Resolution Nuclear Magnetic Resonance Spectra Research Articles

Orientation and Structure of Pyrazine, Pyrimidine, and Pyridazine in a Lyotropic Liquid Crystal

Abstract High resolution proton magnetic resonance spectra of pyrazine, pyrimidine, and pyridazine partially oriented in a lyotropic mesophase have been obtained and analyzed. The distance ratios calculated from the direct dipole-dipole couplings are compared with those reported for microwave and X-ray studies in the gaseous and solid state as well as those determined from NMR investigations of the above solutes in thermotropic nematic phases. The average orientation of the diazines in the oriented lyotropic phase is such that the applied magnetic field is parallel to the molecular plane. In contrast, the average orientation of benzene, furan, thiophene, p-dithiin, and tetrafluoro-1,3-dithietane is such that the molecular ring is on the average perpendicular to the applied magnetic field. Sine benzene, furan, thiophene, and the diazines orient with the molecular plane parallel to the magnetic field in thermotropic nematic phases, contributions from different orienting factors appear to be operative in the...

A study of the 220 MHz proton resonance spectra for a series of biologically important acridine‐type heterocycles

AbstractThe high resolution proton magnetic resonance spectra of acridine, phenazine, phenoxazine and phenothiazine have been measured at 220 MHz in various solvents and at several temperatures. Spectra for all four molecules were analyzed iteratively and good agreement was obtained between observed and calculated spectra.Significant differences are noted between the coupling constants and chemical shifts of acridine and phenazine on the one hand and phenoxazine and phenothiazine on the other. These differences are attributed to a nonplanar heterocyclic ring conformation for phenoxazine and phenothiazine in solution. Moreover, it is concluded from the temperature measurements that the heterocyclic rings of phenoxazine and phenothiazine are conformationally flexible in the range covered. Possible mechanisms are suggested for the chemical shift and coupling constant changes.

Investigation of exchangeable protons and the extent of base pairings in yeast phenylalanine transfer RNA by high resolution nuclear magnetic resonance

High resolution nuclear magnetic resonance spectra were obtained for yeast transfer RNA Phe and unfractionated yeast tRNA in aqueous solutions. Resonances due to the free amino protons, hydrogen-bonded amino protons and hydrogen-bonded ring N—H protons were observed in three non-overlapping spectral regions. The assignment of the exchangeable proton resonances in yeast tRNA Phe is based on their pH and temperature dependence, spectral position, and their disappearance in deuterium oxide solvent. The number of each kind of exchangeable proton was determined and compared with the number expected on the basis of the cloverleaf model. The total number of base pairs observed by nuclear magnetic resonance (20±2) agrees well with the number computed from the cloverleaf model (20±1) depending upon whether or not one 2MeG.C and one G.U base pair are included. Furthermore, the separate numbers of A.U and G.C base pairs, 7 to 8 and 11 to 12, respectively, determined by nuclear magnetic resonance, are in good agreement with the 8 and 12 respective pairs expected from the cloverleaf model. The nuclear magnetic resonance spectra provided little evidence for additional strong Watson-Crick type base pairs resulting from tertiary structure such as are proposed in several models of tRNA. Resonances observed in the −9 to −11 parts per million region appear to be related to tertiary structure, but their assignment remains open at this tune. The results obtained with unfractionated yeast tRNA parallel those obtained with yeast tRNA Phe and therefore indicate that the interpretation of these latter data may be applicable to other tRNA species. The high resolution nuclear magnetic resonance data on the behavior of the exchangeable protons in tRNA are compared with the results of recent tritium exchange experiments. The anomalous behavior of hydrogen-bonded amino protons in both the nuclear magnetic resonance and tritium exchange experiments is discussed.

High resolution nuclear magnetic resonance spectra of hemoglobin: II. Ligated tetramers

The high resolution nuclear magnetic resonance spectra of hemoglobin, particularly in the cyanoferric state, have been compared with those of the isolated chains. The paramagnetically shifted resonances in the α 2 β 2 tetramer show clear differences from those in the isolated α and β chains, indicating some structural change around the heme upon the association. From the comparison with those resonances in the valency hybrids such as ( α III CN β II O 2) 2 and ( α II O 2 β III CN) 2, contributions of the α and β subunits in the Hb IIICN spectra are easily recognized. The spectra of the tetramer show a pH titration and the apparent p K at pH 7.2 in phosphate buffer is shifted to a lower value in phosphate-free buffer. The state of protonation of some histidine(s) seems to be responsible for the effects of the solvent conditions upon the resonance spectra. These spectral changes are characteristic responses of the fully ligated hemoglobin tetramer.

