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J C Martins

Publications and source records attributed to J C Martins.

At least 19 recordsLinked to original sources
PubMed2001

Complexation of triorganotin derivatives of [18]crown-6- and [15]crown-5-(benzo-4-carboxylate) with alkali thiocyanates.

Investigations of the simultaneous complexation of tri-n-butyltin and triphenyltin derivatives of [18]crown-6- or [15]crown-5-(benzo-4-carboxylate) by the anion and cation from NaSCN or KSCN are reported. The crystal structure of [Na+ is included in [15]crown-5-C6H3-4-COOSn(C6H5)3NCS]-], 4 NaSCN, displays an unusual zwitterionic nature with one intramolecular charge separation characterized by an Na-Sn distance of 9.29(1) A, and several intermolecular charge separations, the shortest being 5.48(1) A. Similar distances (9.70(2), 9.28(2), intramolecular; 5.40(2) and 5.41(2) A, shortest intermolecular) are observed in the more complicated structure of the corresponding [18]crown-6-(benzo-4-carboxylate) derivative, 3 NaSCN, with two independent molecules in the asymmetric unit. For the tri-n-butyltin analogues 1 and 2, complex equilibria were observed in acetone and unraveled by variable temperature 13C, 117Sn. and 23Na NMR studies. Their complexes with KSCN and NaSCN undergo decomposition in acetone solution, as evidenced by the transformation of [K+ is included in [18]crown-6-[C6H3-4-COOSn(nBu)3NCS]-], into tri-n-butyltin isothiocyanate and a novel dimeric potassium [18]crown-6-(benzo-4-carboxylate), the structure of which was elucidated by X-ray diffraction analysis.

Journal Article↗
PubMed2001

Arsenate reductase from S. aureus plasmid pI258 is a phosphatase drafted for redox duty.

Arsenate reductase (ArsC) from Staphylococcus aureus plasmid pI258 plays a role in bacterial heavy metal resistance and catalyzes the reduction of arsenate to arsenite. The structures of the oxidized and reduced forms of ArsC were solved. ArsC has the PTPase I fold typical for low molecular weight tyrosine phosphatases (LMW PTPases). Remarkably, kinetic experiments show that pI258 ArsC also catalyzes the tyrosine phosphatase reaction in addition to arsenate reduction. These results provide evidence that ArsC from pI258 evolved from LMW PTPase by the grafting of a redox function onto a pre-existing catalytic site and that its evolutionary origin is different from those of arsenate reductases from Escherichia coli plasmid R773 and from Saccharomyces cerevisiae. The mechanism proposed here for the catalysis of arsenate reduction by pI258 ArsC involves a nucleophilic attack by Cys 10 on arsenate, the formation of a covalent intermediate and the transport of oxidative equivalents by a disulfide cascade. The reaction is associated with major structural changes in the ArsC.

Adenosine Triphosphatases↗
PubMed2001

Solution structure of the main alpha-amylase inhibitor from amaranth seeds.

The most abundant alpha-amylase inhibitor (AAI) present in the seeds of Amaranthus hypochondriacus, a variety of the Mexican crop plant amaranth, is the smallest polypeptide (32 residues) known to inhibit alpha-amylase activity of insect larvae while leaving that of mammals unaffected. In solution, 1H NMR reveals that AAI isolated from amaranth seeds adopts a major trans (70%) and minor cis (30%) conformation, resulting from slow cis-trans isomerization of the Val15-Pro16 peptide bond. Both solution structures have been determined using 2D 1H-NMR spectroscopy and XPLOR followed by restrained energy refinement in the consistent-valence force field. For the major isomer, a total of 563 distance restraints, including 55 medium-range and 173 long-range ones, were available from the NOESY spectra. This rather large number of constraints from a protein of such a small size results from a compact fold, imposed through three disulfide bridges arranged in a cysteine-knot motif. The structure of the minor cis isomer has also been determined using a smaller constraint set. It reveals a different backbone conformation in the Pro10-Pro20 segment, while preserving the overall global fold. The energy-refined ensemble of the major isomer, consisting of 20 low-energy conformers with an average backbone rmsd of 0.29 +/- 0.19 A and no violations larger than 0.4 A, represents a considerable improvement in precision over a previously reported and independently performed calculation on AAI obtained through solid-phase synthesis, which was determined with only half the number of medium-range and long-range restraints reported here, and featured the trans isomer only. The resulting differences in ensemble precision have been quantified locally and globally, indicating that, for regions of the backbone and a good fraction of the side chains, the conformation is better defined in the new solution structure. Structural comparison of the solution structure with the X-ray structure of the inhibitor when bound to its alpha-amylase target in Tenebrio molitor shows that the backbone conformation is only slightly adjusted on complexation, while that of the side chains involved in protein-protein contacts is similar to those present in solution. Therefore, the overall conformation of AAI appears to be predisposed to binding to its target alpha-amylase, confirming the view that it acts as a lid on top of the alpha-amylase active site.

