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M Avalos

Publications and source records attributed to M Avalos.

At least 19 recordsLinked to original sources
PubMed2001

Can we predict the conformational preference of amides?

To what extent, if any, is the conformation of secondary amides revealed by theory? This question has now been addressed by computational methods using calculations at the B3LYP/6-31G level of theory and (1)H NMR spectroscopy. Both gas-phase and solvent studies predict a Z-anti conformation to be the lowest in energy for an evaluated series of acetamides. Moreover, Z-anti conformations may also be inferred from the chemical shifts of the N-CH alpha protons determined by NMR spectroscopy. Thus, a proton situated anti to the N-H proton consistently appears approximately 0.8 ppm further downfield than a proton situated gauche to the N-H proton. This finding, which could only be derived by using the DFT calculations of conformational preference as a guide to interpret the NMR data, might prove to be useful as a simple and convenient methodology for establishing amide conformation experimentally.

Journal Article↗
PubMed2001

Synergic effect of vicinal stereocenters in [3 + 2] cycloadditions of carbohydrate azadipolarophiles and mesoionic dipoles: origin of diastereofacial selectivity.

The intermolecular [3 + 2] cycloaddition of carbohydrate-derived 1,2-diaza-1,3-butadienes and 1,3-thiazolium-4-olates provides a conceptual basis for the problem of diastereofacial preference in the acyclic series of unsaturated sugars. Experimental results employing a side chain of D-arabino configuration have shown the stereodifferentiation exerted by the first stereogenic center that renders the Re,Re face of the acyclic sugar-chain azadiene eligible for cycloaddition (J. Org. Chem. 2000, 65, 5089). The results of the present work, now utilizing an alternative framework of D-lyxo configuration, evidence the discriminating power of the second stereogenic carbon, which induces the preferential approach to the Re,Si face of the heterocyclic dipole. This scheme of face selectivity is also grounded in theoretical calculations at a semiempirical level. In addition to dihydrothiophenes, which are the expected products of the [3 + 2] cycloaddition, bicyclic systems based on dihydrothieno[2,3-c]piperidine skeleton can also be obtained.

Journal Article↗
PubMed2001

Three- and four-membered rings from cycloadditions of 1,3-thiazolium-4-olates and aldehydes.

2-Aminothioisomünchnones, a well-known family of masked dipoles, react with aromatic aldehydes in a domino cascade reaction that produces episulfides (thiiranes) or beta-lactams (2-azetidinones). This sequence is initiated by a [3+2] dipolar cycloaddition followed by ring opening of cycloadducts and intramolecular rearrangement to afford these unusual ring contractions. The nature of the reaction products depends on the structural characteristics of the starting dipole and the experimental conditions. Episulfides are obtained selectively as cis isomers with respect to both aryl groups, whereas beta-lactams are produced as cis/trans mixtures. These structural features were determined unequivocally by X-ray crystallographic analysis. The beta-lactams still possessed a flexible acyclic chain containing sulfur, a salient lead modification of the bioactive cyclic penems and cephems. The preferential production of exo transition structures was rationalized with the aid of computational calculations at the B3LYP/6-31G* level.

Journal Article↗
PubMed2001

Remarkably reduced transplant-related complications by dibromomannitol non-myeloablative conditioning before allogeneic bone marrow transplantation in chronic myeloid leukemia.

A non-myeloablative conditioning protocol containing dibromomannitol (DBM/cytosine arabinoside/cyclophosphamide) has been applied to 36 chronic myeloid leukemia (CML) patients followed by bone marrow transplantation (BMT) from sibling donors. Risk factors include: accelerated phase (10 patients), older age (17 patients over >40 years) and long interval between diagnosis and BMT (27 months on average). Severe mucositis did not occur. Venoocclusive liver disease was absent. Infectious complications were rare. Although grade II-IV acute graft-versus-host disease (GVHD) was present in 9 (25%) cases, there were only 2 serious (III-IV) ones. Chronic GVHD occurred in 25 (69%) cases, preceded by acute GVHD in 9 of the 25 affected patients. Early hematological relapse, 7-29 weeks after BMT, developed in 6 patients (17.6%). No relapse was noted in the completely chimeric patients, however molecular genetic residual disease was observed in 6 patients, in most of them after transient short-term mixed chimeric state. Overall actual survival rate is 83.3% for the 36 cases, and leukemia-free survival is 72.2% for the 34 engrafted patients.

Adolescent↗
PubMed2000

Reactions of 2-amino-2-thiazolines with isocyanates and isothiocyanates. Chemical and computational studies on the regioselectivity, adduct rearrangement, and mechanistic pathways.

