Analogous Fragments Research Articles

Molecular descriptors of amino acids for the evaluation of the physicochemical parameters and biological activity of peptides

Statistically significant correlation structure-activity relationships for two groups of oligopeptides (48 dipeptide fragments of analogues of inhibitors of the angiotensin-converting enzyme and 21 pentapeptide structural analogues of bradykinin) were obtained by the methods of regression analysis. Geometric, hydrophobic, and electron descriptors of the component amino acids were applied to the characteristics of the molecular structure of the peptides. The electron properties (electronegative charge, polarity of the molecules, and ability to form hydrogen bonds) were shown to have the greatest influence on the bioactivity of the studied compounds. A satisfactory fit between the calculated and experimental values of the bioactivity (IgA) of the peptides proved the validity of the four-parametrical equation for the evaluation of the unknown IgA values of oligopeptides on the basis of the physicochemical parameters of the participating amino acids.

Purification and Characterization of a 44-kDa Recombinant Collagen I α 1 Fragment from Corn Grain

This paper demonstrates that a fibrous, repetitive amino acid sequence collagen-related protein, a 44-kDa fragment of human collagen I alpha 1 (CIalpha1), was expressed in corn grain molecularly equivalent to that produced in recombinant yeast. The recombinant CIalpha1 was extracted and purified from early generation plants having low levels of recombinant protein accumulation. It was selectively extracted at low pH and purified by ion exchange and gel filtration chromatography, resulting in a 44-kDa CIalpha1 with >70% purity and 60% recovery. The N-terminal sequence, amino acid composition, and immunoreactivity closely matched those of an analogous 44-kDa CIalpha1 fragment produced by the yeast Pichia . The corn-derived 44-kDa CIalpha1 had an intact protein mass of 44088 Da, which is within 0.2% of the mass calculated from the expected sequence. Tandem mass spectrometry confirmed the primary sequence with 78% coverage. The amino acid composition analysis indicated a low level of prolyl hydroxylation. Glycoprotein staining revealed no glycosylation.

A fragmentary model of vitreous As2S3

The X-ray scattering intensity curves for vitreous As2S3 are experimentally measured using soft (copper) and hard (molybdenum) X rays. The inclusion of all the specific features revealed in the experimental scattering intensity curves makes it possible to obtain the experimental atomic radial distribution function (ARDF) in the ordering range up to ∼1 nm. The experimental ARDF is interpreted in the framework of the fragmentary model. A comparison of the experimental ARDF with the model ARDF calculated from the crystal structure data obtained for orpiment demonstrates that the corrugated layers inherent in crystalline As2S3 are retained in the glass structure but the characteristic interlayer interatomic distances are absent. The layers are joined together through the As4S5 molecular clusters. Microcrystals consisting of several unit cells are absent in the glass structure, which involves only the structural fragments of both crystalline analogs. One of the possible variants of their joining is proposed.

Oligomer formation during gas-phase ozonolysis of small alkenes and enol ethers: new evidence for the central role of the Criegee Intermediate as oligomer chain unit

