Chemical Shifts Of Methyl Protons Research Articles

NMR Assignment of Methyl Groups in Immobilized Proteins Using Multiple-Bond 13C Homonuclear Transfers, Proton Detection, and Very Fast MAS.

In nuclear magnetic resonance spectroscopy of proteins, methyl protons play a particular role as extremely sensitive reporters on dynamics, allosteric effects, and protein–protein interactions, accessible even in high-molecular-weight systems approaching 1 MDa. The notorious issue of their chemical shift assignment is addressed here by a joint use of solid-state 1H-detected methods at very fast (nearly 100 kHz) magic-angle spinning, partial deuteration, and high-magnetic fields. The suitability of a series of RF schemes is evaluated for the efficient coherence transfer across entire 13C side chains of methyl-containing residues, which is key for establishing connection between methyl and backbone 1H resonances. The performance of ten methods for recoupling of either isotropic 13C–13C scalar or anisotropic dipolar interactions (five variants of TOBSY, FLOPSY, DIPSI, WALTZ, RFDR, and DREAM) is evaluated experimentally at two state-of-the-art magic-angle spinning (55 and 94.5 kHz) and static magnetic field conditions (18.8 and 23.5 T). Model isotopically labeled compounds (alanine and Met-Leu-Phe tripeptide) and ILV-methyl and amide-selectively protonated, and otherwise deuterated chicken α-spectrin SH3 protein are used as convenient reference systems. Spin dynamics simulations in SIMPSON are performed to determine optimal parameters of these RF schemes, up to recently experimentally attained spinning frequencies (200 kHz) and B 0 field strengths (28.2 T). The concept of linearization of 13C side chain by appropriate isotope labeling is revisited and showed to significantly increase sensitivity of methyl-to-backbone correlations. A resolution enhancement provided by 4D spectroscopy with non-uniform (sparse) sampling is demonstrated to remove ambiguities in simultaneous resonance assignment of methyl proton and carbon chemical shifts.

Doubly Strapped Redox-Switchable 5,10,15,20-Tetraaryl-5,15-diazaporphyrinoids: Promising Platforms for the Evaluation of Paratropic and Diatropic Ring-Current Effects.

This paper presents a novel series of chemically stable and redox-switchable 20π, 19π, and 18π 5,10,15,20-tetraaryl-5,15-diazaporphyrinoids (TADAPs) that have two alkyl-chain straps above and below the diazaporphyrin ring. Three types of doubly strapped TADAPs were prepared as nickel(II) complexes using meso-N-(2,6-dihydroxyphenyl)-substituted TADAP and the corresponding aliphatic diacids as precursors. Theoretical calculations revealed that regardless of their oxidation states, all strapped TADAPs had essentially flat π-planes. It was found that the alkyl-chain straps slightly affected the optical and electrochemical properties of the DAP rings, particularly in the oxidized forms. 1H NMR spectroscopy was used to evaluate the antiaromatic character of the 20π TADAPs and the aromatic character of the 18π TADAP dications, and it was observed that they displayed paratropic and diatropic ring-current effects, respectively, on the chemical shifts of methylene protons in the spatially separated alkyl chains. The degree of shielding and deshielding depended on the position of the methylene units; it decreased with increase in separation from the π-plane and central axis of the porphyrin ring. The NMR experiments also revealed that the degree of the diatropic ring currents was clearly related to the π-electron density of the porphyrin ring; the ring-current effects decreased as the charge increased from 0 to +2. These findings are also qualitatively supported by the nucleus-independent chemical shifts.

Origins of Small Proton Chemical Shift Differences in Monodeuterated Methyl Groups.

We have recently shown that the small proton chemical shift difference in 2-methyl-1-(methyl-d)piperidine supports a long-lived nuclear spin state. To identify additional candidate molecules with CH2D groups exhibiting accessible long-lived states, and to investigate the factors governing the magnitude of the shift differences, we report a computational and experimental investigation of methyl rotational equilibria and proton chemical shifts in a variety of 2-substituted 1-(methyl-d)piperidines. The polarity and size of the 2-substituent affect the 1,2-stereoisomeric relationship, and consequently, the strength of the rotational asymmetry within the CH2D group. Nonpolar and large 2-substituents prefer the equatorial position, and relatively large shift differences (i.e., > 13 ppb) are observed. Polar and small substituents, however, increasingly prefer the axial position, and medium to small shift differences (i.e., 0 to 9 ppb) are observed. In addition, the diastereotopic CH2D proton chemical shift difference for tricarbonyl(1-chloro-2-deuteriomethylbenzene) chromium(0) was computed, showing that reasonable predictions of these small shift differences can be extended to more complex, organometallic species.