2-Amino-2-thiazoline derivatives bearing alkyl or aryl substituents at exocyclic nitrogen have been condensed with different isocyanates and isothiocyanates. The addition occurs at ring endocyclic nitrogen in a regiospecific manner to afford kinetic and enthalpy-favored adducts. The unequivocal assignment of these structures has been confirmed by X-ray diffraction analyses of several compounds. The endo adducts do not rearrange on heating with the sole exception of adducts in which the exocyclic nitrogen remains unsubstituted. Trapping experiments in the presence of other isocyanates or isothiocyanates produce the formation of new endo adducts by acyl exchange in the reaction mixture. Semiempirical PM3 calculations full corroborate the higher stability of endo or exo adducts depending on the substitution pattern. The formation of adducts is compatible with a stepwise reaction mechanism, for which semiempirical transition structures could be located in the potential energy surface, and the global energetics of the process have been determined. The formation of the endo adducts proceeds with a smaller activation barrier.

Crystallography, X-Ray↗
PubMed2000

The first density functional study on the

The hetero-Diels-Alder reactions of 1,2-diaza-1,3-butadiene with ethylene, methyl vinyl ether, and methyl acrylate have been investigated theoretically with the aid of density functional theory using the Becke3LYP/6-31G(d) computational level. In the reactions with substituted alkenes, the transition states are concerted but asynchronous; the shortest bond-forming distance involves the nonsubstituted carbon of the alkene. In agreement with the experimental results, the reaction with methyl vinyl ether proceeds with high endo stereoselectivity and with complete regioselectivity to form the 6-methoxy cycloadduct. The conformational study of the transition states shows a sharp s-trans preference. In contrast, the [4 + 2]-cycloaddition of 1,2-diaza-1,3-butadiene with methyl acrylate have been found to occur with low regio- and stereoselectivity but with a s-cis preference in the transition structures.

Journal Article↗
PubMed2000

Computational studies on the BF3-catalyzed cycloaddition of furan with methyl vinyl ketone: a new look at Lewis acid catalysis

Transition structures for both uncatalyzed and BF3-catalyzed Diels-Alder reactions involving furan and methyl vinyl ketone have been determined at the hybrid DFT (B3LYP/6-31G*) level of theory. The transition structures are predicted to be relatively concerted and highly asynchronous in all cases. A subsequent bond-order analysis has been carried out at the MP2/6-31G*//B3LYP/6-31G*. The role of the Lewis acid and the origin of the endo selectivity have been discussed in terms of the nature and number of interactions present in the four possible transition structures. The partition of the potential energy barrier has also been used to estimate the contributions of the pure deformation energy and the differential interaction between the reaction partners on passing from the ground state to the saddle point. This analysis reveals that the major influence arises from the heterodiene-dienophile interaction instead of that corresponding to a BF3-dienophile interaction.

Journal Article↗
PubMed2000

Diastereoselective cycloadditions of 1,3-thiazolium-4-olates with chiral 1,2-diaza-1,3-butadienes

A series of 2-(N-methyl)benzylamino-1,3-thiazolium-4-olates (2-aminothioisomunchnones) react with chiral 1,2-diaza-1,3-butadienes derived from carbohydrates to afford a diastereomeric mixture of (4R,5S)- and (4R,5R)-4,5-dihydrothiophenes. These substrate-controlled cycloadditions are chemoselective, regiospecific, and proceed with a high facial diastereoselection. A theoretical rationale at semiempirical level does justify the stereochemical outcome observed in the experiments.

Journal Article↗
PubMed2000

Understanding diastereofacial selection in carbohydrate-based domino cycloadditions: semiempirical and DFT calculations.

The sequential cycloaddition of nitroalkenes with methyl vinyl ether was investigated by semiempirical (PM3) and density functional methods (B3LYP/6-31G*). The asymmetric version was also examined with a threoconfigured carbohydrate auxiliary. This produces a larger, more flexible system that complicates the calculation. Most transition structures were then fully optimized at the PM3 level and further refinement was done at ab initio levels. This study represents a model case that enables the rationalization of the high facial selectivity observed in carbohydrate-based nitrone- and nitronate-alkene cycloadditions. The selective endo orientation of the [4+2] pathway results from Coulombic attraction and secondary orbital interactions in the transition state. The stereochemical outcome is largely influenced by a combination of steric shielding from the bulky chiral substituent at C4 and the anomeric effect that places the nitronate C6-alkoxy group in a pseudoaxial arrangement. The resulting conformation favors the subsequent exo approach of methyl vinyl ether to the less hindered re face of the nitronate. It is also remarkable to note that solvation energies stabilize significantly a particular transition structure, thereby explaining the marked stereoselection observed in a polar medium.