Abstract. An important fraction of secondary organic aerosol (SOA) formed by atmospheric oxidation of diverse volatile organic compounds (VOC) has recently been shown to consist of high-molecular weight oligomeric species. In our previous study (Sadezky et al., 2006), we reported the identification and characterization of oligomers as main constituents of SOA from gas-phase ozonolysis of small enol ethers. These oligomers contained repeated chain units of the same chemical composition as the main Criegee Intermediates (CI) formed during the ozonolysis reaction, which were CH2O2 (mass 46) for alkyl vinyl ethers (AVE) and C2H4O2 (mass 60) for ethyl propenyl ether (EPE). In the present work, we extend our previous study to another enol ether (ethyl butenyl ether EBE) and a variety of structurally related small alkenes (trans-3-hexene, trans-4-octene and 2,3-dimethyl-2-butene). Experiments have been carried out in a 570 l spherical glass reactor at atmospheric conditions in the absence of seed aerosol. SOA formation was measured by a scanning mobility particle sizer (SMPS). SOA filter samples were collected and chemically characterized off-line by ESI(+)/TOF MS and ESI(+)/TOF MS/MS, and elemental compositions were determined by ESI(+)/FTICR MS and ESI(+)/FTICR MS/MS. The results for all investigated unsaturated compounds are in excellent agreement with the observations of our previous study. Analysis of the collected SOA filter samples reveal the presence of oligomeric compounds in the mass range 200 to 800 u as major constituents. The repeated chain units of these oligomers are shown to systematically have the same chemical composition as the respective main Criegee Intermediate (CI) formed during ozonolysis of the unsaturated compounds, which is C3H6O2 (mass 74) for ethyl butenyl ether (EBE), trans-3-hexene, and 2,3-dimethyl-2-butene, and C4H8O2 (mass 88) for \\textit{trans}-4-octene. Analogous fragmentation pathways among the oligomers formed by gas-phase ozonolysis of the different alkenes and enol ethers in our present and previous study, characterized by successive losses of the respective CI-like chain unit as a neutral fragment, indicate a similar principal structure. In this work, we confirm the basic structure of a linear oligoperoxide – [CH(R)-O-O]n – for all detected oligomers, with the repeated chain unit CH(R)OO corresponding to the respective major CI. The elemental compositions of parent ions, fragment ions and fragmented neutrals determined by accurate mass measurements with the FTICR technique allow us to assign a complete structure to the oligomer molecules. We suggest that the formation of the oligoperoxidic chain units occurs through a new gas-phase reaction mechanism observed for the first time in our present work, which involves the addition of stabilized CI to organic peroxy radicals. Furthermore, copolymerization of CI simultaneously formed in the gas phase from two different unsaturated compounds is shown to occur during the ozonolysis of a mixture of \\textit{trans}-3-hexene and ethyl vinyl ether (EVE), leading to formation of oligomers with mixed chain units C3H6O2 (mass 74) and CH2O2 (mass 46). We therefore suggest oligoperoxide formation by repeated peroxy radical-stabilized CI addition to be a general reaction pathway of small stabilized CI in the gas phase, which represents an alternative way to high-molecular products and thus contributes to SOA formation.

Conformational studies of [Abu3, 11]‐SFTI‐1, an analogue of the trypsin inhibitor isolated from sunflower seeds

With only 14 amino acid residues, the trypsin inhibitor SFTI-1 is the smallest naturally occurring serine proteinase inhibitor. It consists of two cyclic fragments (with head-to-tail cyclization and a disulfide bridge). In our previous paper, we showed that the removal of the disulfide bridge produced 2.4-fold lower activity. Here, we present the total conformational analysis of the [Abu(3, 11)]-SFTI-1 analog by means of 2D NMR spectroscopy in conjunction with theoretical methods. The peptide was synthesized by Fmoc SPPS. It was cyclized with PyBop and DIPEA in DMF. The NMR studies were performed in DMSO-d(6) at 303 K. Conformations of the peptide studied were calculated by the following three approaches: distance geometry (DG), molecular dynamics (MD) and determination of the statistical weights of conformations. The first two algorithms use a CHARMM force field, whereas the last uses an ECEPP/3 force field. Our calculations resulted in three sets of conformers with 7, 9 and 6 representatives, respectively. All our results were compared with published ones. It was found that the peptide has an ill-defined structure. Despite its conformational flexibility, the binding loop (3-11 fragment) displayed geometry similar to the corresponding fragments of the other SFTI-1 analogs and to the inhibitor itself. Furthermore, the peptide bond between the Ile7 and Pro8 residues adopts cis geometry, which is essential for inhibitory activity.

The brain 5‐HT1A receptor gene expression in hibernation

Hibernation is a unique physiological state characterized by profound reversible sleep-like state, depression in body temperature and metabolism. The serotonin 5-hydroxytryptamine(1A) (5-HT(1A)) receptor gene sequence in typical seasonal hibernator, ground squirrel (Spermophilus undulatus), was specified. It was found that the fragment encoding the fifth transmembrane domain showed 93.6% of homology with the analogous fragment of the mouse and rat genes and displayed 88.5% homology with the human 5-HT(1A) receptor gene. Using primers designed on the basis of obtained sequence, the expression of 5-HT(1A) receptor gene in the brain regions in active, entering into hibernation, hibernating and coming out of hibernation ground squirrels was investigated. Significant structure-specific changes were revealed in the 5-HT(1A) messenger RNA (mRNA) level in entry into hibernation and in arousal. An increase in the 5-HT(1A) gene expression was found in the hippocampus during the prehibernation period and in ground squirrels coming out of hibernation, thus confirming the idea of the hippocampus trigger role in the hibernation. Significant decrease in 5-HT(1A) receptor mRNA level in the midbrain was found in animals coming out of hibernation. There was no considerable changes in 5-HT(1A) receptor mRNA level in different stages of sleep-wake cycle in the frontal cortex. Despite drastically decreased body temperature in hibernating animals (about 37 degrees C in active and 4-5 degrees C in hibernation), 5-HT(1A) receptor mRNA level in all examined brain regions remained relatively high, suggesting the essential role of this 5-HT receptor subtype in the regulation of hibernation and associated hypothermia.

