RESUMEN
We achieved the reconstruction of VS1-analogues containing a substitute for the fifth residue, gamma-oxo-Pip (Pip = pipecolic acid), starting from VS1-pentapeptide (VS5P;3) the latter being prepared by a two-step degradation process of the native antibiotic VS1 (1a). Protecting groups during the procedure were chosen in order to realize a minimal number of steps. Most of these gave excellent yields, including final cyclization between the fourth and fifth residue. In total, four analogues were synthesized with Ala, Asp, Asn and Lys (1b) replacing gamma-oxo-Pip. Among these, [Lys5(Tfa salt)]-VS1 is water-soluble, which is an important characteristic for eventual application of VS1 as a pharmaceutical agent. In the proposed reaction sequence, we made sure that residues 4 (MePhe) and 6 (Phg) became partially epimerised. We therefore obtained each time after cyclization a total of four epimers that have been separated by preparative TLC. The chiral identity of the final residues was realized by GC (Chirasil Val-III) on the total hydrolysates.
Asunto(s)
Virginiamicina/análogos & derivados , Secuencia de Aminoácidos , Espectroscopía de Resonancia Magnética , Métodos , Datos de Secuencia Molecular , Estructura Molecular , Estereoisomerismo , Virginiamicina/síntesis química , Virginiamicina/químicaRESUMEN
Detailed analysis of the proton and carbon-13 NMR spectra of cyclo(L-prolyl-L-N-methylphenylalanyl) in chloroform and methanol in relation to its nonmethylated analog provided information on the conformation of the title compound in solution as well as on the effect of N-methylation and solvation. The X-ray structure of the title compound in the crystalline state showed the same conformational features as the solution structure. The phenyl group folds over the diketopiperazine ring which resembles a flattened half-chair. Both amide bonds are considerably nonplanar. The pyrrolidine ring of proline shows a strong pucker at the ring junction with the largest chi 5 value hitherto observed.
Asunto(s)
Dipéptidos/química , Péptidos Cíclicos/química , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Estructura Molecular , Conformación Proteica , Soluciones , Difracción de Rayos XRESUMEN
cyclo(D-Phenylalanyl-trans-4-fluoro-D-prolyl), c(D-Phe-D-FPro), was synthesized and its conformation determined both in solution and in the solid state by 1H NMR and X-ray analysis, respectively. In the crystals the 2.5-diketopiperazine (DKP) ring assumes the uncommon conformation, for cyclodipeptides containing Pro residue, of a flattened chair, which seemingly results from a compromise between, on the one hand, the DKP-aromatic intramolecular ring-ring attraction (folding), requiring the C alpha--C beta bond of the Phe to be axial, and, on the other hand, the intrinsic tendency of the Pro residue to have its C alpha--C beta bond equatorial. Unlike the solid state, the 1H NMR data in CDCl3 and C6D6 demonstrate that in both solutions the DKP ring assumes a boat-like shape, typical for the Pro-containing cyclodipeptides, with the equatorial C alpha--C beta bonds in both amino acid residues, which preclude ring-ring folding. A similar conformation was encountered in the closest analog of c(D-Phe-D-FPro), viz, in c(Phe-Pro), both in solution (21, 22, 26) and in the solid state (12). A subtle interplay of intramolecular interactions introduced into a cyclodipeptide by a Pro-type and a Phe-type residue is emphasized.
Asunto(s)
Péptidos Cíclicos/química , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Péptidos Cíclicos/síntesis química , Conformación Proteica , Difracción de Rayos XRESUMEN
The structure and the stereochemistry of afrocurarine (a new unsymmetrical bisindole alkaloid) previously isolated from S. USAMBARENSIS Gilg. are established especially from (1)H-NMR spectroscopic data. The separation of quaternary alkaloids has always been very tedious. Ion-pair reversed-phase column chromatography applied to the purification of afrocurarine is proved very useful and seems to be a suitable technique in quaternary alkaloid research.
RESUMEN
Three quaternary alkaloids have been isolated from root barks of STRYCHNOS USAMBARENSIS Gilg from Rwanda: malindine, isomalindine and Nb-methyl-antirhine. The structure elucidation and the stereochemistry of the new isomalindine has been proposed based on its spectral data and their comparison with those of malindine, its isomer.
RESUMEN
The experimental results of 1H- and 13C-NMR studies of allohydroxy-, and of deoxyvirginiamycin S strongly confirm the conformation that was proposed earlier for the parent virginiamycin S (Anteunis, Callens and Tavernier (1975) Eur. J. Biochem. 58, 259--268). The changing nature of dipole-induced dipole interaction is responsible for the specific gradually increasing libration of the N-MePhe side chain along the series virginiamycin S, allohydroxy-, deoxyvirginiamycin S. Previous methods for the estimation of rotameric populations around the alpha, beta bonds are critically evaluated and compared to the present results obtained from interpretation of geminal 2J (beta) coupling constants.
