Helix-coil transition in polypeptides having linear alkyl side-chain and ester group,and some conformational aspects observed by 13C NMR

¹³C NMR has been used to study the helix-coil transition induced in polypeptides by trifluoroacetic acid. An attempt is made to separate the influences of conformational change and direct solvent interaction. For poly(γ-methyl-L -glutamate), interactions between trifluoroacetic acid and the ester group of the side chain occur in the helical region. Poly(L -norvaline) and poly(L -norleucine) are in the form of “partially distorted helical conformation”. Conformational change and the interaction between trifluoroacetic acid and the peptide group which accompany the helix-coil transition, affect the chemical shift difference up to C^δ or C^β of poly(L -norvaline) and poly(L -norleucine), On the contrary, in the case of polyglutamates and polyaspartate, these effects are minor for the carbons in the ester portion. The chemical shift differences of these polypeptides can be well interpreted qualitatively by the results for some butyrates, poly(L -norvaline), and poly(L -norleucine).     

Solvent effect on the 13C NMR chemical shifts of meso and racemic 2,4-dichloropentanes as the model compounds of poly(vinyl chloride)

The ¹³C NMR chemical shifts of meso and racemic 2,4-dichloropentanes as the model compounds of poly(vinyl chloride) in various solvents were observed, and were well interpreted theoretically by means of “solvaton”-MO theory. The temperature dependence of the chemical shifts in pentachloroethane and dimethyl sulfoxide solutions were also well interpreted theoretically. Further, the similarity between these behaviours of the dimers and the polymer was discussed.     

Preparation and 13C NMR spectra of model compounds for poly(4-bromostyrene)

The preparations of 2,4-bis(4-bromophenyl)pentane ( 1b ) and 2,4,6-tris(4-bromophenyl)heptane ( 2b ) are described. The study of the different isomers of 1b and 2b by ¹³C NMR allows a comparison of the various configurational sequences of these compounds in terms of their chemical shifts. Methyl, methylene, and aromatic C¹ carbons of 1b and 2b could be deduced from the values obtained from corresponding polystyrene model compounds by a mere translation. From the aromatic C¹ chemical shifts it was possible to propose an assignment of the corresponding triads of poly(4-bromostyrene).     

Calculation of the carbon-13 chemical shifts of the methylene carbons of meso and racemic 2,4-dimethoxypentanes as model compounds of poly(vinyl methyl ether)

The ¹³C chemical shifts of the methylene carbons of meso and racemic 2,4-dimethoxypentanes were calculated by the CNDO/2 method. The results were in fairly well accordance with the experimental results of poly(vinyl methyl ether). The cis form of the side chain was more stable than the trans form.     

13C NMR chemical shifts and stereochemical structures in poly(vinyl chloride)

The ¹³C NMR chemical shifts of triad and tetrad configurations in poly(vinyl chloride) have been calculated by the average excitation energy approximation using the CNDO/2 MO method. On the basis of these calculations, both Inoue's and Carman's assignments for the observations of the triad and tetrad configurations have been examined and the partial discrepancy between them ascribed to the solvent effect. Further, the tacticity dependences of the calculated chemical shifts for triad and tetrad configurations are discussed.     

Multilabeled 13C substrates as probes in in vivo 13C and 1H NMR spectroscopy

The potential use of 13C multilabeled substrates has been studied in biological applications using in vivo and in vitro proton and 13C NMR spectroscopy. In 13C NMR spectroscopy, multilabeled compounds allow the simultaneous observation of several nuclei or increase distinctly the signal to noise ratio due to a higher degree of enrichment. Contiguous labeling of substrates leads to homonuclear 13C-13C spin couplings and provides a simple means to distinguish between endogenous stores of metabolites and metabolites derived from added substrates.     

Characterization of polyesters resulting from 1,2-propanediol and phthalic anhydride and their model compounds by 1H and 13C NMR spectroscopy

In order to study the side reactions taking place during the synthesis of polyesters, the structures of several model compounds were determined. Diesters or polyesters from 1,2-propanediol and phthalic acid were used as models. Their synthesis and characterization are described. The interpretation of their ¹H and ¹³C NMR spectra is reported; resonances due to the various end groups could be fully assigned.     