Leptospira Immunoglobulin-Like Proteins as a Serodiagnostic Marker for Acute Leptospirosis

There is an urgent need for improved diagnosis of leptospirosis, an emerging infectious disease which imparts a large disease burden in developing countries. We evaluated the use of Leptospira immunoglobulin (Ig)-like (Lig) proteins as a serodiagnostic marker for leptospirosis. Lig proteins have bacterial immunoglobulin-like (Big) tandem repeat domains, a moiety found in virulence factors in other pathogens. Sera from patients identified during urban outbreaks in Brazil reacted strongly with immunoblots of a recombinant fragment comprised of the second to sixth Big domains of LigB from L. interrogans serovar Copenhageni, the principal agent for transmission in this setting. Furthermore, the sera recognized an analogous LigB fragment derived from L. kirschneri serovar Grippotyphosa, a pathogenic serovar which is not endemic to the study area. The immunoblot assay detected anti-LigB IgM antibodies in sera from 92% (95% confidence interval, 85 to 96%) of patients during acute-phase leptospirosis. The assay had a sensitivity of 81% for sera from patients with less than 7 days of illness. Anti-LigB antibodies were found in sera from 57% of the patients who did not have detectable anti-whole-Leptospira responses as detected by IgM enzyme-linked immunosorbent assay and microagglutination test. The specificities of the assay were 93 to 100% and 90 to 97% among sera from healthy individuals and patients with diseases that have clinical presentations that overlap with those of leptospirosis, respectively. These findings indicate that the antibody response to this putative virulence determinant is a sensitive and specific marker for acute infection. The use of this marker may aid the prompt and timely diagnosis required to reduce the high mortality associated with severe forms of the disease.

LTBP-2 specifically interacts with the amino-terminal region of fibrillin-1 and competes with LTBP-1 for binding to this microfibrillar protein

LTBP-2 is a matrix protein of unknown function since, unlike other LTBPs, it does not form covalent complexes with latent TGF-β. We have previously shown that LTBP-2 has widespread association with fibrillin-containing microfibrils in developing aorta and other tissues. We have now shown that full-length human recombinant LTBP-2 specifically binds to the amino-terminal region of fibrillin-1, but not to fibrillin-2, in solid phase assays and overlay blotting. The binding was enhanced by the inclusion of 2 mM Ca 2+ ions in the assay buffer and abolished by 5 mM EDTA indicating that the interaction was directly or indirectly Ca 2+ ion dependent. The K d for the interaction was calculated from the specific binding curve as 9.4 nM. A recombinant carboxyl-terminal fragment of LTBP-2 was shown to a) bind the amino-terminal fragment of fibrillin-1 and b) block completely the binding of full length LTBP-2 to fibrillin-1. This result indicates that the major fibrillin-1 binding site resides close to the carboxyl-terminus of LTBP-2. Further competitive binding studies showed that an analogous carboxyl terminal fragment of LTBP-1 was able to block the binding of LTBP-2 to fibrillin-1 and that the C-terminal fragment of LTBP-2 could block the interaction of the LTBP-1 fragment with the fibrillin. Thus the binding site for LTBP-2 on fibrillin-1 appears to be the same or in close proximity to that for LTBP-1. Immunohistochemical analysis of developing human aorta showed distinctive but extensively overlapping distributions for LTBPs-1 and -2. Both LTBPs showed extensive co-localization with fibrillin-1 and elastic lamellae but LTBP-2 had extensive signal throughout the medial layer whereas LTBP-1 showed strong localization only in the outer medial layer. The finding indicates that there is a possibility for LTBP-2 to compete with LTBP-1 for binding to fibrillin-containing microfibrils throughout the aortic wall but particularly in the outer medial region where the LTBP-1 is predominantly located. Overall, the results support the concept that that LTBP-2 may be an indirect negative modulator for storage of the large latent TGF-β complex on microfibrils in aorta and other fibrillin-rich tissues.

