15N NMR spectroscopy, 2. Detection of tacticity in polypeptides (original) (raw)
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Biopolymers, 1982
L-Phe), were prepared, 40.55-MHz l5N-nmr spectra were measured in various solvents. The signal patterns depend strongly on the nature of the solvent, yet in most cases at least four signals are resolved, representing the four enantiomeric pairs of triads L-L-L (D-D-D), L-D-L (D-L-D), L-L-D (D-D-L), and D-L-L (L-D-D). Numerous copolypeptides of the general structure (A),-B*-(A) , (the asterisk denotes 40-50% 15N enrichment) were synthesized and measured as models for syndiotactic sequences in the spectra of poly(D,L-amino acids). In this way unambiguous assignments for both isotactic and syndiotactic trials were obtained. A spectroscopic rule was established "isotactic sequences absorb downfield of syndiotactic ones." Furthermore, the spectra of various types of stereocopolypeptides such as (L-Leu/L-Val), and (L-Leu/D-Val),, were investigated, including the ternary systems (L-Leu/L-Ala/D-Ala
15N NMR spectroscopy, 9. Solvent effects on polypeptides and polyamides
1978
Natural abundance ' 5N NMR spectra of Nylon-2 to Nylon-8 were measured in 2,2,2-trifluoroethanol, formic acid, trifluoroacetic acid, and fluorosulfonic acid. The "N NMR spectra of several sequence polypeptides containing Gly-Gly units were measured in the same solvents depending on their solubility and chemical stability. The shifts of these polymers were compared with each other and strong downfield shifts (up to 20ppm) were found with increasing acidity of the solvents. The downfield shift was more pronounced in the case ofw-aminoacyl units when compared with a-amino acid residues. c-Caprolactam shows shift effects that parallel those of Nylond. Polysarcosine, poly(L-lysine) (S), iso-poly(L4ysine) (6) and the sequence polymer (Tau-c-Aca). (7) were measured in dimethyl sulfoxide, water, formic acid, and trifluoroacetic acid. Polysarcosine, like polyglycine, shows comparatively small shift effects on changing the solvent, and polylysine as well as isopolylysine behave also similarly to other polypeptides, despite their charged side chains. With respect to solvent effects the sulfonamide group of 7 behaves differently from all other amide groups. The solvent effects are mostly explained by hydrogen bonds and protonation of the amide group.
15N NMR spectroscopy 14. Neighboring residue effects in glycine-containing polypeptides
1979
Gly-Gly), were synthesized by known methods, and their natural abundance "N NMR spectra were measured in trifluoroacetic acid. These spectra were compared with those of other previously described sequence polypeptides and with the corresponding homopolypeptides. The spectra of all sequence polypeptides exhibit neighboring residue effects, so that glycine nitrogen atoms acylated by other 2-(or w-)amino acids have chemical shifts different from that of the Gly-Gly bond. These neighboring residue effects cannot be summarized or explained by simple rules; however, they are useful for the characterization ofcopolypeptides. Such an application was tested in the case of four sequence polypeptides consisting of isomeric sequences of identical monomeric units. All isomeric sequences can be distinguished from each other, and the observed shift effects could be related to the neighboring residue effects of other copolypeptides.
13C NMR sequence analysis, 15. Copolymerization of alanine-NCA with other α-amino acid NCAs
1978
Alanine-N-carboxylic acid anhydride (Ala-NCA) was copolymerized with glycine-NCA, phenylalanine-NCA, valine-NCA, leucine-NCA, and sarcosine-NCA in various solvents and with various catalysts; 22,6MHz 13C NMR spectra and 90 MHz 'H NMR spectra of the resulting copolypeptides were measured in trifluoroacetic acid and from the signal intensities the rate of incorporation was estimated. The copolypeptides poly(Ala/Gly) and poly(Ala/Phe) contained both monomeric units in an almost equal concentration, while the low reactivity of Val-NCA and Leu-NCA resulted in a lower concentration of these residues compared with Ala in poly(Ala/Val) and poly(Ala/Leu). The I3C NMR spectra of poly(Ala/Gly) exhibit four CO-signals which could be assigned by comparison with the corresponding homopolypeptides and with the sequence polypeptides (Ala-Ala-Gly-Gly),, (Ala-Ala-Gly),, and (Ala-Gly-Gly).. From the intensities of these CO-signals the average length of the homogeneous blocks was calculated. Both block lengths and rate of incorporation proved to be almost independent of solvent, catalyst, and reaction temperature. Poly(Ala/Phe) and poly(Ala/Sar) show also four CO-signals, but poly(Ala/Val) only two and poly(Ala/Leu) one. The number and shape of the CO-signals allow one to decide whether random copolypeptides were obtained or not.