Carbohydrates↗
PubMed2000

Immunological importance of chimerism in transplantation: new conditioning protocol in BMT and the development of chimeric state.

Chimerism is an exceptional immunogenetic state, characterized by the survival and collaboration of cell populations originated from two different individuals. The prerequisits to induce chimerism are immuno-suppression, myeloablation, or severe immunodeficiency of the recipients on the one side and donor originated immuno-hematopoietic cells in the graft on the other. The pathologic or special immunogenetic conditions to establish chimerism are combined with bone marrow transplantation, transfusion, and various kinds of solid organ grafting. Different types of chimerism are known including complete, mixed and mosaic, or split chimerism. There are various methods used to detect the type of chimera state, depending on the immunogenetic differences between the donor and recipient. The induction of complete or mixed chimerism is first determinated by the effect of myeloablative therapy. The chimera state seems to be one of the leading factors to influence the course of the post-transplant period, the frequency and severity of GVHD, and the rate of relapse. However, the most important contribution of the chimeric state is in development of graft versus leukemia effect. A new conditioning protocol (DBM/Ara-C/Cy) for allogeneic BMT in CML patients and its consequence on chimera state and GVL effect is demonstrated.

Antineoplastic Agents, Alkylating↗
PubMed1998

Glycine-enhanced inhibition of rat liver nucleotide pyrophosphatase/phosphodiesterase-I by EDTA: a full account of the reported inhibition by commercial preparations of acidic fibroblast growth factor (FGF-1).

The earlier reported inhibition of rat liver nucleotide pyrophosphatase/phosphodiesterase I (EC 3.1.6.9/EC 3.1.4.1; NPP/PDE) by culture-grade acidic fibroblast growth factor (FGF-1) correlates with a low-Mr contaminant. 1H-NMR analyses revealed EDTA in the total-volume fractions of a gel-filtration experiment, where all the inhibitory activity of the FGF-1 preparation was recovered. NPP/PDE inhibition by EDTA (and by unfractionated FGF-1 or the EDTA-containing fractions) was time-dependent, blocked by the substrate p-nitrophenyl-dTMP, and strongly enhanced by glycine. The use of glycine buffers in earlier work was critical to the apparent inhibition by FGF-1. The results point to a conformational change favored by glycine that may be relevant to the biological role of NPP/PDE.

Animals↗
PubMed1997

Identification of rat liver glucose-3-phosphatase as an inositol monophosphatase inhibited by lithium.

Glucose-3-phosphatase (Glc3Pase) from rat liver has been purified 780-fold with a 4% recovery. The substrate specificity of the purified enzyme agreed with that of inositol monophosphatase (EC 3.1.3.25). D-Glucose 3-phosphate (D-Glc(3)P; K(m) = 200 microM) was hydrolyzed with an efficiency similar to DL-myo-inositol 1-monophosphate (DL-Ins(1)P; K(m) = 80 microM), since the ratio V(max)/K(m) was similar for both substrates. Purification data, coelution of activities, thermal inactivation curves, optimal pH, bivalent cation requirements, inhibition by Li+, molecular weight, and isoelectric pH comparisons supported that the hydrolysis of D-Glc(3)P and DL-Ins(1)P was catalyzed by a unique phosphohydrolase identified as a hepatic form of the lithium-sensitive inositol monophosphatase. That the hydrolysis of D-Glc(3)P is a genuine feature of inositol monophosphatases was confirmed because the enzyme purified from bovine brain showed also Glc3Pase activity, and inspection of published 3D models of inositol monophosphatase complexes with D(L)-Ins(1)P or D(L)-Ins(4)P indicated that beta(alpha)-D-Glc(3)P in a pyranose conformation with all (but one) the hydroxy groups in equatorial orientation would fit in the active site as other good substrates do. The results of this work are suggestive of possible relationships between inositol and sugar 3-phosphate metabolism.

Animals↗
PubMed1997

Isolation of new nonconditional Saccharomyces cerevisiae mutants defective in asparagine-linked glycosylation.

