Ru4(μ3-Se)μ-C4H4)(CO)11]

Abstract The trinuclear carbobyl clusters [Fe 3 (CO) 12 ], [Ru 3 (CO) 12 ], [Os 3 (CO) 11 (MeCN)], and [Os 3 (CO) 10 (MeCN) 2 ] react with selenophene and tellurophene ( cyclo -C 4 H 4 X, X = Se or Te) under mild conditions to give compounds containing the open-chain ligands CHCHCHCHX or the fragments X, C 4 H 4 , C 4 H 3 , or H as bridging ligands. The following compounds were isolated and characterised: [Os 3 (CO) 10 (C 4 H 4 X)], 1 where X = Se (X-ray structure reported previously) and 8 where X = Te, [Os 6 H(Se)(C 4 H 3 )(CO) 20 ], 2 , [Os 2 (CO) 6 (C 4 H 4 Se)], 3 , [Ru 2 (CO) 6 (C 4 H 4 )], 4 , [Ru 4 (CO) 6 (C 4 H 4 )], 4 , [Ru 4 (Se)(CO) 11 (C 4 H 4 )], 5 , [Fe 2 (CO) 6 (C 4 H 4 )], 6 , and [Fe 2 (CO) 6 (C 4 H 4 Se)], 7 . The clusters 2 and 5 were shown by single-crystal X-ray diffraction methods to have had both SeC bonds broken to give μ 3 -Se ligands in each case. Compound 2 contains an interesting μ 4 -C 4 H 3 ligand linking two Os 3 units through a μ-alkylidyne bridge, a σ-OsC bond, and an η 3 -allyl component. Compound 5 has a μ-C 4 H 4 ligand of a type previously found to be formed by alkyne coupling. The other compounds were characterised spectroscopically.

Chemical and spectroscopic studies of Cercidium praecox gum exudate.

The structure of the polysaccharide from Cercidium praecox (R&P) Harms gum exudate has been studied by Smith degradation, by sugar and methylation analyses, and by 13C NMR spectroscopy. The results showed a (1-->4)-xylan core. Some xylose residues are substituted at O-2 by alpha-D-glucuronic acid and 4-O-methyl-alpha-D-glucuronic acid residues. beta-D-Glucuronic acid is present, probably as terminal residues. The arabinose is present as alpha-L-furanose and beta-L-pyranose.

Comparison of three anacardiaceae gum exudates

The gum exudates from Anacardium occidentale, Spondias purpurea and Spondias mombin are very soluble in water. Analytical data for a Venezuelan gum sample of A. occidentale showed that the intrinsic viscosity limit is lower than those reported for the Papuan and Indian gum exudates. On the other hand, the galactose/arabinose ratio (1:6) appears lower than that reported (4:3). The gums studied contained galactose, arabinose, mannose, xylose and rhamnose. Xylose was not evidenced in S. mombin. The cationic composition of the ash of Anacardiaceae gums studied have high calcium, potassium, sodium and magnesium contents. 13C-NMR spectral studies are according with the chemical results. The resonances attributed to galactose and uronic acids are inequivocally assigned. There are evidences of arabinofuranose residues as structural features of the Spondias gums.

Carbon–hydrogen cleavage versus ring opening in the oxidative addition reactions of furan, thiophene, selenophene and tellurophene with [Os3(CO)10(MeCN)2]

Reactions of the cluster [Os3(CO)10(MeCN)2] with furan, thiophene, selenophene or tellurophene lead to compounds of corresponding stoichiometry, [Os3(CO)10(C4H4E)](E = O, S, Se or Te), but, whereas for E = O or S these are furyl or thienyl hydrido compounds formed by C–H cleavage, the Se and Te clusters are formed by E–C cleavage (X-ray structure for the E = Se cluster).

Chemical and spectroscopic studies of the gum polysaccharide from Acacia macracantha

Abstract The polysaccharide isolated from Acacia macracantha gum, a Venezuelan Gummiferae spp., contains galactose, arabinose, rhamnose, glucuronic acid and its 4-O-methyl analogue. 13C-NMR spectroscopy, in combination with chemical methods, was applied in this study. A complex fragment, constituted of galactose, arabinose and glucuronic acid (3:2:1), was obtained from the original polysaccharide and its degradation products. This fragment appears to be an important structural feature of the gum. The backbone of the structure (degraded gum B) is a β-(1→3) galactan, although glucuronic acid and arabinose residues are still present in the core of the structure. Stereochemistry of the molecule may work against periodate oxidation of these sugar residues. Arabinose may be present as internal residues and as short side-chains while glucuronic acid exists mainly as a terminal residue.