Tryptophan activation activity of a minimal TrpRS catalytic domain

A ~130 residue fragment of B. stearothermophilus tryptophanyl-tRNA synthetase (TrpRS) has been constructed by deleting the first connecting peptide, CP1, from the Rossmann fold and the C-terminal anticodon-binding domain. Moreover, a cycle of protein design has been used to “heal” scars created by removing the CP1 peptide. Characterizing the residual catalytic activity of this redesigned fragment is complicated by the fact that its activity is ~10−3 times that of the intact enzyme, making detection against contaminating chromosomal TrpRS problematic, even using FLAG-tagged fragments. We have addressed this problem in two ways. First, we have renatured the fragment from inclusion bodies, to purify it away from contaminating native activity. Second, we have mutated active-site residue D146 to alanine, to implicate the fragment directly. Both the redesigned fragment and the D146A mutant have catalytic activity above the essentially flat background measured for material renatured from inclusion bodies made with cells transformed with an empty vector. Moreover, the D146A mutant activity differs from that of the wild-type fragment. Significantly different specific activities for these two minimal catalytic domain fragments would provide strong evidence that analogous fragments could have had sufficient amino acid activating activity to support natural selection of a primitive translation apparatus. Supported by NIGMS 48519.

Differential Recognition of the Type I and II H Antigen Acceptors by the Human ABO(H) Blood Group A and B Glycosyltransferases

The human ABO(H) blood group A and B antigens are generated by the homologous glycosyltransferases A (GTA) and B (GTB), which add the monosaccharides GalNAc and Gal, respectively, to the cell-surface H antigens. In the first comprehensive structural study of the recognition by a glycosyltransferase of a panel of substrates corresponding to acceptor fragments, 14 high resolution crystal structures of GTA and GTB have been determined in the presence of oligosaccharides corresponding to different segments of the type I (alpha-l-Fucp-(1-->2)-beta-D-Galp-(1-->3)-beta-D-GlcNAcp-OR, where R is a glycoprotein or glycolipid in natural acceptors) and type II (alpha-l-Fucp-(1-->2)-beta-D-Galp-(1-->4)-beta-d-GlcNAcp-OR) H antigen trisaccharides. GTA and GTB differ in only four "critical" amino acid residues (Arg/Gly-176, Gly/Ser-235, Leu/Met-266, and Gly/Ala-268). As these enzymes both utilize the H antigen acceptors, the four critical residues had been thought to be involved strictly in donor recognition; however, we now report that acceptor binding and subsequent transfer are significantly influenced by two of these residues: Gly/Ser-235 and Leu/Met-266. Furthermore, these structures show that acceptor recognition is dominated by the central Gal residue despite the fact that the L-Fuc residue is required for efficient catalysis and give direct insight into the design of model inhibitors for GTA and GTB.

Bifunctional Bioorganometallic Iridium(III)–Platinum(II) Complexes Incorporating Both Intercalative and Covalent DNA Binding Capabilities

