C-13 nuclear magnetic resonance spectroscopy of humic substances

Summary Evidence is presented from a preliminary study showing that C-13 N.M.R. spectroscopy is a useful new tool for the elucidation of the structure of soil organic matter.     

Carbon-13 nuclear magnetic resonance spectra of lignins

From the ¹³C-nmr spectra of a large number of dimeric and monomeric lignin model compounds the chemical shifts of the carbon atoms of the C₉-units in lignin with different substitution patterns were determined. The absorption peaks of the carbon-13 spectra of two lignins (beech and spruce) could be assigned by comparison (Table 3).     

Steady-state carbon-13 nuclear magnetic resonance spectra of acyl-alpha-chymotrypsin

When [13C]carbonyl-enriched p-nitrophenyl 5-n-propyl-2-furoate is incubated with alpha-chymotrypsin, a new peak appears in the 13C NMR spectrum. On the basis of its position and the fact that it is "chased" with unlabeled substrate, we conclude that this new signal is due to the acyl-enzyme intermediate. In spectra taken during steady-state turnover, the acyl-enzyme ester carbonyl 13C chemical shift displays a pH dependence that fits to a titration curve with an apparent pK of 7.1 (0.1). The apparent pK of the kcat vs. pH curve for enzyme-catalyzed hydrolysis of the same substrate under conditions differing only in reactant concentration is 7.0 (0.1). We have found no spectral evidence for a tetrahedral intermediate.     

Carbon-13 nuclear magnetic resonance spectra of carbohydrate oxirane derivatives

The ¹³C-chemical shifts and ¹J_C,H values of two series of carbohydrate oxirane derivatives, namely methyl 2,3-anhydro-ribo- and -lyxofuranosides and methyl 2,3-anhydro-4,6-O-benzylidene-manno- and -allopyranosides have been determined. The assignment of ¹³C resonances has been established mainly by the examination of the proton-coupled and the selective proton-decoupled spectra. The effect of the oxirane rings on the chemical shifts of β and γ carbon atoms (from the oxirane ring oxygen atom) has been observed. Large ¹J_C,H values associated with cis CH bonds adjacent to the oxirane rings relative to those of trans counterparts have been found.     

The carbon-13 nuclear magnetic resonance spectra of four eudesmane sesquiterpenols

The ¹³C NMR spectra of geosmin, selina-4(14),7(11)-diene-99-ol and two dihydroeudesmol isomers have been obtained and the individual resonances assigned. Several different empirical correlations developed by others have been combined in simple calculations to predict chemical shift values for sesquiterpenols of the eudesmane group.     

13C nuclear magnetic resonance spectra of 1,2-dioctadec-cis-enoyl-sn-glycero-3-phosphorylcholines.

The 13C NMR spectra of all sixteen 1,2-dioctade-cis-enoyl-sn-glycero-3-phosphorylcholines have been obtained. Resonance lines of the olefinic, methylene, methyl and carboxyl carbon nuclei are sufficiently characteristic to permit unequivocal designation of double bond position for each isomer. Two resonances of the sn-glycero-3-phosphorylcholine structure have been reassigned.     

Proton and C-13 nuclear magnetic resonance spectra of some benzoylated aldohexoses

Chemical shifts and coupling constants of ¹H-n.m.r. spectra of the perbenzoates of α-d-glucopyranose (1), β-d-glucopyranose (2), α-d-galactopyranose (3), α-d-mannopyranose (4), β-d-mannopyranose (5), and α-d-galactofuranose (6) are reported. The ¹³C-n.m.r. chemical shifts of compounds 1-3 and 6, and of penta-O-benzoyl-β-d-galactofuranose (7) are given. Mass spectra were used to differentiate the furanoses 6 and 7 from the pyranose 3.     

Carbon-13 nuclear magnetic resonance spectra of D-homoannulated 17-hydroxypregnan-20-ones

The 13C-NMR spectra of several groups of isomeric D-homoannulated 17 alpha-hydroxypregnan-20-ones have been recorded. The chemical shifts of the various carbon atoms have been correlated with the structures of the different isomers.     

Fluorine nuclear magnetic resonance spectra of rabbit carbonmonoxyhemoglobin

Carbonmonoxyhemoglobin prepared from protein isolated from rabbits maintained on a diet supplemented with 4-fluorophenylalanine (Phe (4F)) has been studied by fluorine NMR spectroscopy. Substitution of Phe(4F) appears to take place randomly at the sixteen nonequivalent phenylalanine positions of the globins; examination of hybrid hemoglobins in which only one type of globin chain contained the fluorinated amino acid, as well as changes in the spectrum upon exposure to oxygen, aided in the assignment of fluorine resonances from the alpha- or beta-globin chains. The effects of modification of Cys-beta-93 with a spin label and variation of pH and sample temperature on the spectrum were also examined. These data in association with theoretical estimates of aromatic ring current and van der Waals effects on chemical shifts were used to support tentative assignments of several signals observed to specific amino acid residues. Evidence suggesting the presence of two conformational forms of the protein, possibly due to disorder of the heme groups, is described.     

H and 13C nuclear magnetic resonance spectra of some peptides with fibrinogen-like reactivity.

The 1H and 13C spectra of four peptides, L-Phe-Val-Arg-pNA, D-Phe-Val-Arg-pNA, L-Phe-Pip-Arg-pNA and D-Phe-Pip-Arg-pNA (pNA = p-nitroaniline, Pip = pipecolic acid residue), have been examined, and deductions made about their conformation in solution. The D-Phe peptides, which are cleaved especially rapidly by thrombin in water, have structures (in deuterated DMSO) in which the aromatic ring of the D-Phe residue is folded back over the Val or Pip residue. This arrangement is not found in the L-Phe peptides. It is proposed that this feature (in which Phe could be situated near Val and near the Arg-Gly bond of the A alpha chain in the three-dimensional structure of fibrinogen) may be especially advantageous for binding to the enzyme.     

Carbon-13 nuclear magnetic resonance studies of cobalamins

The carbon-13 nuclear magnetic resonance spectra of aquocobalamin, adenosylcobalamin, methylcobalamin, and (carboxymethyl)cobalamin have been interpreted. The assignments were made by a comparison of the spectra with that of cyanocobalamin, by a study of the pH dependence of the chemical shifts, by an analysis of the effect of the axial ligands on the carbon atoms of the corrin ring, and by a study of the specific line broadening effect of the paramagnetic ions Mn2+ and Gd3+. The chemical shift changes that accompany the "base-on"----"base-off" conversion of the organocobalamins demonstrate that the conformation of the "western" half of the corrin ring and the conformations of the a, b, c, d, f, and g side chains are relatively constant. In contrast, the conformations of the "eastern" half of the corrin ring and the e propionamide side chain are highly variable.     

High-resolution solid-state nuclear magnetic resonance spectra of dentin collagen

Insoluble collagen of bovine dentin was characterized by high-resolution solid-state 13C nuclear magnetic resonance (NMR) spectroscopy using a cross-polarization magic angle spinning procedure. A downfield shift was observed in the signal of hydroxyproline C beta compared with that in skin collagen, indicating a distortion in the hydroxyproline structure. A signal of 31P NMR was detected in dentin collagen that was compatible with the presence of matrix-associated phosphoprotein.