Amaranth Dye↗
PubMed2000

Convenient synthesis of 3,4-methano-1,2,3, 4-tetrahydroisoquinoline-3-carboxylic acid and its derivatives as doubly constrained nonproteinogenic amino acid derivatives.

Three strategies for the synthesis of the novel, doubly constrained, 3,4-methano-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid and its derivatives were evaluated. Only cyclocondensation of the mono(triphenyl)phosphonium salt derived from 1, 2-bis(bromomethyl)benzene with N-alkoxycarbonyloxamates in 1, 2-dimethoxyethane in the presence of potassium carbonate and subsequent cyclopropanation with dimethylsulfoxonium methylide in dimethyl sulfoxide furnished suitable O- and N-protected derivatives of 3,4-methano-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid in a convenient way. A detailed 2D DQF-COSY and 2D NOESY NMR analysis of the rotational isomerism of the latter bicyclic amino acid derivatives was performed. Various O- and N-protection protocols were worked out to afford access to a whole range of new derivatives of the title amino acid, suitable for peptide synthesis.

Amino Acids↗
PubMed1999

A gene encoding a hevein-like protein from elderberry fruits is homologous to PR-4 and class V chitinase genes.

We isolated SN-HLPf (Sambucus nigra hevein-like fruit protein), a hevein-like chitin-binding protein, from mature elderberry fruits. Cloning of the corresponding gene demonstrated that SN-HLPf is synthesized as a chimeric precursor consisting of an N-terminal chitin-binding domain corresponding to the mature elderberry protein and an unrelated C-terminal domain. Sequence comparisons indicated that the N-terminal domain of this precursor has high sequence similarity with the N-terminal domain of class I PR-4 (pathogenesis-related) proteins, whereas the C terminus is most closely related to that of class V chitinases. On the basis of these sequence homologies the gene encoding SN-HLPf can be considered a hybrid between a PR-4 and a class V chitinase gene.

Amino Acid Sequence↗
PubMed1998

Reconsidering the energetics of ribonuclease catalysed RNA hydrolysis.

In principle, all biochemical reactions are reversible, though some are more reversible than others. The classical ribonuclease mechanism involves a reversible transphosphorylation step, followed by quasi irreversible hydrolysis of the cyclic intermediate. We performed isotope-exchange and intermediate-trapping experiments showing that the second hydrolysis step is readily reversible in the presence of RNase A or RNase T1. As a consequence, the equilibrium between a phosphodiester and a 2',3'-cyclophosphate accounts for all catalysed reactions, even if the leaving/attacking group is a water molecule. Therefore, ribonucleases are transferases rather than hydrolases. The equilibrium constant for the catalysed interconversion is close to 1 M. From this result, we estimate the effective concentration of the 2'-hydroxyl nucleophile in the cyclization step to be 10(7) M. The high effective concentration of the vicinal hydroxyl group balances the strain-associated and solvation-associated instability of the pentacyclic phosphodiester.

Catalysis↗
PubMed1996

H NMR study of the solution structure of Ac-AMP2, a sugar binding antimicrobial protein isolated from Amaranthus caudatus.

