Temperature dependence of protein backbone motion from carbonyl 13C and amide 15N NMR relaxation

The NMR spin-lattice relaxation rate (R1) and the rotating-frame spin-lattice relaxation rate (R1rho) of amide 15N and carbonyl 13C (13C') of the uniformly 13C- and 15N-labeled ubiquitin were measured at different temperatures and field strengths to investigate the temperature dependence of overall rotational diffusion and local backbone motion. Correlation between the order parameter of the N-H vector, SNH2, and that of the carbonyl carbon, S2C', was investigated. The effective S2C' was estimated from the direct fit of the experimental relaxation rates and from the slope of 2R2-R1 vs. B2 using Lipari-Szabo formalism. The average SNH2 decreased by 5.9%, while the average S2C' decreased by 4.6% from 15 to 47 degrees C. At the extreme low and high temperatures the difference in the temperature dependence of the order parameters vanishes. At the intermediate temperatures they do not change by the same amount but they follow the same trend. On the same peptide plane along the protein sequence, S2C' and SNH2 are highly correlated. The results suggest that fast local motion experienced at the site of the N-H vector and carbonyl nucleus is more complicated than previously thought and it cannot be easily described by one single type of motion in a broad range of temperature.     

High-resolution NMR spectra in inhomogeneous and unstable fields via the three-pulse method

In modern solution nuclear magnetic resonance (NMR), the spectral resolution is mainly dependent on the spatial homogeneity and temporal stability of the magnetic field. The spectral linewidths are usually proportional to the overall field homogeneity and the stability experienced by the sample. Many high-resolution NMR methods have been developed, but few are applicable in inhomogeneous and unstable fields. In this paper, a high-resolution three-pulse method based on intermolecular zero-quantum coherences (iZQCs) is proposed. Since this method is insensitive to field inhomogeneity and instability, spectral information such as the chemical shift can be retained in the resulting spectra. In comparison with the CPMG-HOMOGENIZED method, the new method provides almost pure solvent–solute iZQC signals.     

A statistical study of the analysis of congested spectra by total spectrum fitting

Heavily overlapped, or congested spectra often display much structure but few individual “lines.” Methods have been devised for analyzing such spectra through nonlinear least-squares fitting of the intensity as a function of wavelength or wavenumber. Such total spectrum fitting (TSF) methods are examined statistically for a simple diatomic model and compared with the standard “measure-assign-fit” (MAF) approach in use since the dawn of spectroscopy. Monte Carlo computations on typically 1000 synthetic spectra at a time verify that the predictions of the variance-covariance matrix are reliable under many circumstances. However in regions where the P and R branches double up, the predicted standard errors in the key spectroscopic constants rise sharply and greatly exceed estimates from the Monte Carlo ensemble statistics. In the same regions, the MAF method actually gives better precision. However, for imperfectly overlapped R and P branches, the MAF standard errors are typically three times larger than the TSF values; moreover, the MAF statistical errors are dwarfed by bias. The TSF approach, while clearly superior in these tests, has a practical drawback: it, too, can give significant bias if the spectra are analyzed with an incorrect model, as illustrated here through calculations employing the wrong function to describe the spectral lineshape.     

NMR spectra of alkylphenols

Journal of Applied Spectroscopy -     

High-frequency modes in magnetic spectra of carbonyl iron

Magnetic spectra of composites filled with carbonyl iron powder were measured at frequencies of up to 30 GHz. Two previously unknown intensive high-frequency peaks of magnetic absorption have been found at frequencies surpassing the ferromagnetic resonance frequency. The peaks may be attributed to Aharoni exchange resonance modes. The resonances are excited inside magnetically isolated nanocrystallites that constitute carbonyl iron grains. Features of homogeneous ferromagnetic resonance in carbonyl iron powder grains are also discussed.     