AbstractThe DNA binding of bifunctional IrIII–PtII complexes containing [(η5‐Cp*)Ir(dppz)]2+ and [Pt(terpy)]2+ or trans‐[PtL2L′]2+ fragments (L = NH3, L′ = DMF; L = H2O, L′ = NH3) bridged by flexible κS:κN(amino)‐coordinated methionine‐containing peptides has been studied by UV/Vis and CD spectroscopy. Stable intercalative binding of the IrIII fragment of the complexes [{(η5‐Cp*)Ir(dppz)}(μ‐peptide‐κS:κN){Pt(terpy)}](CF3SO3)4 (5) (peptide = H‐Gly‐Gly‐Phe‐Met‐OH; H‐Gly‐OH = glycine, H‐Phe‐OH = L‐phenylalanine, H‐Met‐OH = L‐methionine) and 6 [peptide = H‐(Ala)4‐Met‐OH; H‐Ala‐OH = L‐alanine] is indicated by their steady decrease in absorbance at maxima between 350 and 390 nm on titration with CT DNA and by the bathochromic shifts of these maxima. Binding constants Kb of 1.4(4) × 106 M–1 for both 5 and 6 and site sizes s of 2.8(1) and 2.2(1), respectively, are in accordance with this monofunctional mode. Both peptide chain length and the site(s) of the labile PtII substituents DMF or H2O play an important role in determining the degree of intercalation of the IrIII fragment and the extent of helix distortion caused by simultaneous covalent binding of PtII centres for the DNA interaction of the complexes [{(η5‐Cp*)Ir(dppz)}(μ‐peptide‐κS:κN){trans‐(PtL2L′)}]4+ 7–10. Whereas both 9 and 10 (L = H2O, L′ = NH3; peptide = H‐Gly‐Met‐OH or H‐Gly‐Gly‐Met‐OH) exhibit a high degree of bifunctional binding [9, Kb = 1.1(5) × 106 M–1, s = 1.3(1); 10, Kb = 1.4(7) × 106 M–1, s = 2.1(1)], CD spectroscopy indicates that a more pronounced reorganisation of the DNA helix is required for the former complex with its shorter peptide. Effective intercalation is also observed for 8 (L = NH3, L′ = DMF; peptide = H‐Gly‐Gly‐Met‐OH) but not for 7 with the analogous PtII fragment but a shorter bridging peptide (H‐Gly‐Met‐OH). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

KARMA: a web server application for comparing and annotating heterogeneous microarray platforms.

We have developed a universal web server application (KARMA) that allows comparison and annotation of user-defined pairs of microarray platforms based on diverse types of genome annotation data (across different species) collected from multiple sources. The application is an effective tool for diverse microarray platforms, including arrays that are provided by (i) the Keck Microarray Resource at Yale, (ii) commercially available Affymetrix GeneChips and spotted arrays and (iii) custom arrays made by individual academics. The tool provides a web interface that allows users to input pairs of test files that represent diverse array platforms for either single or multiple species. The program dynamically identifies analogous DNA fragments spotted or synthesized on multiple microarray platforms based on the following types of information: (i) NCBI-Unigene identifiers, if the platforms being compared are within the same species or (ii) NCBI-Homologene data, if they are cross-species. The single-species comparison is implemented based on set operations: intersection, union and difference. Other forms of retrievable annotation data, including LocusLink, SwissProt and Gene Ontology (GO), are collected from multiple remote sites and stored in an integrated fashion using an Oracle database. The KARMA database, which is updated periodically, is available on line at the following URL: http://ymd.med.yale.edu/karma/cgi-bin/karma.pl.

Active domains of salivary statherin on apatitic surfaces for binding to Fusobacterium nucleatum cells.

Fusobacterium nucleatum can bind to saliva-coated tooth surfaces. However, the nature of the domains of salivary protein that interact with F. nucleatum remains unclear. The ability of individual proteins in human submandibular-sublingual saliva (HSMSL) to bind F. nucleatum cells was examined by dot blot assay; statherin displayed the strongest binding activity. Statherin binding sites were determined based on binding of (125)I-labelled F. nucleatum to statherin-coated hydroxyapatite (sHAP) beads via inhibition assays using synthetic analogous peptide fragments of whole statherin. Analogous peptides corresponding to residues 19-26 and 32-39 of statherin inhibited binding by 77 % and 68 %, respectively. Synthetic peptides were also prepared by serial deletions of individual residues from N- and C-termini of the peptides GPYQPVPE (aa 19-26) and QPYQPQYQ (aa 32-39). The inhibitory effects of peptides YQPVPE (aa 21-26) and PYQPQYQ (aa 33-39) were very similar to those of GPYQPVPE and QPYQPQYQ, respectively. However, additional deletion of residues resulted in significant reduction of the inhibitory effect. Alanine-scan analysis of YQPVPE revealed that all tested peptides retained inhibitory activity; only YAPVPE exhibited significantly decreased inhibitory activity. These findings suggest that YQPVPE and PYQPQYQ may represent the minimal active segments of statherin for binding to F. nucleatum; moreover, Gln may be a key amino acid in the active segment.

Assistance of maltose binding protein to the in vivo folding of the disulfide-rich C-terminal fragment from Plasmodium falciparum merozoite surface protein 1 expressed in Escherichia coli.