The conformation in water of antimicrobial protein 2 from Amaranthus caudatus (Ac-AMP2) was determined using 1H NMR, DIANA and restrained molecular modeling. Ac-AMP2 is a 30 amino acid residue, lectin-like protein that specifically binds to chitin, a polymer of beta-1,4-N-acetyl-D-glucosamine. After sequence specific resonance assignments, a total of 198 distance restraints were collected from 2D NOESY buildup spectra at 500 MHz at pH 2, supplemented by a 2D NOESY spectrum at 600 MHz. The location of the three previously unassigned disulfide bridges was determined from preliminary DIANA structures, using a statistical analysis of intercystinyl distances. The solution structure of Ac-AMP2 is presented as a set of 26 DIANA structures, further refined by restrained molecular dynamics using a simulated annealing protocol in the AMBER force field, with a backbone r.m.s.d. for the well defined Glu3-Cys28 segment of 0.69(+/-0.12) angstroms. The main structural element is an antiparallel beta-sheet from Met13 to Lys23 including a betaI-turn over Gln17-Phel8 with a beta bulge at Gly19. In addition, a beta'I turn over Arg6-Gly7, a beta'III turn over Ser11-Gly12 and a helical turn from Gly24 to Cys28 are identified. This structure is very similar to the equivalent regions of the X-ray structure of wheat germ agglutinin and the NMR structure of hevein.

Amino Acid Sequence↗
PubMed1996

Conformational features of a synthetic cyclic peptide corresponding to the complete V3 loop of the RF HIV-1 strain in water and water/trifluoroethanol solutions.

The disulfide-bridge-closed cyclic peptide corresponding to the whole V3 loop of the RF HIV-1 strain was examined by proton two-dimensional NMR spectroscopy in water and water/trifluoroethanol solutions. Although most of the peptide is conformationally averaged in water, the NOE data support a beta-turn conformation for the central conservative GPGR region and the presence of nascent helix. Upon addition of trifluoroethanol, helix formation in the C-terminal part becomes apparent. This is confirmed by CD data. NOEs indicative of multiple and transient beta-turns around the Asn6 glycosylation site and NOEs fitting X-ray data on a linear V3 peptide-Fab complex also emerge. The C-terminal helix is shown to have amphipathic character and might thus assist in the infection process.

Amino Acid Sequence↗
PubMed1995

Determination of the three-dimensional solution structure of scyllatoxin by 1H nuclear magnetic resonance.

The three-dimensional solution structure of Scyllatoxin (leiurotoxin I) a venom peptide from the scorpion Leiurus quinquestriatus hebraeus was determined at 1 A resolution by homonuclear proton n.m.r. methods at 500 MHz. Data analysis and structure calculation followed conventional protocols inherent to DIANA and related programs with two exceptions. First, distance constraints were obtained from two-dimensional nuclear Overhauser spectra by a previously described partial relaxation matrix approach. Second, since the pairing pattern of the six cysteine residues was not established a priori, the unequivocal assignment of the disulfide bridges was achieved exclusively from the n.m.r. data by a statistical analysis of preliminary DIANA structures. In the final calculation we used 227 upper distance constraints, 67 torsional constraints from vicinal coupling constants and ten stereospecific assignments of beta-methylene protons. Scyllatoxin folds into a compact ellipsoidal shape. An alpha-helix (defined with 0.24 A resolution) spanning 2.5 turns from Leu5 till Ser14 is stabilized by Cys8-Cys26 and Cys12-Cys28 disulfide bridges to the carboxy-terminal strand of an anti-parallel beta-sheet. The antiparallel beta-sheet (defined at 0.66 A resolution) extends from Leu18 to Val29 with a tight turn at Gly23-Asp24 and displays a right-handed twist. Scyllatoxin competes with the toxins apamin from Apis mellifera mellifera and P05 from Androctonus mauretanicus mauretanicus for the same or similar high conductance calcium-activated potassium channels. Consideration of presently known biological activities and three-dimensional structures of these toxins suggest a different toxin-receptor interaction of scyllatoxin as compared to apamin and P05.

Amino Acid Sequence↗
PubMed1995

1H NMR study of the interaction of N,N',N"-triacetyl chitotriose with Ac-AMP2, a sugar binding antimicrobial protein isolated from Amaranthus caudatus.