Probing slow timescale dynamics in proteins using methyl 1H CEST.

Although 15N- and 13C-based chemical exchange saturation transfer (CEST) experiments have assumed an important role in studies of biomolecular conformational exchange, 1H CEST experiments are only beginning to emerge. We present a methyl-TROSY 1H CEST experiment that eliminates deleterious 1H-1H NOE dips so that CEST profiles can be analyzed robustly to extract methyl proton chemical shifts of rare protein conformers. The utility of the experiment, along with a version that is optimized for 13CHD2 labeled proteins, is established through studies of exchanging protein systems. A comparison between methyl 1H CEST and methyl 1H CPMG approaches is presented to highlight the complementarity of the two experiments.

Specific 13C labeling of leucine, valine and isoleucine methyl groups for unambiguous detection of long-range restraints in protein solid-state NMR studies

Here we present an isotopic labeling strategy to easily obtain unambiguous long-range distance restraints in protein solid-state NMR studies. The method is based on the inclusion of two biosynthetic precursors in the bacterial growth medium, α-ketoisovalerate and α-ketobutyrate, leading to the production of leucine, valine and isoleucine residues that are exclusively 13C labeled on methyl groups. The resulting spectral simplification facilitates the collection of distance restraints, the verification of carbon chemical shift assignments and the measurement of methyl group dynamics. This approach is demonstrated on the type-three secretion system needle of Shigella flexneri, where 49 methyl–methyl and methyl–nitrogen distance restraints including 10 unambiguous long-range distance restraints could be collected. By combining this labeling scheme with ultra-fast MAS and proton detection, the assignment of methyl proton chemical shifts was achieved.

PPM: a side-chain and backbone chemical shift predictor for the assessment of protein conformational ensembles

The combination of the wide availability of protein backbone and side-chain NMR chemical shifts with advances in understanding of their relationship to protein structure makes these parameters useful for the assessment of structural-dynamic protein models. A new chemical shift predictor (PPM) is introduced, which is solely based on physical-chemical contributions to the chemical shifts for both the protein backbone and methyl-bearing amino-acid side chains. To explicitly account for the effects of protein dynamics on chemical shifts, PPM was directly refined against 100 ns long molecular dynamics (MD) simulations of 35 proteins with known experimental NMR chemical shifts. It is found that the prediction of methyl-proton chemical shifts by PPM from MD ensembles is improved over other methods, while backbone Cα, Cβ, C', N, and H(N) chemical shifts are predicted at an accuracy comparable to the latest generation of chemical shift prediction programs. PPM is particularly suitable for the rapid evaluation of large protein conformational ensembles on their consistency with experimental NMR data and the possible improvement of protein force fields from chemical shifts.

DFT and experimental prediction of negative chemical shifts of methyl protons in some piperidines

The high resolution 1H NMR spectra of four 3-ethyl-4-hydroxy-4-phenylpiperidines 1–4 have been recorded in CDCl3, C6D6, and (CD3)2CO and analysed. These compounds exist in chair conformation with axial orientation of hydroxyl group at C(4) and equatorial orientation of the remaining substituents. The chemical shifts of methyl protons of ethyl group are quiet novel and surprising and are closer to TMS in CDCl3 and somewhat higher in C6D6. Changing the solvent from CDCl3 to (CD3)2CO alters the chemical shifts of these protons and they are very closer to TMS (0.01ppm) in 3–4 and negative in 2 i.e., less than zero. The results are interpreted in terms of the magnetic anisotropic effect of the phenyl rings at C(2) and C(4) which in turn depends on the conformations of the ethyl group at C(3) and hydroxyl group at C(4). DFT studies were also performed to predict the favoured conformations of ethyl group at C(3) and hydroxyl group at C(4). Chemical shifts were also computed theoretically in the favoured conformation and they also support negative chemical shifts of 2 in acetone-d6.