We describe the isolation and partial characterization of Saccharomyces cerevisiae nonconditional mutants that show defects in N-glycosylation of proteins. The selection method is based on the reduction of affinity for the ion exchanger QAE-Sephadex as a consequence of the decrease in the negative charge of the cell surface. This characteristic reflects a decrease in the incorporation of mannosylphosphate units into the N-linked oligosaccharides of the mannoproteins. The mutants exhibit low affinity for the basic dye alcian blue and for that reason we have called them Idb (low dye binding) mutants. Eight of the complementation groups seem to be new as shown by complementation studies with previously isolated mutants of similar phenotype. Four of the groups showed a significant reduction in the number and/or size of the N-linked oligosaccharides attached to secreted invertase. We have analyzed the N-linked oligosaccharides of Idb1 and Idb2, the mutants that show the most drastic reduction in the affinity for the alcian blue dye. In both cases, the purified endo H-released oligosaccharides from the mannoproteins lacked detectable amounts of phosphate groups as shown by ion exchange chromatography and the 1H NMR spectra. In addition, Ibd1 synthesizes a truncated and unbranched outer chain lacking any alpha (1,2) linked mannoses attached to the alpha (1,6) linear backbone.

Alcian Blue↗
PubMed1996

Improved models for pharmacological null experiments: calculation of drug efficacy at recombinant D1A dopamine receptors stably expressed in clonal cell lines.

Modern drug discovery demands accurate knowledge of the drug properties of affinity and efficacy at specific receptor proteins. Furthermore, drugs with well defined properties make better tools with which to explore and understand receptor regulation. The use of clonal cell lines stably expressing a given recombinant receptor may provide a highly useful model in which drug effects may be studied on one receptor subtype at a time. The present report was designed to evaluate the utility of a general method in which a clonal cell line stably expressing a recombinant D1A dopamine receptor was used as a model system for studying drug actions by null models. The null model for receptor occlusion (to calculate agonist Ka) and the null model for relative efficacy (to calculate test agonist affinity and epsilon r) were evaluated in these studies. To initiate these studies, rat C6 glioma cells that do not normally express DA receptors have been modified by stable transfection with the primate D1A DA receptor [Machida et al., 1992 (Molec. Pharmacol. 41: 652-659)] to a density of approximately equal to fmol/mg protein. The recombinant receptors show robust stimulation of cAMP in the stably transfected C6 cells. Calculation of agonist Ka from dose-response data requires that a portion of the cell's receptors be occluded in the absence of changes in post-receptor events leading to the response. Receptor reserve is typically reduced by alkylation, thereby lowering maximal response. Unfortunately, most of the currently available alkylating agents are not selective either for a particular receptor or for receptors vs other proteins within a signaling pathway. Short-term agonist treatment offers a possible complement to the use of non-selective or poorly characterized alkylating drugs for reducing maximum response in appropriate cell systems. The null method of receptor occlusion was used to determine the Ka for dopamine when maximum response was decreased by alkylation vs short-term agonist treatment. Direct non-linear curve fitting was used to analyze the data. In addition to DA, two other compounds were used to reduce receptor reserve to validate the method: fenoldopam (relatively high efficacy) and SKF38393 (low efficacy). Analyses indicated that the affinity of DA was similar whether calculated by alkylation (1.1 +/- 0.58 microM), 75 min DA treatment (0.57 +/- 0.16 microM) or 45 min treatment with DA (0.86 +/- 0.11 microM). Short-term agonist treatment experiments using multiple concentrations of DA, fenoldopam, or SKF38393 to decrease receptor reserve provided additional support for the validity of the Ka determinations using this procedure. Other experiments were conducted according to the null model for relative efficacy in which the affinity for DA is calculated by comparing the DA response before and after receptor occlusion, and the affinity and relative intrinsic efficacy of the test agonist are determined as a function of its actions relative to DA. We used the following four test drugs: + Br-APB, a novel agent with potential dopamine agonist properties, and three high-affinity DA agonists, fenoldopam, R-(-)-apomorphine (APO), and SKF38393. Intrinsic efficacy values relative to that of DA (1.0) were as follows: fenoldopam, 0.46 +/- 0.11; APO, 0.19 +/- 0.13; SKF38393, 0.07 +/- 0.01; and +Br-APB, 0.26 +/- 0.40. The agonist affinities (Ka) were: fenoldopam, 0.018 +/- 0.008 microM; APO, 0.80 +/- 0.18 microM; SKF38393, 0.16 +/- 0.04 microM; BR-APB, 0.43 +/- 0.29 microM; and DA, 0.58 +/- 0.17 microM. EC50/Ka ratios were consistent with relative intrinsic efficacies and Ka values were similar to KL values reported for membrane binding studies. Finally, Monte Carlo simulations were conducted to determine the precision of the parameter estimates...