Asunto(s)
Virginiamicina , Fenómenos Químicos , Química , Hidroxilación , Espectroscopía de Resonancia Magnética , Conformación Molecular , SolucionesRESUMEN
Although the main characteristics of the parent virginiamycin S conformation i.e. a bend of type VI (Lewis, P.N., Momamy, F.A. and Scheraga, H.A. (1973) Biochim. Biophys. Acta 303, 211--229) formed by the Pro-N-MePhe-X-PhGly sequence is still present in patricin A, the substitution of X = pipecolic acid by proline in the latter results in a destabilization of the tertiary structure of the depsipeptide, since two isomeric states of a peptidic bond appear in C2HC13 solution. Addition of +/- 30% (v:v) (C2H3)2SO totally shifts this equilibrium in favor of the major parent isomer. These results completely fit with what is known up to now on the occurrence and structure of turns (Chou and Fasman (1977) J. Mol. Biol. 115,135--175).
Asunto(s)
Virginiamicina , Modelos Químicos , Conformación Molecular , Prolina , Soluciones , Análisis EspectralRESUMEN
Racemic trans-3,4-dideutero proline has been prepared by catalytic deuteration of 3-pyrroline-2-carboxylic acid. The stereospecificity of the reduction (i.e. trans to the carboxylic group) was ascertained after transformation of the dideutero compound into the diketopiperazide c/Pro, Aib/, for which unambiguous proton assignments in the 1H n.m.r. spectrum had been obtained previously. The identification of the configuration of the dideuteroproline allows for the affirmation of the correctness of proton assignments as previously proposed in literature.
Asunto(s)
Prolina/análogos & derivados , Fenómenos Químicos , Química , Deuterio , Isomerismo , Conformación Molecular , Prolina/síntesis química , Protones , Análisis EspectralRESUMEN
The interpretation of the 300 MHz 1H-nmr spectra of septamycin (1) and its sodium salt (1+) allow one to extract most of the parameters revealing their resemblant conformations in solution. The backbone forms a pseudocyclic structure closed by head-to-tail hydrogen bonding between the carboxylate fragment and the OH-14. In 1+, the sodium ion is trapped in a central hole by coordinating around it six to seven oxygen atoms (including COO-). The external lipophilic zone of the molecule keeps the sodium ion away from the surroundings. The dangling sugar-like fragment does not participate in the metal binding. It was not possible to detect any water molecule participating in the cyclization of septamycin-free acid, nor in septamycin-Na+, as was found in an X-ray study for the p-bromophenacyl ester.
Asunto(s)
Antibacterianos , Antibacterianos/análisis , Fenómenos Químicos , Química , Éteres , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Conformación MolecularRESUMEN
It is found that methionine-enkephalin has at least two different conformations in aqueous solution, one at low and one at high pH. From inspection of titration curves and coupling constant values, it seems reasonable to conclude that these conformations are characterized by a folding so as to bring the two ends of the molecule in close proximity. This behavior parallels that found recently in (CD3)2SO as the solvent. It follows that the Phe-Met region of the molecule constitutes a relatively rigid region, but that the chain possesses flexibility around the first Gly residue. Possible implications of this behavior with respect to the receptor site are discussed.
Asunto(s)
Oligopéptidos , Concentración de Iones de Hidrógeno , Espectroscopía de Resonancia Magnética , Metionina , Conformación ProteicaRESUMEN
From the 1H-NMR study of Tyr-Gly-Gly and Phe-Gly-Gly in H2O and 2H2O as a function of pH it follows that these tripeptides display at least two and probably three conformational zones. Under slow exchange conditions of the peptidic NH-protons, coupling constants 3J(NH, CaH) may be extracted as the probe. At higher pH values shift values and 3J(a, beta) of the side chain and the titration curves are indicative for these conformational alterations.
Asunto(s)
Oligopéptidos , Glicina , Concentración de Iones de Hidrógeno , Espectroscopía de Resonancia Magnética , Fenilalanina , Conformación Proteica , TirosinaRESUMEN
The 1H (at 300 MHz) and 13C nuclear magnetic resonance spectra of virginiamycins S and S4 and vernamycin Balpha have been unravelled and analyzed. Together with model building and theoretical considerations, this allows the detailed description of their solution conformations. The depside bond can rotate and gives to the backbone some conformational mobility. The orientation of the depsicarbonyl bond depends on the surrounding. Apparent discrepancies between the different methods that are applicable for the disclosure of the nature of peptide H-bonding, have found a rational explanation.
Asunto(s)
Virginiamicina/análisis , Enlace de Hidrógeno , Espectroscopía de Resonancia Magnética , Modelos Moleculares , Conformación Molecular , Oligopéptidos/análisisRESUMEN
1. After hydrolysis with glusulase of bile from rats treated with papaverine, four metabolites (A, B, C and D) were separated. 2. The structures A, B and C were established as monodemethylated compounds, 4'-desmethyl-, 7-desmethyl-, and 6-desmethylpapaverine, respectively. 3. D was formed from papaverine by bis-desmethylation. Since it was found in cat bile after administering either A or C, but not B, it was identified as 4', 6-desmethylpapaverine.