The C-terminal fragment of Plasmodium falciparum merozoite surface protein 1 (F19) is a leading candidate for the development of a malaria vaccine. Successful vaccination trials on primates, immunochemistry, and structural studies have shown the importance of its native conformation for its protective role against infection. F19 is a disulfide-rich protein, and the correct pairing of its 12 half-cystines is required for the native state of the protein. F19 has been produced in the Escherichia coli periplasm, which has an oxidative environment favorable for the formation of disulfide bonds. F19 was either expressed as a fusion with the maltose binding protein (MBP) or directly addressed to the periplasm by fusing it with the MBP signal peptide. Direct expression of F19 in the periplasm led to a misfolded protein with a heterogeneous distribution of disulfide bridges. On the contrary, when produced as a fusion protein with E. coli MBP, the F19 moiety was natively folded. Indeed, after proteolysis of the fusion protein, the resulting F19 possesses the structural characteristics and the immunochemical reactivity of the analogous fragment produced either in baculovirus-infected insect cells or in yeast. These results demonstrate that the positive effect of MBP in assisting the folding of passenger proteins extends to the correct formation of disulfide bridges in vivo. Although proteins or protein fragments fused to MBP have been frequently expressed with success, our comparative study evidences for the first time the helping property of MBP in the oxidative folding of a disulfide-rich protein.

The gamma -secretase-cleaved C-terminal fragment of amyloid precursor protein mediates signaling to the nucleus.

Sequential processing of the amyloid precursor protein (APP) by beta- and gamma-secretases generates the Abeta peptide, a major constituent of the senile plaques observed in Alzheimer's disease. The cleavage by gamma-secretase also results in the cytoplasmic release of a 59- or 57-residue-long C-terminal fragment (Cgamma). This processing resembles regulated intramembrane proteolysis of transmembrane proteins such as Notch, where the released cytoplasmic fragments enter the nucleus and modulate gene expression. Here, we examined whether the analogous Cgamma fragments of APP also exert effects in the nucleus. We find that ectopically expressed Cgamma is present both in the cytoplasm and in the nucleus. Interestingly, expression of Cgamma59 causes disappearance of PAT1, a protein that interacts with the APP cytoplasmic domain, from the nucleus and induces its proteosomal degradation. Treatment of cells with lactacystin prevents PAT1 degradation and retains its nuclear localization. By contrast, Cgamma57, a minor product of gamma-cleavage, is only marginally effective in PAT1 degradation. Furthermore, Cgamma59 but not Cgamma57 potently represses retinoic acid-responsive gene expression. Thus, our studies provide the evidence that, as predicted by the regulated intramembrane proteolysis mechanism, Cgamma seems to function in the nucleus.

Evidence of a Role for C4 in Modulating Interstitial Inflammation in Experimental Glomerulonephritis

Activation of C4 releases into the fluid phase a fragment of the alpha chain, C4a. Unlike the analogous fragments of C3 and C2, there is no evidence for an anaphylatoxic effect of C4a. There is actually some in vitro evidence that it could have a modulating effect on inflammation by inhibiting monocyte chemotaxis. We induced an immune complex glomerulonephritis in wild-type (WT) and C4 knock out (C4KO) mice. Although the glomerular component of the disease did not differ in the two groups of animals, there were marked differences in the accompanying tubulo-interstitial injury. Compared to WT animals, the C4KO mice had significantly more infiltrating interstitial cells (1910 vs 2720/mm2), foci of tubular atrophy (6.3 vs 14.8/section), and interstitial space (22 vs 30% of cortex). C4 is expressed constitutively by renal tubular epithelial cells. These data support a role for such local C4 in modulating interstitial inflammation, consistent with in vitro experiments.