Lignin as a cross-linker of acrylic acid-grafted carboxymethyl lignocellulose

Cunninghamia lanceolata wood meal samples with different lignin contents after delignification with an acidic NaClO2 system were carboxymethylated, and the degree of substitution (DS) and the distribution of the carboxymethyl (CM) groups were investigated by proton nuclear magnetic resonance (1H NMR) spectroscopy. Cellulose samples prepared from bleached kraft softwood pulp, food-grade konjac mannan, and commercial oat xylan (containing 10% arabinosyl and 15% glucosyl residues) were also investigated. The chemical shift of methylene protons in 1H NMR spectra of CM groups of carboxymethyl konjac mannan and commercial oat xylan appeared in the same region as those of carboxymethylcellulose. The DS of carboxymethyl lignocellulose (CMLC) increased slightly from 1.36 to 1.48 with decreasing lignin content, but the water solubility of CMLC clearly increased with decreasing lignin content. It was suggested that the covalent linkages between lignin and cell-wall polysaccharides play the role of cross-linker in CMLC. Water absorbents were synthesized by graft-copolymerization of acrylic acid onto CMLC samples with different lignin contents. The highest level of water absorbency was obtained from CMLC containing 14% of lignin, suggesting the importance of lignin as the cross-linker.

1H-NMR studies on the ternary complexes of rare-earth ions with thenoyltrifluoroacetone and polyethers in dichloromethane

The structures of the ternary complexes of lanthanoid and yttrium (Ln3+)-thenoyltrifluoroacetonates (tta-) with polyether (POE) in organic phase were investigated by 1H-NMR spectroscopy, where the POEs are crown ethers (18-crown-6 and benzo-18-crown-6) and monodispersed linear polyethers (DEOn: HO-(CH2CH2O-)nC12H25, where n=4, 6, 8). The changes in chemical shift of methylene protons of POE by addition of the adduct complex [Ln(tta)3(POE)] were measured at various Ln3+-to-POE concentration ratios. Chemical shift changes for each proton of POE by the formation of [Ln(tta)3(POE)] were determined. Results revealed that oxygen atoms at the hydroxyl terminal of linear POE have higher tendency to coordinate to the metal ion in [Ln(tta)3] complex. Three (for La3+) or two (for Lu3+ or Y3+) oxygen atoms of the POE coordinate to the metal ion without substitution of tta- ligands to satisfy the metal ion's coordination number of nine or eight, respectively. In the case of 18-membered crown ether complexes, La3+ is incorporated inside the cavity of the POE, displacing one of the three tta- from the inner coordination sphere while the other two remain coordinated to the metal ion. On the other hand, for the adduct of Y3+ complex with crown ether, all three tta- ligands are directly coordinating to the metal ion.

Preferential binding and structural distortion by Fe2+ at RGGG-containing DNA sequences correlates with enhanced oxidative cleavage at such sequences

Certain DNA sequences are known to be unusually sensitive to nicking via the Fe2+-mediated Fenton reaction. Most notable are a purine nucleotide followed by three or more G residues, RGGG, and purine nucleotides flanking a TG combination, RTGR. Our laboratory previously demonstrated that nicking in the RGGG sequences occurs preferentially 5′ to a G residue with the nicking probability decreasing from the 5′ to 3′end of these sequences. Using 1H NMR to characterize Fe2+ binding within the duplex CGAGTTAGGGTAGC/GCTACCCTAACTCG and 7-deazaguanine-containing (Z) variants of it, we show that Fe2+ binds preferentially at the GGG sequence, most strongly towards its 5′ end. Substitutions of individual guanines with Z indicate that the high affinity Fe2+ binding at AGGG involves two adjacent guanine N7 moieties. Binding is accompanied by large changes in specific imino, aromatic and methyl proton chemical shifts, indicating that a locally distorted structure forms at the binding site that affects the conformation of the two base pairs 3′ to the GGG sequence. The binding of Fe2+ to RGGG contrasts with that previously observed for the RTGR sequence, which binds Fe2+ with negligible structural rearrangements.

Student-Determined Values for the Calculation of Chemical Shifts of Methylene Protons in Different Chemical Environments

Many tables of values exist for the rapid calculation of approximate chemical shifts of protons in organic molecules. These tables are usually simply handed to students to use without discussion of the source of such data. Herein we describe a laboratory experiment in which students themselves determine values for the calculation of chemical shifts of methylene protons in a variety of chemical environments. Although these values come from single determinations of representative compounds from several different classes, the alpha-parameter values obtained are in close agreement with literature data. Students also gain valuable hands-on experience preparing samples, operating the NMR spectrometer, and interpreting the 1H NMR spectra of a variety of compounds.

Assignment of heme methyl 1H-NMR resonances of high-spin and low-spin ferric complexes of cytochrome p450cam using one-dimensional and two-dimensional paramagnetic signals enhancement (PASE) magnetization transfer experiments.