Additive rules for the 13C NMR shifts of methyl-substituted alkanes and ethylene-propylene copolymers

Detailed ¹³C NMR chemical shift rules are devised for ethylene and propylene polymers and for low-molecular-weight analogs. The rules are additive and account for substituent effects as well as for configurational sequences. The agreement between the observed and the predicted shifts is excellent. A computer program has been written to obtain the predicted shifts quickly and with minimum effort.     

Stereochemical configuration of polyfumarates as studied by 13C nuclear magnetic resonance spectroscopy

The 125 MHz ¹³C NMR spectra of some poly(dialkyl fumarates) (PDAF) and poly(dicycloalkyl fumarates) (PDCF) prepared by radical polymerization are measured in CDCl₃ solution at 60°C. The multiplet resonances of the main-chain carbon atoms and the ester carbonyl carbon atoms are assigned to stereochemical triad and pentad sequences, respectively, using model compounds as well as computer simulation. From these results it is found that PDAF and PDCF exhibit essentially atactic structures with meso addition favoured over racemic addition, and that the ratio of meso to racemic content varies with different kinds of alkyl and cycloalkyl groups. The tactic sequence proportions of the polymers cannot be explained by one-parameter Bernoullian statistics, but can be successfully explained by modified Bernoullian trial.     

Role of glial metabolism in diabetic encephalopathy as detected by high resolution 13C NMR

The roles of glial energetics and of the glutamine cycle in diabetic encephalopathy have been investigated ex vivo by (13)C NMR in extracts of adult rat brain. Streptozotocin-induced diabetic or euglycemic animals received intravenous infusions of (1-(13)C) glucose in the absence and presence of trifluoroacetic acid or methionine sulfoximine, two selective inhibitors of the glial tricarboxylic acid cycle or of glutamine synthase, respectively. (1-(13)C) glucose infusions resulted in smaller (13)C incorporation in all carbons of cerebral glutamate, glutamine and GABA in the diabetic animals. Co-infusion of trifluoroacetic acid with (1-(13)C) glucose further reduced the (13)C enrichments in cerebral glutamate and glutamine, the decrease being larger in the diabetic animals than in the corresponding euglycemic controls. Methionine sulfoximine decreased to undetectable levels the fractional (13)C enrichment in the carbons of cerebral glutamine in both groups and had no significant effect on (13)C incorporation in glutamate and GABA, suggesting that glutamine is not the main precursor of glutamate and GABA. Additional animals were infused with (1,2-(13)C(2)) acetate, a major substrate of glial metabolism. In this case, (1,2-(13)C(2)) acetate infusions resulted in increased (13)C incorporation in all carbons of glutamate, glutamine and GABA in the diabetic animals. Together, these results reveal that diabetic encephalopathy has an important effect in astroglial metabolism, decreasing glucose transport and metabolism and increasing the relative contribution of glial oxidative metabolism to the support of glutamatergic and GABAergic neurotransmissions.     

Microbial production of C13-norisoprenoids and other aroma compounds via carotenoid cleavage

Carotenoids are important precursors of a variety of compounds: the C(20)-retinoids, the C(15)-phytohormones, and the C(9)- to C(13)-aromas. Among the last type, C(13)-carotenoid-derived compounds (norterpenoids/norisoprenoids) such as ionones and damascones, constitute an essential aroma note in tea, grapes, roses, tobacco, and wine. Extraction of carotenoid-derived aroma compounds from plant sources is not economically realistic or considerably expensive. The biotechnological production of aroma compounds represents a feasible alternative and offers the production of enantiomerically pure molecules which can be labeled as "natural." To date, research in the production of ionones or the C(10)-compound, safranal, has mainly been focused on plant dioxygenases that cleave carotenoids in the positions between carbons 9 and 10 (9'-10') or 7 and 8 (7'-8'), respectively. Although relatively little is known about the microbial conversion of carotenoids into compounds with aroma due to the well known advantages of manipulating microorganisms, the aim of this work is to review the current state of the research in microbial production of norisoprenoids and other aroma compounds derived from carotenoid cleavage.     