The interaction between Ac-AMP2, a lectin-like small protein with antimicrobial and antifungal activity isolated from Amaranthus caudatus, and N,N',N"-triacetyl chitotriose was studied using 1H NMR spectroscopy. Changes in chemical shift and line width upon increasing concentration of N,N',N"-triacetyl chitotriose to Ac-AMP2 solutions at pH 6.9 and 2.4 were used to determine the interaction site and the association constant Ka. The most pronounced shifts occur mainly in the C-terminal half of the sequence. They involve the aromatic residues Phe18, Tyr20 and Tyr27 together with their surrounding residues, as well as the N-terminal Val-Gly-Glu segment. Several NOEs between Ac-AMP2 and the N,N',N"-triacetyl chitotriose resonances are reported.

Amino Acid Sequence↗
PubMed1995

A comparative structural study on the steroids [1,2,5]oxadiazolo[3', 4':3,4]-5 alpha-pregn-16-en-20-one oxime (HS998), [1,2,5]oxadiazolo[3, 4':3,4]-5 beta-pregn-16-en-20-one (HS973) by NMR, molecular modeling, and X-ray investigations.

The molecular structure of the steroids [1,2,5]oxadiazolo[3', 4':3,4]-5 alpha-pregn-16-en-20-one oxime, [1,2,5]oxadiazolo[3',4':3, 4':3,4]-5 alpha-pregn-16-en-20-one and [1,2,5]oxadiazole]3',4':3,4]-5 beta-pregn-16-en-20-one has been determined. The proton-proton distances in the solid state from previous crystallographic studies are compared with the corresponding distances from novel and previous solution NMR as well as from novel in vacuo modeling studies.

Crystallization↗
PubMed1992

[Living with an ostomy: a preliminary study].

This study reveals some aspects of ostomy patients life experience. The data were obtained by using the participant observation technique during the monthly meeting session of the Ostomy Patients Association from July 1989 to August 1991. The findings showed that the ostomized patients were concerned with: 1) the ostomy pouch (how to get it); 2) other persons opinion about ostomized patients; 3) their sexuality, and stoma care. The patients perceived themselves as physically disable and inferior persons. Some of them perceived themselves as having a normal life. Other patients also had to cope with the stigma of cancer.

Adaptation, Psychological↗
PubMed1990

Solution conformation of leiurotoxin I (scyllatoxin) by 1H nuclear magnetic resonance. Resonance assignment and secondary structure.

A proton NMR study at 500 MHz of leiurotoxin I in water is presented. Nearly complete sequence-specific assignments of the individual backbone and side-chain proton resonances were achieved using through-bond and through-space connectivities obtained from standard two-dimensional NMR techniques. The secondary structure of this toxin is inferred from a combination of short-range nuclear Overhauser enhancements, scalar couplings and proton/deuteron exchange rates. Three disulfide bridges locate the N-terminal part (that is alpha-helical from residue 6 to 16) on one side of a C-terminal two stranded antiparallel beta sheet (from Leu18 to Val29). The latter features a tight turn at Gly23-Asp24.

Apamin↗
PubMed1990

Assessment of gastro-esophageal reflux disease: comparison of reflux scintigraphy with endoscopy biopsy and esophageal pH monitoring.

In a study designed to evaluate reflux scintigraphy in 79 patients with gastro-esophageal reflux disease, quantitated reflux scintigraphy was found to have predictive values of 73% in detecting reflux as judged by esophageal pH monitoring, of 63% in detecting esophagitis as judged by esophagoscopy and biopsy, and of 77% in detecting the presence of either esophagitis or reflux. However, its sensitivity was only 48, 58 and 42%, respectively. Gastro-esophageal reflux scanning has little value in the routine clinical diagnosis of mild to moderate gastroesophageal reflux disease, but none of the other tests (biopsy, endoscopic appearances of 24 hour esophageal pH monitoring) were sufficiently specific to be taken as a sole diagnostic criterion ("gold standard"). Investigation of gastro-esophageal reflux disease could probably best be limited to endoscopy (especially to detect ulceration or metaplasia and to exclude neoplasia) with 24 hour pH monitoring in addition for those patients with suspected reflux disease but negative endoscopy.

Adult↗