Animals↗
PubMed1993

Alcohol esterification reactions and mechanisms of snake venom 5'-nucleotide phosphodiesterase.

In a previous study we have shown that snake venom 5'-nucleotide phosphodiesterase (SVP) catalyzes methanol-esterification reactions [García-Díaz, M., Avalos, M. & Cameselle, J. C. (1991) Eur. J. Biochem. 196, 451-457]. Now we have demonstrated that SVP catalyzes AMP transfer from ATP to propanol, ethanol, methanol, ethylene glycol, glycerol, 2-chloroethanol or 2,2-dichloroethanol. The AMP-O-alkyl ester products were identified by HPLC, enzyme analysis, ultraviolet and NMR spectroscopy. Those results show the potential of SVP as a tool to prepare 5'-nucleotide esters and agree with the formation of a covalent 5'-nucleotidyl-SVP intermediate susceptible to nucleophilic attack by short-chain (poly)alcohols as acceptors alternative to water. To test the kinetic influence of the solvent nucleophile in SVP mechanisms, initial rates of ATP solvolysis were assayed in different water/alcohol mixtures. Relatively high alcohol concentrations inactivated SVP but lower concentrations gave proportional rates of alcoholysis. An efficiency parameter (EA), defined as the ratio of the mole fraction of AMP-O-alkyl ester as a product to that of alcohol as an acceptor in water/alcohol mixtures, made possible the comparison of alcohols and water as AMP acceptors at low concentrations, as it could be reasoned that EA = 1 for water. Rates of hydrolysis (VH) of substrates yielding AMP and different leaving groups were also assayed. The higher EA and VH values corresponded, respectively, to those acceptors and leaving-group conjugate acids with lower pKa and higher polar-substituent constants (sigma*). The results support the occurrence of general acid-base catalysis in the active center of SVP and the identification of rate-limiting steps. A model is proposed for the mechanisms of SVP-catalyzed hydrolysis and alcoholysis which accounts for the influence of the acid-base properties of alcohols on the kinetic profile of SVP reaction sequences.

Adenosine Triphosphate↗
PubMed1991

Methanol esterification reactions catalyzed by snake venom and bovine intestinal 5'-nucleotide phosphodiesterases. Formation of nucleoside 5'-monophosphate methyl esters from guanosine 5'-triphosphate and other nucleoside 5'-polyphosphates.

It is not known whether the enzymes 5'-nucleotide phosphodiesterase/nucleotide pyrophosphatase (EC 3.1.4.1/EC 3.6.1.9) catalyze the transfer of nucleotides to acceptors other than water. We have investigated the action of snake venom and bovine intestinal mucosa phosphodiesterases on nucleoside 5'-polyphosphates in the presence of methanol. In those conditions, GTP was converted by snake venom phosphodiesterase to a mixture of GMP and another compound with a different retention time in reverse-phase high-performance liquid chromatography. That compound, by ultraviolet, 1H- and 13C-nuclear magnetic resonance spectroscopic analysis, and by enzyme analysis, was characterized as the methyl ester of GMP (GMP-OMe). The molar fraction [GMP-OMe]/[GMP + GMP-OMe] formed was higher than the molar fraction of methanol as a solvent in reaction mixtures. Similar reactions took place at comparable rates with snake venom and bovine intestinal mucosa phosphodiesterases using several nucleoside 5'-polyphosphates as substrates. The ability of 5'-nucleotide phosphodiesterases to catalyze transfer reactions to a non-water acceptor is relevant to the mechanism of the enzymes, to their use as analytical tools, and to their possible use/role in the preparative/in vivo synthesis of nucleotide esters.

Animals↗
PubMed1989

Low-dose arabinosyl cytosine therapy of AML may be effective after failure of high or conventional doses.

In 3 patients with ANLL, chemotherapy including high or conventional doses of Ara-C had been ineffective. However subsequent treatment with low-dose (LD) Ara-C induced a PR or CR which were sustained with regular LD Ara-C reinduction courses. Survival of the 2 patients with PR was 24 and 8 months respectively. The patient with CR was alive 18 months after a bone marrow graft. In 2/3 cases, the cytogenetic abnormalities initially detected disappeared with treatment. These observations suggest that the cellular mechanism of action of LD Ara-C is different from that of HD Ara-C, but do not favour a differentiating action of LD Ara-C. LD Ara-C can be prescribed as a salvage therapy even after failure of high or conventional doses.

Adult↗