High resolution nuclear magnetic resonance spectra of hemoglobin: I. The cyanide complexes of α and β chains

The high resolution nuclear magnetic resonance spectra of the isolated chains of human hemoglobin have been studied. Interest is mainly focussed on the paramagnetically shifted resonances in the cyanoferric state of these chains. Two resonance peaks of the heme methyls are easily observable, together with several single proton peaks in the downfield region. The strong peaks in the high-field region are due to two methyl resonances of some amino-acid residue near the periphery of the heme, shifted up-field by the pseudocontact interaction. Effects of pH and of a modification of the protein on those resonances are presented. The exchangeable NH protons of the ring and of the peptide of the proximal histidine have resonance positions very similar to those in cyanoferric myoglobin, although the methyl resonances of the heme group are at somewhat different positions.

Identification of Polychlorobiphenyls by High Resolution Proton Magnetic Resonance

Abstract Mass spectrometry allows the number of chlorine atoms per polychlorobiphenyl molecule to be determined but not the positions of substitution. A method of identifying PCB components from high resolution proton magnetic resonance spectra is illustrated by application to 2,5,2′,5′-tetrachloro- and 2,3,4,2′,4′,- 5′-hexachlorobiphenyls separated by gas chromatography from a commercial PCB mixture.

High resolution nuclear magnetic resonance spectra of hemoglobin: III. The half-ligated state and allosteric interactions

The high resolution proton nuclear magnetic resonance spectra of the halfligated valency hybrids ( α III CN β II ) 2 and ( α II β III CN) 2 of human hemoglobin have been studied at 220 MHz. It has been possible to resolve and identify resonances from the α III CN or the β III CN subunits and from the deoxygenated subunits. Depending upon solution conditions, the cyanoferric subunits of both hybrids gave spectra which were either very similar to or definitely changed from those observed in fully ligated hemoglobin. For example, in the ( α III CN β II ) 2 hybrid at pH > 7.3 in the absence of phosphates, the α III CN subunit spectrum looked very much like that observed in ( α III CN β II O 2) 2. Upon addition of one mole of 2, 3-diphosphoglycerate per tetramer of hemoglobin the nuclear magnetic resonance spectrum changed. A similar change was observed at pH 7.1 in 0.1 m-phosphate. The other hybrid ( α II β III CN) 2 did not change its spectrum significantly under these conditions but did change with the addition of a slight molar excess of inositol hexaphosphate. Under certain conditions, the changed and unchanged spectra of the ( α III CN β II ) 2 hybrid were only slowly exchanging with each other, since in the presence of less than stoichiometric amounts of inositol hexaphosphate the two types of spectra were superimposed. These two distinct nuclear magnetic resonance spectra of the hybrids were interpreted as reflecting the two quaternary structures. The forms with the changed spectra, reflecting the deoxy quaternary, were stabilized by the organic phosphates which preferentially bind to deoxyhemoglobin. The stabilization of the deoxy quaternary in the ( α II β III CN) 2 hybrid was more difficult than in the ( α III CN β II ) 2 hybrid, reflecting differences between the a and β subunits, and therefore, a stronger allosteric effector, inositol hexaphosphate, was necessary to switch the structure. Comparison of the two types of nuclear magnetic resonance spectra with the rate of CO combination in these hybrids ( Cassoly, Gibson, 1971, Ogawa & Shulman, 1971) showed that the changed spectra arose from a form with slow CO combination rate, characteristic of the first ligand binding to hemoglobin and the unchanged spectra corresponded to a fast rate characteristic of the fourth ligand. From these experiments, we concluded that the two quaternary forms of hemoglobin can be observed for the halfligated states of hemoglobin and that the switch between the two forms is responsible for the co-operativity. It is also shown that the change of ligand binding energy during co-operativity does not show up in the iron-ligand bond of the ligated heme.