An 1H-NMR study of ferric cytochrome P450cam in different paramagnetic states was performed. Assignment of three heme methyl resonances of the isocyanide adduct of cytochrome P450 in the ferric low-spin state was recently performed using electron exchange in the presence of putidaredoxin [Mouro, C., Bondon, A., Jung, C., Hui Bon Hoa, G., De Certaines, J.D., Spencer, R.G.S. & Simonneaux, G. (1999) FEBS Lett. 455, 302-306]. In this study, heme methyl protons of cytochrome P450 in the native high-spin and low-spin states were assigned through one-dimensional and two-dimensional magnetization transfer spectroscopy using the paramagnetic signals enhancement (PASE) method. The order of the methyl proton chemical shifts is inverted between high-spin and low-spin states. The methyl order observed in the ferric low-spin isocyanide complexes is related to the orientation of the cysteinate ligand.

A DFT Study of the Camphene Hydrochloride Rearrangement.

The rearrangement of camphene hydrochloride to isobornyl chloride is one of the best studied among the many early examples of Wagner-Meerwein rearrangement processes. However, many aspects of the reaction remain to be elucidated. A summary of early mechanistic studies is given. A proposal of new mechanistic possibilities is set forth and then tested by density functional theory (DFT) calculations. The uncertainties in configurations of camphene hydrochloride and isobornyl chloride are laid to rest by a comparison of computed and experimental carbon-13 and methyl proton chemical shifts.

Mono-, bi- and polynuclear complexes of dimethyldiphenylsilane, diphenylether and biphenyl with Cr, Co and Ru. Synthesis and 1H, 13C, 17O and 29Si NMR spectroscopic characterization

Mono- and bimetallic transition metal carbonyls of dimethyldiphenylsilane, Ph 2Si(CH 3) 2 (L1), diphenylether, Ph 2O (L2) and biphenyl, Ph 2 (L3): Ph 2Si(CH 3) 2Cr(CO) 3 ( 1), Ph 2Si(CH 3) 2[Cr(CO) 3] 2 ( 2), Ph 2Si(CH 3) 2Co 4(CO) 9 ( 3), Ph 2Si(CH 3) 2[Co 4(CO) 9] 2 ( 4), Ph 2Si(CH 3) 2Cr(CO) 3Co 4(CO) 9 ( 5), Ph 2OCr(CO) 3 ( 6), Ph 2OCo 4(CO) 9 ( 7), Ph 2ORu 6C(CO) 14 ( 8), Ph 2OCr(CO) 3Co 4(CO) 9 ( 9) and Ph 2Ru 6C(CO) 14 ( 10) have been prepared and characterized by means of 1H, 13C, 17O and 29Si NMR spectroscopy. The same order of the shielding effects induced by the transition metal (Co < Ru < Cr) on the 1H NMR chemical shifts of the η 6-coordinated ring as observed earlier for diphenylmethane is also valid for the present bisarene aromatic ligands containing hetero-atoms in their structures. Differing from the behaviour of the 1H NMR chemical shifts of methylene protons of the diphenylmethane complexes, δ( 1H) of Si(CH 3) 2 group in dimethyldiphenylsilane always shows deshielded values consistent with π-coordination. Similarly, δ( 29Si) of Si(CH 3) 2 group is monotonously deshielded with the increasing degree of the complexation. As for diphenylmethane derivatives, δ( 17OC) of the present chromium tricarbonyl derivatives always shows easy-to-detect spectral characteristics, the shifts varying between 372.8 and 374.5 ppm. The only value obtained for tetracobalt nonacarbonyl derivatives was that of 7, δ( 17OC) = 349 ppm. The same order of δ( 17OC) as observed for the mesitylene transition metal carbonyls (Co < Cr < Ru) is also valid for the present polyarene compounds. The inverse relation between δ( 13CO) and δ( 17OC) in Ru and Co derivatives can be explained by the effect of π-backbonding. Diphenylether derivatives were not thermally stable enough to determine accurately δ( 17O) of the ethereal oxygen for all derivatives prepared. From the synthetic point of view, the present bisarene ligands were not so easily transformed to their π-coordinated derivatives as diphenylmethane.

N‐acetyl‐L‐aspartate and acetate 1H NMR signal overlapping under mild acidic pH conditions

The pH dependence of methyl proton chemical shifts of acetate, acetoacetate, N-acetyl-L-aspartate (NAA), and N-acetyl-L-aspartyl-L-glutamate (NAAG) were studied from pH 3 to pH 9. Only slight shifts of acetoacetate, NAA, and NAAG methyl signals were observed, whereas the acetate signal was largely shifted as a result of the titration of its acidic function. At pH 4.7, acetate and NAA methyl signals overlapped, whereas at more acidic pH, the acetate signal appeared downfield when compared to that of NAA. Results are discussed in terms of spectra misinterpretation risks linked to uncontrolled sample pH, on the one hand, and in terms of pH control and contamination by exogenous acetate during perchloric acid cell extract preparation, on the other.