Composition of Cercidium praecox gum exudates

Abstract Venezuelan gum exudates from nine specimens of Cercidium praecox (Leguminosae) have been examined. The samples, very soluble in water and laevorotatory, have viscosity comparable to that of “arabic gum” ( Acacia senegal ). Sugar composition shows the presence of xylose, arabinose and glucuronic acid and its 4- O -methyl analogue. The most interesting feature of the analyses reported concerns the absence of galactose.

Composition of Acacia macracantha gum exudates

Abstract Venezuelan gum exudates from eight specimens of Acacia macracantha were examined. Minor variations in the characteristic features of the gum specimens studied were recorded. The constituent sugars found were galactose, arabinose, glucuronic acid, 4- O -methylglucuronic acid and rhamnose. The negative specific rotation and the ratio Gal/Ara > 1 are atypical features for an Acacia species assigned to the series Gummiferae.

The polysaccharide gum from Accia tortuosa

Abstract The polysaccharide gum obtained from Venezuelan Acacia tortuosa contains galactose, arabinose, xylose as traces, glucuronic acid and its 4- O -methyl analogue. 13 C NMR spectroscopy, in combination with chemical methods, was applied in this study. The preparation of degraded products, via hydrolytic- and Smith-degradations, indicated interesting structural features of the polysaccharide. The core of the structure is essentially a branched β-(1 → 3) galactan. Arabinose, glucuronic acid and its 4- O -methyl analogue could not be totally removed from the core. Arabinose (as furanose and pyranose residues) exist as terminal and 3- O -linked, while xylose, glucuronic acid and its 4- O -methyl derivative are probably terminal residues. The high content of proteinaceous material in A. tortuosa gum must be investigated in order to determine whether there are many polysaccharide-protein covalent linkages.

Influence of charge and metal coordination of meso-substituted porphyrins on bacterial photoinactivation.

The photodynamic effect of meso-substituted porphyrins with different charges and metal ions: meso-tetraphenylporphyrin tetrasulfonate 1, its nickel 2 and zinc complexes 3; meso-tetranaphthylporphyrin tetrasulfonate 4, and its zinc complex Zn 5; and tetra piridyl ethylacetate porphirins 6 and their nickel 7 and zinc 8 complexes, were synthesized and studied their antimicrobial activity against Escherichia coli. Fluorescence quantum yields (ΦF) were measured in water using reference TPPS4, obtaining higher values for complexes 3 and 4. The singlet oxygen ΦΔ were measured using histidine as trapping singlet oxygen and Rose Bengal as a reference standard. Complexes 1, 2 and 6 have the highest quantum yields of singlet oxygen formation, showing no relation with the peripheral charges and efficiency as Type II photosensitizers. Meanwhile complexes 3, 8 and 4 were the most efficient in producing radical species, determined with their reaction with NADH. The photoinduced antibacterial activity of complex was investigated at different concentrations of the photosensitizers with an irradiation time of 30 min. The higher antibacterial activities were obtained for the complexes 1-3 that are those with greater production of ROS and minor structural deformations. Complexes 7 and 8 had moderate activity, while 4-6 a low activity. Thus, in this work demonstrates that the production of ROS and structural deformations due to peripheral substituents and metal coordination, influence the activity of the complexes studied. Therefore, is important to perform comprehensive study physics and structurally when predicting or explain such activity.

Synthesis, Photochemical and Photoinduced Antibacterial Activity Studies of meso-Tetra(pyren-1-yl)porphyrin and its Ni, Cu and Zn Complexes

The synthesis of the meso-tetra(pyren-1-yl)porphyrin (1) was successfully accomplished by means of the pyrrole condensation with pyrene-1-carb-aldehyde in acidic media. Its metallization was carried out in an almost quantitative yield to obtain the corresponding complexes of Ni(II) (2), Cu(II) (3) and Zn (4). Their photophysical properties such as fluorescence quantum yield and energy transfer to oxygen for an efficient generation of singlet oxygen were determined. Their photophysical and photochemical properties were compared with those of other similar porphyrin derivatives such as tetraphenylporphyrin and tetranaphthylporphyrin. Photochemical studies on their effectiveness as photosensitizer were carried out by means of the photoinduced oxidation of aromatic alcohols like α-naphthol to naphthoquinone. The antibacterial photoactivity assay for compounds 1–4 was testeted against Escherichia coli (ATCC 8739) and its proliferation and viability were measured by chemiluminescence. An efficient inactivation of E. coli was observed. This was more efficient for compounds 2 and 3, following the direct relationship to high generation of singlet oxygen by these compounds.