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D Cowburn

Publications and source records attributed to D Cowburn.

At least 73 records · Page 4Linked to original sources
PubMed1988

13C NMR assignments of the protonated carbons of [d(TAGCGCTA)]2 by two-dimensional proton-detected heteronuclear correlation.

The resonances of the protonated carbons of [d(TAGCGCTA)]2 have been assigned by the two-dimensional proton-detected double-quantum heteronuclear correlation experiment [( 1H-13C]-DQCOSY). 13C-coupled and 13C-decoupled versions of the experiment were used. The assignment method is discussed in detail. The deoxyribose cross peaks segregate into five well-resolved regions, and the base cross peaks have distinct features that are helpful for assignments. The cross peaks from the 1H-13C pairs at the Cyd5, Ado2 and ThdCH3 base positions fall in separate regions of the spectrum from each other; they also are resolved from the closely spaced Ado8, Guo8, Cyd6 and Thd6. Additional parameters for distinction of the base signals are their differing J-coupling values and long-range coupling patterns.

Carbon Isotopes↗
PubMed1987

Binding of oxytocin and 8-arginine-vasopressin to neurophysin studied by 15N NMR using magnetization transfer and indirect detection via protons.

NMR was used to monitor the binding to neurophysin of oxytocin and 8-arginine-vasopressin, 15N labeling being used to identify specific backbone 15N and 1H signals. The most significant effects of binding were large downfield shifts in the amino nitrogen resonance of Phe-3 of vasopressin and in its associated proton, providing evidence that the peptide bond between residues 2 and 3 of the hormones is hydrogen-bonded to the protein within hormone-neurophysin complexes. Suggestive evidence of hydrogen bonding of the amino nitrogen of Tyr-2 was also obtained in the form of decreased proton exchange rates on binding; however, the chemical shift changes of this nitrogen and its associated proton indicated that such hydrogen bonding, if present, is probably weak. Shifts in the amino nitrogen of Asn-5 and in the -NH protons of both Asn-5 and Cys-6 demonstrated that these residues are significantly perturbed by binding, suggesting conformational changes of the ring on binding and/or the presence of binding sites on the hormone outside the 1-3 region. No support was obtained for the thesis that there is a significant second binding site for vasopressin on each neurophysin chain. The behavior of both oxytocin and vasopressin on binding was consistent with formation of 1:1 complexes in slow exchange with the free state under most pH conditions. At low pH there was evidence of an increased exchange rate. Additionally, broadening of 15N resonances in the bound state at low pH occurred without a corresponding change in the resonances of equilibrating free hormone.(ABSTRACT TRUNCATED AT 250 WORDS)

Arginine Vasopressin↗
PubMed1986

Trypsin-catalysed formation of pig des-(23-63)-proinsulin from desoctapeptide-(B23-30)-insulin.

Incubation of pig desoctapeptide-(B23-30)-insulin with trypsin in solvent systems consisting of dimethyl sulphoxide, butane-1,4-diol and Tris buffer resulted in the formation of an extra peptide bond between Arg-B22 and Gly-A1 in the DOPI molecule. This DOPI derivative can also be regarded as pig des-(23-63)-proinsulin. The structure of the new, previously unreported, proinsulin analogue was determined on the basis of amino acid analysis, dansylation and digestion with Staphylococcus aureus V8 proteinase. Receptor-binding ability of des-(23-63)-proinsulin was 20% of that of pig desoctapeptide-(B23-30)-insulin and 0.02% of that of pig insulin.

Amino Acids↗
PubMed1986

Enzymatic semisynthesis of porcine despentapeptide (B26-30) insulin using unprotected desoctapeptide (B23-30) insulin as a substrate. Model studies.

Unprotected porcine desoctapeptide(B23-30) insulin (DOPI) and the synthetic Gly-Phe-Phe were used as substrates for the trypsin-catalyzed synthesis of despentapeptide(B26-30) insulin (DPPI). The DPPI synthesis was accompanied by a moderate oligomerization and by the formation of a side produce which was identified as a DOPI derivative having an extra peptide bond between the Gly(A1) and Arg(B22) and which was named des(23-63) proinsulin (1). Despite side reactions, the conditions were found where the overall DPPI yields were comparable to those obtained via di-Boc DOPI, and these procedures were faster and simpler since the Boc protection and deprotection steps were omitted. The reaction progress was directly monitored by HPLC.

Animals↗
PubMed1985

Evidence for the cerebral uptake in vivo from two pools of glucose and the role of glucose-6-phosphatase in removing excess substrate from brain.