Physicochemical Analyses of the Bovine Milk Fat Globule Membrane. III. Proton Magnetic Resonance Spectroscopy

High resolution nuclear magnetic resonance spectra were from freeze-dried milk fat globule membranes in D2O. The effect of increasing temperature on spectra of the acetone-extracted membrane was related to thermal behavior of its lipid components. At 33.4C the spectrum of the membrane reflected acetonesoluble lipids. At 50C the spectrum of acetone-extracted membrane was still dominated by signals from lipids, which increased in intensities at from 60 to 120C. No signals attributed to proteins were observed before and after treatment with 8M urea and sodium deoxycholate. However, treatment with a more powerful denaturing solvent, trifluoroacetic acid, resulted in sharp resonances attributable to amino acids. This observation indicated ordered configuration for membrane proteins. Data presented suggested little or no restriction in molecular freedom of protons of the membrane constituents. In this regard bovine milk fat globule membrane contrasted with erythrocyte membrane.

Diffusible fraction of serum cholesterol and atherogenesis.

CHOLESTEROL is found in the blood as a structural component of lipoproteins concerned with the transport of other lipids1. The high resolution nuclear magnetic resonance spectra of high density serum lipoproteins are similar to that observed when lipids are dissolved in organic solvents, or dispersed in water by bile salts or detergents, or in sonicated form. The lipid component in lipoproteins is therefore probably in an extremely fluid condition2. If human serum is mixed with paraffin oil, some of the cholesterol diffuses into the oil without affecting the ultraviolet absorption spectrum of serum proteins. This procedure avoids any protein denaturing action used for cholesterol extraction3–5. It therefore seems that serum cholesterol has two fractions, one strongly bound by lipoprotein structures, and the other loosely bound and diffusible in an oil phase. In this article I designate the loosely bound fraction “diffusible”.

NMR studies of furan and thiophene partially oriented in a lyotropic liquid crystal

High resolution nuclear magnetic resonance spectra have been obtained and analyzed for the solutes, furan and thiophene, oriented in a Lyotropic mesophase. The structural parameters calculated for both molecules agree well with microwave geometries. The signs of the isotropic coupling constants are positive for furan and thiophene. The orientation of both molecules is such that the planes containing the protons are on the average perpendicular to the magnetic field direction. A comparison of structural parameters obtained employing thermotropic as well as lyotropic liquid crystals is presented.

Some Recent Developments in High Resolution Nuclear Magnetic Resonance

Techniques for studying high resolution nuclear magnetic resonance spectra have been considerably broadened in recent years. The most far reaching development—pulse Fourier transform (FT) methods—is discussed in detail. Applications of FT techniques to measurement of relaxation times and to enhancement of weak signals, especially from natural abundance 13C, are reviewed. Double resonance methods, particularly the nuclear Overhauser effect, and the use of lanthanide shift reagents are also covered in this “mini-review.”

Carbon‐13 nuclear magnetic resonance spectroscopy of polystyrene and poly‐α‐methylstyrene

AbstractThe carbon‐13 high resolution nuclear magnetic resonance spectra with proton spin decoupling of polystyrene and poly‐α‐methylstyrene were measured. The spectra clearly split corresponding to triad, tetrad, and partially pentad placements, and these peaks were tentatively assigned. By comparison of the triad tacticity of poly‐α‐methylstyrene determined from carbon‐13 spectra with that from proton spectra, it was confirmed that the nuclear OVERHAUSER effect due to proton decoupling has little effect on the relative intensities of the peaks assigned to chemically equivalent carbons except for stereochemical configuration.The stereospecific polymerization mechanisms are discussed on the basis of the microtacticity determined from carbon‐13 spectra. As a result, it is concluded that poly‐α‐methylstyrene polymerized with cationic and anionic catalysts can be described by BERNOULLIAN statistics, while the atactic polystyrene does not obey BERNOULLIAN but first or second order MARKOVIAN statistics.