1H Nmr Spectra of Penylated Flavonoids and Pyranoflavonoids

1 H Nmr examination of prenylated flavonoids has shown that the presence of the prenyl group at C-3, C-6, or C-8 position of flavone can be deduced from the chemical shift of methylene protons of prenyl group measured in acetone-d 6 . The chemical shifts of olefinic proton on pyran ring and A ring proton can be used to distinguish between linear type pyranoflavone and angular type pyranoflavone. The application of this 1 H nmr method to the identification of Atalantia flavone and Artocarpus flavones is discussed

Fluorine-19 nuclear magnetic resonance studies of mixed bis(η 6-arene)chromium(0) complexes

19F NMR studies have been conducted on mixed bis(η 6-arene)chromium(0) complexes containing (CH 3) m ( m=1, 2 or 3) substituents in one of the partner arenes and CF 3 or F n , ( n=1 or 2) substituents in the other arene. There is deshielding of the 19F in the (C 6H 6− m (CH 3) m )-(C 6H 5CF 3)Cr complex ( I) relative to that in (C 6H 5CF 3) 2Cr( II). However, increased shielding of 19F is observed in the (C 6H 6− m (CH 3) m)-(C 6H 6− n F n )Cr complex ( III) relative to that in (C 6H 6− n F n ) 2Cr ( IV). It appears that the fluorines in CF 3 take on an unique behaviour which is affected by a complex series of electron charge movements mainly due to more back-donation towards the (C 6H 6− m (CH 3) m arene and relaying of effects of substituents of one arene on to those of the partner arene by the chromium centre. On the other hand, the upfield shift of the 19F resonance in III, is rationalized partly on the basis of greater back-donation to the F n-containing arene and electron-releasing effect of the (CH 3) m that is transmitted by the metal centre. Evidence from two different methyl proton chemical shifts in (1,2,4-C 6H 3(CH 3) 3)(1,2-C 6H 4F 2)Cr suggests that the through space dipole interactions are an insignificant factor in the electron charge perturbations in these mixed-arene complexes.

Conformation of ring A in triterpenoid and 4,4-dimethylsteroid 3-ketones. Chemical shifts of methyl protons and lanthanide and benzene induced shifts

Chemical shifts of signals due to methyl groups in position 10β (in CDCl3) and 4α and 4β (in C6D6) in 1H NMR spectra of pentacyclic triterpene 3-oxo derivatives (V, VIII, IX, and XII) are suitable for estimation of chair-boat equilibrium in the ring A. Benzene and lanthanide induced shifts of 4α and 4β-methyl protons were also used for this purpose. The results obtained with 2α-methyl-3-ketones (III, X) and 2β-methyl-3-ketones (IV, XI) as the respective chair and boat models agree well with those derived from other physical data (about 40% boat). The same methods were applied to 4,4-dimethylsteroid 3-ketones XV-XVII.

S-substituted mercaptoacetic esters as anticorrosion additives to lubricating oils

1. 1. S-substituted mercaptoacetic esters possess high anticorrosion properties and are more effective than corresponding derivatives of mercaptopropionic esters and unsubstituted sulphides. 2. 2. The linear dependence of chemical shifts of methylene protons of the —SCH 2— COO group of S-substituted mercaptoacetic esters on the induction constants of the substituent has been found. This is characterized by a fairly high correlation coefficient ( r=0·98) and indicates that the effect of substituent R at the sulphur atom on screening of methylene protons is communicated by the system of chemical bonds mainly by an induction mechanism. 3. 3. The combined presence of the sulphide sulphur atom and an ester group leads to an increase in the anticorrosion activity of the sulphides on account of the electronic interactions arising between these groups and occurring by a π-induction mechanism.

Proton magnetic resonance studies of substituted 3,5‐dimethylbenzenes: Influence of substituents on the methyl proton chemical shifts

AbstractProton NMR data for a number of 1‐X‐4‐Y‐3,5‐dimethylbenzenes and 1‐X‐2‐Y‐4‐Z‐3,5‐dimethylbenzenes are presented. The proton chemical shifts of the methyl groups at positions 3 and 5 are mainly affected by the ring current effect of the substituents. This conclusion was strongly supported by the proton chemical shifts calculated using additivity rules.