Journal of Polymer Science: Polymer Chemistry Edition
The tripeptides Phe-Gly-Gly, 8-Ala-Gly-Gly, and c-Aca-Gly-Gly as well as the peptide derivatives 6-isothiocyanatovaleroyl-Gly-Gly and e-isothiocyanatocaproyl-Gly-Gly were synthesized by using known methods so that the peptide nitrogen between the two glycyl residues was isotopically enriched in 15N to a level of 0.8-0.9%. These monomer units were then used to produce the sequence polymers (Phe-Gly-Gly),, (0-Ala-Gly-Gly),, (6-Ava-Gly-Gly),, and (e-Aca-Gly-Gly),. The 18.24 MHz 15N-NMR spectra of the oligo-and polypeptides were obtained by using trifluoroacetic acid as solvent, since the solutions have relatively low,viscosity and exhibit a strong negative nuclear Overhauser enhancement of the 15N signals. For comparison, 15N-NMR spectra of the homopolymers (Gly),, (0-Ala),, (y-Abu),, (a-Ava),, and (e-Aca), were also recorded. The 15N signals from the w-aminoacyl residues in the sequence polymers appear up to 11 ppm upfield of the signals observed for the homopolyamides. The l5N signals from the two glycyl residues are separated by 3-7 ppm. Comparison with the 1%-NMR spectra of the same polymers indicates that 15N-NMR is better suited for the characterization and sequence analysis of these types of polymers.
Journal of Molecular Structure, 1990
The i3C cross-polarization, magic-angle-spinning (CP-MAS) NMR spectra of poly(L-phenylalanine) and oligopeptides containing a L-phenylalanine or L-tyrosine residue in the solid state have been measured, in order to elucidate the side-chain conformational features of the phenylalanine and tyrosine residues. From these data it was found that the W chemical-shift difference between C, (main-chain carbon) and C!, (aromatic carbon) carbons in the L-phenylalanine and L-tyrosine residues in the oligopeptides of which the side-chain conformations in the solid state have already been determined from X-ray diffraction, depends significantly on the side-chain conformation. Using these results, the side-chain conformation of poly(~-phenylalanine) in the a-helix, w-helix and P-sheet forms was determined. Furthermore, an FPT INDO MO calculation was carried out in order to allow detailed discussion of the side-chain conformation dependence of the i3C chemical shift.
A simple method for determining protic end-groups of synthetic polymers by 1H NMR spectroscopy
Polymer, 2006
A simple method for the determination of protic end-groups (–XH) in synthetic polymers involves in situ derivatization with trichloroacetyl isocyanate (TAI) in an NMR tube and observation of the imidic hydrogens of the derivatized products [–X–C(O)–NH–COCCl3] by 1H NMR spectroscopy. In this paper, we report that the method is effective for the quantitative determination of hydroxy, primary amino and carboxy end-groups of polymers with . It may also be applied to detect chain ends in higher molecular weight polymers. The signals for the imidic (and, in the case of amines, amidic) hydrogens appear in a region (δ 7.5–11) that is clear of other signals in the case of most aliphatic polymers and many aromatic polymers such as polystyrene and poly(ethylene terephthalate). The method has been applied in the characterization of polymers formed by conventional and living radical polymerization (RAFT, ATRP, NMP), to end functional poly(ethylene oxide) and to polyethylene-block-poly(ethylene oxide). The method appears less effective in the case of sulfanyl end-groups. The chemical shift of the imidic hydrogen shows remarkable sensitivity to the microenvironment of chain end. Thus, the imidic hydrogens of TAI derivatized polyethylene-block-poly(ethylene oxide) [PE-(EO)mOC(O)NHC(O)CCl3] are at least partially resolved for m=0, 1, 2, 3 and ≧4 in the 400 MHz 1H NMR spectrum. It is also sensitive to the chain end tacticity of, for example, amino-end-functional polystyrenes and thus to the relative configuration of groups removed from the chain-end by two or more monomer units. TAI derivatization also facilitates analysis of amine functional polymers by gel permeation chromatography (GPC) which is often rendered difficult by specific interactions between the amine group and the GPC column packing.