15N NMR spectroscopy, 2. Detection of tacticity in polypeptides (original) (raw)
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.
Journal of the American Chemical Society, 1990
A series of polypeptides [Ala*,X], containing I5N-labeled L-alanine (Ala*) and other amino acids (X; natural abundance of 15N) such as glycine, L-alanine, palanine, L-leucine, &benzyl L-aspartate, y-benzyl L-glutamate, y-methyl L-glutamate, L-valine, L-isoleucine, and sarcosine were synthesized by the a-amino acid N-carboxy anhydride (NCA) method. Conformations of these polypeptides in the solid state were characterized on the basis of conformation-dependent I3C chemical shifts in the I3C cross-polarization-magic-angle spinning (CP-MAS) NMR spectra and of the characteristic bands in the infrared (IR) and far-IR spectra. Further, isotropic 15N chemical shift (ab) and chemical shift tensors (ull, u2, and uaa) of the polypeptides were measured by the 15N CP-MAS and the 15N CP-static (powder pattern) methods, respectively. It was found that ai, is useful for the conformational study of homopolypeptides and copolypeptides with identical primary structures (amino acid sequences). In addition, it was demonstrated that the u22 value of the Ala* residue in copolypeptide is closely related to the main-chain conformations (such as the right-handed and left-handed a-helices, and the &sheet forms) rather than the amino acid sequence. Consequently, the uz2 value is very useful for conformational analysis of solid copolypeptides.
Determination of the conformation and stability of simple homopolypeptides using solid-state NMR
Solid State Nuclear Magnetic Resonance, 2003
15 N CPMAS, 13 C CPMAS and 1 H CRAMPS spectra of several polypeptide samples were compared to determine the useful features of each technique. 13 C CPMAS is the most wellestablished technique and is useful for quick determination of secondary structure. The 15 N nucleus is more sensitive to exact hydrogen-bonding parameters, which complicates interpretation of the spectra. However, it is better for resolving end effects and structural types in short oligomers. 1 H CRAMPS spectra are similar to 13 C CPMAS in the information obtained, but the resolution is not as good. Using 13 C CPMAS, the conformation of polyglycine was investigated in detail. Precipitation from solvents such as DCA or TFA resulted in the rippled b-sheet structure (PG I), while 3 1-helix (PG II) was formed by precipitation from aqueous solutions of LiBr. Grinding the sample resulted in an increase in the amount of PG I, indicating that this form is more stable in the solid state. These results agree with previous work on poly(l-alanine) showing that the b-sheet form is more stable in the solid state. Homopolypeptides with larger side chains did not change conformation upon grinding due to the greater difficulty in disrupting van der Waals interactions and inertia of the large side chains.
1H NMR spectroscopic criteria for the configuration of N‐acyl‐α, β‐dehydro‐α‐amino acid esters
Magnetic Resonance …, 2005
The diagnostic values of the following three spectral criteria for the configuration of N-acyl-a,b-dehydroa-amino acid esters were examined: (i) the proton at the b-position at the double bond of a Z-isomer is shielded if compared with the respective E-isomer (d Z b hd E b ); (ii) the proton at the nitrogen atom is shielded in a Z-isomer in comparison with the corresponding E-isomer (d Z NH hd E NH ); and (iii) changing of the solvent from CDCl 3 to deuterated trifluoroacetic acid (TFA) causes shielding of the H b vinylic proton of an E-isomer or deshielding of the respective proton of the Z-isomer (d E CDCl 3 id E TFA or d Z CDCl 3 hd Z
1978
Natural abundance "N NMR spectra of various aliphatic lactams and of the corresponding polyamides were measured in trifluoroacetic acid. Cis and trans amide bonds were found to have nearly identical shifts and identical 'JNH coupling constants. Increasing chain length of the monomeric units and increasing ring size result in a downfield shift of the "N signals. The I3C NMR spectra display, on the other hand, remarkable shift differences for lactams and Nylons. The polyamides show a monotonic downfield shift of the carbonyl signal with increasing chain length of the monomeric unit in contrast to the lactams. The I3C NMR spectra of the lactams were measured in various solvents; they show a downfield shift of the carbonyl signals with increasing acidity of the solvent. Protic solvents also favor strongly the cis-form of 1-aza-2-cyclononanone over the transform .
Solid-State 17 O NMR of Amino Acids
The Journal of Physical Chemistry B, 2004
17 O solid-state NMR from 14 amino acids is reported here, greatly increasing the number investigated. In most cases well-separated resonances from carbonyl and hydroxyl oxygens with distinct second-order quadrupolar line shapes are observed using a 600 MHz spectrometer with fast magic angle spinning (MAS). This is in contrast to the motionally averaged resonances usually seen from amino acids in solution. For amino acids double-angle rotation (DOR) produces a decrease in the line width by more than a factor of 40, providing very high resolution, ∼1 ppm, spectra. The oxygen lines in alanine and the carbonyl oxygens in L-glutamic acid hydrochloride are assigned using 1 H-decoupled DOR. The NMR interaction parameters for amino acids show a wide variation of Q , from 6.4 to 8.6 MHz, η, from 0.0 to 0.9, and δ iso , from 83 to 353 ppm. The high quality of the MAS NMR line shapes obtained at 14.1 T means that even small changes in parameters can be very accurately deduced, offering the possibility of 17 O NMR as a sensitive probe of structural changes in these and related compounds. The D-and L-forms of glutamic acid hydrochloride are shown to have the same NMR parameters to within error, which are very different from those reported in the literature for the D,L-form. A strong correlation (∼-1200 ppm/Å) is found between δ iso and the C-O bond length of the carbonyl oxygens. On the basis of these data, enriching specific amino acids in more complex polypeptides and proteins could provide site-selective information about the bonding and functionality of different sites in biomolecules. An estimate is made of the possible detection limit for such species.