15N-nmr spectroscopy. 33. Assignments of isotactic and syndiotactic sequences in poly(D,L-amino acids) (original) (raw)
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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.
Journal of Polymer Science: Polymer Chemistry Edition
Alternating copolyamides of various w-amino acids were synthesized by base-catalyzed polycondensation of N-isothiocyanatoacyl w-amino acids in solution. Derivatives of the following amino acids were used glycine, 8-alanine, y-aminobutyric acid, 6-aminovaleric acid, e-aminocaproic acid, D,L-B-aminobutyric acid, trans-4-aminocyclohexane 1-carboxylic acid, 4-aminophenyl acetic acid, 3-aminobenzoic acid, 4-aminobenzoic acid, 3-amino-4-methyl benzoic acid, and 4-amino-3-methyl benzoic acid. The base-catalyzed polycondensation at lower temperatures gave purer products than the bulk condensation at 18&20O0C. '3C-NMR and natural-abundance 15N-NMR spectra measured in trifluoroacetic acid demonstrate that in most cases undisturbed alternating sequences were obtained. Strong neighboring residue effects and long-range sequence effects were found in the 15N-NMR spectra, and structurehhift relationships are discussed. The sequences of copolyamides obtained by copolymerizations of lactams or 8-amino acid N-carboxyanhydrides were investigated by both 15N-NMR and W-NMFt spectroscopy. 13C-NMR spectroscopy was found to be more useful if the copolyamides consist of w-amino acid units of different chain length. However, 15N-NMR spectroscopy is more suited if the monomer units differ exclusively by their substituents.
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.
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.
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.