Gas-Phase and Condensed-Phase Studies on the Reactivity of the Azido(2,2':6',2''-terpyridine)platinum(II) Cation

The recent report of the gas-phase loss of nitrogen from [Pt(dien)N3]+ (dien = diethylenetriamine) under collisioninduced dissociation conditions has prompted us to investigate whether the analogous fragmentation of [Pt(tpy)N3]+ (tpy = 2,2′: 6′,2″-terpyridine) occurs in the gaseous and condensed phase as well as in the solid state. While such a process occurs in the gaseous phase, attempts to fragment [Pt(tpy)N3]+ by refluxing it in MeCN yielded the tetrazolato complex [Pt(tpy)(N4CMe)]+, and solid state thermolysis resulted in decomposition. In order to examine the mode of binding of the tetrazole to platinum, the related complex [Pt(tpy)(N4CPh)]BF4 was synthesized via a similar route. X-ray crystallography revealed that the tetrazolate ligand in this complex binds through the N 2 position. In addition, the gas-phase reactivity of these complexes has been investigated using electrosprayionization tandem mass spectrometry (ESI-MS/MS). [Pt(tpy)N3]+ fragments to form [Pt(tpy)N]+, which readily reacts with alcohols, CH3CN and (CH3)2CO, to form adducts. The [Pt(tpy)N+R′OH]+ adduct ions (R′ = CH3, CH3CH2 and (CH3)2CH) fragment to yield [Pt(tpy)N+H2]+ ions with concomitant oxidation of the alcohols.

Structural analysis of a defective transfer-like region in a staphylococcal aminoglycoside resistance plasmid

Staphylococcus epidermidis clinical isolate MH6502 contained the 51.9-kb nonconjugal plasmid pMH6502, which has homology to a major part of the transfer gene region of a known conjugal plasmid. Plasmid pMH6502 mediates aminoglycoside and ethidium bromide resistance. During restriction digest analysis of pMH6502, a double logarithmic regression of marker data gave a better linear relationship than a semi-logarithmic one. The analysis indicated several differences in the transfer-like region of pMH6502 compared to the analogous region of the S. aureus conjugal plasmid pG01. The transfer-like region was in the opposite orientation compared to pG01. An EcoRI site that is within the transfer-like region of pMH6502, has no analogue in pG01. A HindIII site, located outside a 6.3-kb EcoRI fragment in the transfer gene region of pG01, is inside the analogous fragment of pMH6502. A model is proposed to describe how a conjugal ancestral plasmid of pMH6502 could alter to its present form.

Studies directed toward the synthesis of FS-2: observations on the fragmentation of cyclobutylcarbinyl radicals

Studies directed toward a synthesis of the sesquiterpene FS-2 have permitted an investigation of the fragmentation of a cyclobutylcarbinyl radical that preferentially cleaves to give the more stable of two possible radicals. This observation is contrasted with the results obtained in an analogous cyclobutyloxy radical fragmentation.

Ligand exchange reactions of [Re2(μ-OR)3(CO)6]− (R = H, Me) with sulfur, selenium, phosphorus and nitrogen donor ligands, investigated by electrospray mass spectrometry †

Negative-ion electrospray mass spectrometry has been used to investigate the reactions of the dinuclear rhenium aggregates [Re2(μ-OH)3(CO)6]−1 and [Re2(μ-OMe)3(CO)6]−2 with a range of thiols, benzeneselenol, and some other sulfur-, phosphorus- and nitrogen-based ligands. Typically up to three of the hydroxo ligands are replaced by simple thiolates, giving the series of species [Re2(OH)2(SR)(CO)6]−, [Re2(OH)(SR)2(CO)6]−, and [Re2(SR)3(CO)6]−. Similarly, reaction of 1 with H2S gives the species [Re2(μ-SH)3(CO)6]−, which undergoes an analogous fragmentation process to [Re2(μ-OH)3(CO)6]−, at high cone voltages, by loss of H2S and formation of [Re2(S)(SH)(CO)6]−. With ligands which are good chelating agents (such as dithiocarbamates R2NCS2−, and thiosalicylic acid, HSC6H4CO2H) initial substitution of one or two OH groups readily occurs, but on standing the dimer is cleaved giving [Re(S2CNR2)2(CO)3]− and [Re(SC6H4CO2)(CO)3]−. The different reactivities of the dithiol reagents benzene-1,2- and benzene-1,4-dimethanethiol towards 1 are also described. Complex 1 also reacts with aniline, and with primary (but not secondary) amides RC(O)NH2, giving monosubstituted species [Re2(OH)2(NHPh)(CO)6]− and [Re2(OH)2{NHC(O)R}(CO)6]− respectively. The reactions with adenine and thymine, and with the inorganic anions thiocyanate and thiosulfate, are also described.