The effect of the substituent on the conformation of some five-membered heterocyclic compounds

The high resolution proton magnetic resonance spectra of some unsaturated five-membered heterocycles have been obtained at variable temperature in the range −60 to +110°, and in various solvents, and have been analyzed in terms of chemical shifts and HH coupling constants. The magnitudes of the coupling constants indicate that a conformation with pseudoequatorial substituent is predominant when the rings have the substituent in position 5(I), but this preference is lost when the substituent is in position 4(II).

Carbon‐13 nuclear magnetic resonance spectroscopy of polypropylene

AbstractThe proton decoupled natural abundance 13C (designated as 13C‐{1H}) high resolution nuclear magnetic resonance spectra were measured for polypropylene at 25.1 MHz. The samples used were 4 polymers of different tacticity. The spectra of methylene carbon split corresponding to diad and partially tetrad placements. The spectra of methyl carbon of stereoblock type polymer clearly split corresponding to pentad placements. These splittings were tentatively assigned. The relation between the relative intensities of pentad placements, namely, was not satisfied. As the causes of this inequality, several possibilities were pointed out.The triad microtacticity of highly isotactic polymers determined from 13C‐{1H} spectra were compared with the results of n‐heptane extraction and good correspondence within experimental error were obtained. The discrimination of configurational sequences of polypropylene by 13C‐{1H} spectra at 25.1 MHz is equal or superior to that possible in 1H spectra at 220 MHz. However, the two peaks assigned to methine carbon showed abnormal intensities.

The measurement of frequency separations in high resolution nuclear magnetic resonance spectra using a frequency modulated observing field

A new method for the measurement of frequency separations in high resolution nuclear magnetic resonance spectra is presented. The method, which involves comparison of two points on the line profile, is shown to be capable of an internal precision of at least 2 mHz. Line separations measured in this way are shown to give results consistent with those obtained using a double-lock spectrometer and a nuclear double side-band oscillator.

High resolution nuclear magnetic resonance study of base pairing in four purified transfer RNA molecules

The high resolution nuclear magnetic resonance spectra of hydrogen bonded protons in four purified tRNA molecules are reported. From the temperature and concentration dependence it is shown that these resonances arise from intramolecular hydrogen bonds.

High Resolution Nuclear Magnetic Resonance Spectra of Bifluoride Ion and its Homologues

The high resolution 1H and 19F nuclear magnetic resonance (n.m.r.) spectra of the four bihalide ions have been observed in aprotic solvents. The 1H shielding is, in every case, 14 p.p.m. less than that in the related molecule. The spin–spin coupling in FHF− is 120.5 ± 0.3 Hz.

Platinum-Acetylene complexes II. Implications of high resolution nuclear magnetic resonance spectra

High resolution PMR spectra were recorded on ten zerovalent platinum complexes of terminal acetylenic alcohols and methylacetylenic alcohols. The results indicate that the complexes are essentialy planar in solution; that the methylacetylenic alcohol complexes dissociate and that these complexes undergo oxidative addition reaction with the solvent (CDCl 3).

Solvation Number of the Electron in Liquid Ammonia. A Nitrogen Magnetic Relaxation Study

The high resolution proton magnetic resonance spectrum of a dilute solution of potassium in liquid ammonia shows a large contribution to the line shape from 14N spin–lattice relaxation through magnetic dipolar interaction with unpaired electrons. The predominant 14N-electron interaction occurs through Fermi contact. From total line shape analyses of the proton spectra recorded at 60 MHz and 100 MHz values were obtained for both τ(e−), the average lifetime of a given 14N in a given electron–ammonia complex, and for N, the average solvation number of the electron. The lifetime varies from approximately 1–2 × 10−12 sec over the concentration and temperature range studied. The solvation number ranges from approximately 20 to 40. These results are most consistent with the lattice vacancy model of the solvated electron.

Resolution Enhancement of High Resolution NMR Spectra of Polymers with the Aid of a Computer

A method of resolution enhancement of high resolution nuclear magnetic resonance spectra of polymers which are poorly resolved due to large local viscosity, is proposed. This method is a suitable subtraction of the second derivative of the spectra from original ones, and one which can be carried out automatically with the aid of a chart reader and a computer. A moderate value for the proportionality constant to the second derivative is obtained as K=1–2. The effect of the separation between representative points at the reading to the computer is also discussed. This procedure is applied to the NMR spectra of poly (methyl propenyl ether).