We propose the following scheme for cerebral uptake and overall metabolism of glucose in vivo: that brain selects from two pools of glucose anomers in arterial blood, that it takes up excess glucose, that glucose enters the brain tissue as glucose-6-phosphate through the actions of mutarotase and hexokinase, that some glucose-6-phosphate becomes metabolized to CO2 and some becomes incorporated into brain carbon pools, and that excess glucose-6-phosphate leaves brain through glucose-6-phosphatase and mutarotase activities. This results from our observations in arterio-venous studies for the determination of cerebral metabolism in humans in vivo that the cerebral uptake of [14C]glucose often appeared to differ from that of unlabeled glucose. With rapidly falling arterial radioactivity, unlabeled glucose uptake was more than [14C]glucose. With rising arterial radioactivity, [14C]glucose extraction exceeded unlabeled glucose. Studies with [14C]glucose-6-phosphate suggested that glucose-6-phosphatase in brain removes excess substrate by dephosphorylation. However, when arterial [14C]glucose increased slowly, [14C]glucose uptake varied considerably and the data resembled human cerebral metabolism of glucose anomers. An experiment employing [13C]glucose and NMR provided further support for our proposed scheme.

Animals↗
PubMed1983

Approaches to the complete determination of the rotation axes and flexibility in the methylene segments of phospholipids: deuterium, carbon-13, and proton NMR studies of dipalmitoyl phosphatidyl choline.

Deuterium NMR has been widely used to investigate flexibility in phospholipids. Usually, the measured deuterium quadrupolar couplings are directly related to segmental motion via a single Saupe order parameter. A more complete description avoiding assumptions concerning orientation and modes of motion can be obtained from five order parameters for a particular segment. Employing this approach, we present experimental NMR measurements using isotopic isomers of dipalmitoyl phosphatidyl choline in bilayer dispersions for the two stereospecific deuterium quadrupole couplings, the carbon-proton dipole couplings, and, preliminarily, the proton-proton dipole couplings, for the C-2 methylene segment of the sn-2 chain of dipalmitoyl phosphatidyl choline. In addition, we discuss the relation of these couplings to the significant order parameters, and calculate tentative orientations of the sn-2 alpha methylene segment.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1981

Hydrogen--deuterium exchange kinetics of the amide protons of oxytocin studied by nuclear magnetic resonance.

The contribution of intramolecular hydrogen bonding to the solution structure of oxytocin was evaluated by study of amide hydrogen exchange rates in D2O by Fourier transform 1H NMR spectroscopy. Resolution enhancement filtering was employed in the determination of individual pseudo-first-order rate constants. Apparent barriers to exchange of 0.5 and 0.6 kcal mol-1 were measured for Asn5 and Cys6 peptide NH, respectively. The slowing is best explained by steric hindrance to solvent access in the case of Asn5, while for the Cys6 participation in a weak intramolecular hydrogen bond is possible. Fourfold acceleration of base-catalyzed exchange was observed for Tyr2 NH; it is proposed that this is the result of electronic effects induced by hydrogen bonding of Cys1 C=0, either to Cys6 NH or to the N-terminal amino group. Exchange proceeds near the random coil limit for each of the remaining residues. Comparison with exchange data for the model tripeptide N-acetyl-L-prolyl-L-leucylglycinamide demonstrates no evidence of noncovalent association of the tocin ring with the tripeptide tail of the hormone.

Amides↗
PubMed1981

Proton NMR studies of vesicles incorporating glycophorin.

The effects of incorporation of glycophorin, the major sialoglycoprotein of the human erythrocyte membrane, on the lipid of small vesicles have been studied using proton NMR and electron microscopy. In contrast to the incorporation of other peptides, the major effect is apparently the clustering of vesicles without fusion. The relative mobility of lipids of the vesicle, monitored by changes in proton spin-lattice time, is only moderately effected by the presence of protein. The methylene protons of the lipid chains are subject to a somewhat greater restriction of motion following the incorporation of glycophorin than are the protons of the head group.

Amino Acids↗
PubMed1981

Anomalous exchange kinetics of peptide amide protons in aqueous solutions.

Reports concerning anomalous rates of exchange of some amides in oxytocin, alumichrome, and gramicidin S are reexamined through systematic analysis of the exchange data as a function of pH and primary structure. It is shown that such an analysis can provide useful information on secondary structure when the degree of hydrogen bonding to both the NH undergoing exchange and the neighboring carbonyl group are taken into consideration.

Ferrichrome↗