Two-compartment model for determination of glycolytic rates of solid tumors by in vivo 13C NMR spectroscopy

Carbon-13 NMR spectroscopy of 13C enriched substrates is useful for non-invasively determining metabolic fluxes of cells and tissues. Our study demonstrates that for RIF-1 tumor cells, examined under monolayer culture with continuous perfusion and also grown as solid subcutaneously (sc) implanted tumors in vivo, the levels of intracellular glucose and intermediates of the glycolytic pathway are below the level of detection by NMR spectroscopy. For these tumors, glucose transport into the cell is the most probable rate limiting step of the glycolytic pathway. Under these limiting conditions a simple two-compartment model of glycolysis applies. This model yields two parameters: the average rate of glycolysis and the rate of lactate clearance through the vasculature. For the RIF-1 tumor these parameters were 0.022 +/- 0.01 and 0.034 +/- 0.006 min(-1), respectively.     

Cross polarization/magic angle spinning 13C solid state nuclear magnetic resonance of model compounds related to supported ziegler-natta catalysts

Cross polarization/magic angle spinning ¹³C NMR has been applied to the investigation of some solid organometallic compounds of the general formulae TiCl₄ · n RCOOR′, MgCl₂ · n RCOOR′ (RCOOR′ = p-CH₃? C₆H₄COOCH₃ and C₆H₅COOC₂H₅, n = 1; 2; and 0,15). Compounds with n = 1 and 2 show narrow resonance lines indicative of crystalline regular structures while for MgCl₂ · 0,15 RCOOR′ a disordered structure is found with the organic ligands strongly bonded to the surface of MgCl₂ crystallites. For n = 2 splitting into a doublet is found for some resonances due to the spatial non-equivalence of the two ligands induced by crystalline packing. A shift has been found for some resonances in going from Mg to Ti complexes which reflects the different Lewis acidity of the two metal atoms.     

The synthesis of polypeptides in influenza C virus-infected cells

The synthesis of virus-specific polypeptides was analyzed in MDCK cells infected with the JJ/50 strain of influenza C virus. In addition to three major structural proteins gp88, NP, and M, the synthesis of five polypeptides with molecular weights of 29,500 (C1), 27,500 (C2), 24,000 (C3), 19,000 (C4), and 14,000 (C5) was found in infected cells. None of these polypeptides were detected either in virions or in immunoprecipitates obtained after treatment of infected cell lysates with antiviral serum, suggesting that they are not viral structural proteins. Polypeptides C1-C5 were found to be synthesized in MDCK cells infected with different influenza C virus strains as well as in different host cell types infected with C/JJ/50. Further, it was observed that cellular protein synthesis was greatly reduced under hypertonic conditions, whereas the synthesis of C1-C5 was relatively unaffected. These results suggest that polypeptides C1-C5 are virus coded rather than host cell coded. Peptide mapping studies showed that each of polypeptides C3, C4, and C5 had a peptide composition similar to the M protein. The amount of C2 synthesized in infected cells was insufficient for mapping. This polypeptide was, however, found to rapidly disappear in pulse-chase experiments, suggesting that C2 is probably not unique but biosynthetically related to one of the other proteins. In contrast to these polypeptides, polypeptide C1 showed a map which is largely different from any major structural polypeptide. It therefore appears likely that C1 is a nonstructural protein of influenza C virus similar to the NS1 protein of influenza A and B viruses.     

Model compounds and 13C NMR investigation of isolated ethylene units in ethylene/propene copolymers

The stereochemical environment of the isolated ethylene units, and the arrangement of the neighbouring propylene units, in ethylene/propene copolymers, prepared in the presence of syndiotactic-specific and isotactic-specific catalysts are investigated by comparing ¹³C NMR spectra of selectively ¹³C-enriched copolymers and suitable model compounds. The implications of copolymer structure on polymerization mechanism are considered. In the presence of homogeneous syndiotactic specific catalyst systems both the regiospecificity and the stereospecificity are controlled by the last unit of the growing chain end. Regiospecificity and stereospecificity are unaffected by the ethylene units in the isotactic polymerization.     

Characterization of hydroxy-terminated polybutadienes by 13C NMR

Proton noise-decoupled ¹³C NMR spectra were used to investigated the sequence distribution of isomeric triads in hydroxy-terminated polybutadienes (HTPB). The population of nine transcentred and seven cis-centred triads was calculated from the integrated intensities in the olefinic region. The population of v\documentclass{article}\pagestyle{empty}\begin{document}$\underline{{\rm tc}}$\end{document}/t, c/t\documentclass{article}\pagestyle{empty}\begin{document}$\underline{{\rm tv}}$\end{document} and t\documentclass{article}\pagestyle{empty}\begin{document}$\underline{{\rm tc}}$\end{document}/t triads is higher for HTPB obtained by free radical polymerization than for anionically obtained HTPB. Quantitative data concerning their diad and triad composition were also derived from intensities of the peaks in the aliphatic region. The microstructure calculated from the sum of the population of cis- and trans-centred triads in the olefinic region agrees well with the microstructure computed from the diads and triads of cis(c), trans(t) and vinyl(v) units in the aliphatic region. The methylene and methine carbons in vvv sequences are sensitive to tacticity.     

The polypeptides induced in Drosophila cells by Drosophila C virus (strain Ouarzazate)

The Ouarzazate strain of Drosophila virus (DCV0) was grown in Drosophila melanogaster tissue culture cells, and [35S]methionine-labeled virions were found to contain a group of major structural proteins with a molecular weight of approximately 30,000 as well as several minor proteins of higher molecular weight and a protein of approximately 10,000 daltons. Using a range of pulses, chases and gel systems, examination of the intracellular proteins induced by DCV0 showed the presence of 17 polypeptides not found in uninfected cells. The synthesis of virus-induced polypeptides was extremely asymmetric with a rapid appearance of the major virus structural proteins and a much slower appearance of the lowest molecular weight structural protein (VP4). Processing of virus-induced proteins including the appearance of VP4 was demonstrated using pulse-chase after pulsing with [35S]methionine. While the highest molecular weight induced protein found in infected cells was 146,000, pretreatment of cells with iodoacetamide resulted in the appearance of a protein with a molecular weight of approximately 200,000. The evidence presented in this paper supports the inclusion of DCV0 in the Picornaviridae group.     

Effect of solvent on 13C NMR chemical shifts of tetrad methylene carbons of poly(vinyl chloride)

The ¹³C NMR chemical shifts of tetrad CH₂ carbons of poly(vinyl chloride) are observed in solvents covering a wide range of relative dielectric constants and show extreme sensitivity to the solvent. The chemical shifts are interpreted theoretically by means of the solvaton-CNDO/2 MO theory.     

Potentiation of poly I: C induced viral resistance by polycationic modified polypeptides

The potentiation of polyI:C induced viral resistance by various side group modified polycationic polyglutamic acid derivatives was investigated. It was established that the efficacy of the various polycationic substances depends primarily on their macromolecular properties. Viscosity and the degree of cationic substitution have been found to be of paramount importance. The maximal potentiating efficacy, expressed as the minimum protective dose of polyI:C necessary for complete protection of cells in the presence of the polycations, was exerted by compounds of highest viscosity and degree of subsitution. The potentiating efficacy of the polycationic derivatives tested could only be observed in a relatively narrow range of concentration, depending on their viscosity and degree of substitution. In view of the extremely low minimal protective dose of polyI:C (10(-5) mug/ml) in the presence of our most effective compound it is assumed that the action of a few molecules of polyI:C may be sufficient to render a cell resistant against viral infection. Within the limits of the described experiments, the efficacy of polycationic derivatives did not seem to be influenced by the modification (e.g. quaternarization) of the cationic group.