Frequency-selective heteronuclear dephasing and selective carbonyl labeling to deconvolute crowded spectra of membrane proteins by magic angle spinning NMR

We present a new method that combines carbonyl-selective labeling with frequency-selective heteronuclear recoupling to resolve the spectral overlap of magic angle spinning (MAS) NMR spectra of membrane proteins in fluid lipid membranes with broad lines and high redundancy in the primary sequence. We implemented this approach in both heteronuclear (15)N-(13)C(α) and homonuclear (13)C-(13)C dipolar assisted rotational resonance (DARR) correlation experiments. We demonstrate its efficacy for the membrane protein phospholamban reconstituted in fluid PC/PE/PA lipid bilayers. The main advantage of this method is to discriminate overlapped (13)C(α) resonances by strategically labeling the preceding residue. This method is highly complementary to (13)C(i-1)(')-(15)N(i)-(13)C(i)(α) and (13)C(i-1)(α)-(15)N(i-1)-(13)C(i)(') experiments to distinguish inter-residue spin systems at a minimal cost to signal-to-noise.     

13C NMR in the Structural Assignment of Fused Furoxans

The chemical shifts and multiplicities of the two bridgehead carbons in the ¹³C NMR spectra of various fused furoxans are snown to provide a general method for assigning structure in these tautomeric systems.     

Resolving ambiguities in two-dimensional NMR spectra: the 'TILT' experiment

Ambiguities in two-dimensional nuclear magnetic resonance spectra due to overlap are usually resolved by recording a three-dimensional version of the experiment. It is shown that a simpler solution is to record a tilted projection of the three-dimensional spectrum, derived by Fourier transformation of the time-domain signal acquired while the two evolution parameters are varied simultaneously at the appropriate rates. By avoiding the need to record the full three-dimensional spectrum, this saves an order of magnitude in measurement time. The tilt technique is illustrated by reference to degenerate responses in the TOCSY and NOESY spectra of a small protein, agitoxin, where the 1H and 15N frequencies are incremented in tandem.     

Amino acid type identification in NMR spectra of proteins via beta- and gamma-carbon edited experiments

In this work, we introduce a set of pulse sequences that provide amino acid type identification of the NH correlation signals of proteins. The first pulse sequence is a modification of the CBCA(CO)NH experiment that exploits spin-coupling topologies to differentiate between amino acid types. A set of eight 2D ¹H–¹⁵N correlation spectra is recorded where the sign of the cross-peaks change from one spectrum to another according to the amino acid type of the preceding residue in the protein sequence. Linear combination of these eight data sets produces four subspectra. Taking also into account the sign of the correlation signals, this method allows the classification of the NH signals into six different groups, depending on the character of the preceding residue. This sequence is complemented with a (CGCBCACO)NH experiment that allows the subdivision of the largest of these groups into two smaller ones. Finally, a modification of the CBCANH experiment led to a similar classification of NH signals into six different groups, but now depending on the type of its own amino acid. The set of pulse sequences is demonstrated with two proteins of small to moderate size.     

Crystallographically-based analysis of the NMR spectra of maghemite

All possible iron environments with respect to nearest neighbour vacancies in vacancy-ordered and vacancy-disordered maghemite have been evaluated and used as the foundation for a crystallographically-based analysis of the published NMR spectra of maghemite. The spectral components have been assigned to particular configurations and excellent agreement obtained in comparing predicted spectra with published spectra taken in applied magnetic fields. The broadness of the published NMR lines has been explained by calculations of the magnetic dipole fields at the various iron sites and consideration of the supertransferred hyperfine fields.     

Assignment of bis(methylenedithio)tetrathiafulvalene vibrational spectra

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 52, No. 4, pp. 622–627, April, 1990.     

Zero-field NMR. Calculation of spectra

Zero-field NMR spectra for a number of cases in single crystals and powders are calculated and some factors influencing resolution are discussed.     

Assignment of continuous spectra by photofragment spectroscopy: C state of I2

Photofragment spectroscopy has been used to probe the C state of iodine by determining its symmetry and dissociation products. By crossing a molecular beam with a powerful pulsed laser beam in high vacuum and monitoring the arrival times of the recoiling photofragments with a mass spectrometer detector, the translational energy distribution of the photodis-sociation products is measured. From energy balance the fragment internal energy is then calculated. Our peak indicates that the C state dissociates to one excited and one ground state atom. The symmetry of the transition is determined by rotating the plane of polarization of the laser light and measuring a fragment angular distribution. The transition was found to be 1 ← 0. The results are consistent with Mulliken's assignment of the C state as 1441 $\text{1}{}_{\mathrm{u}}\text{(}{}^3\text{Σ}{}^{\mathrm{+}}\text{)}$, but inconsistent with the earlier assignment of 2332 $\text{0}{}_{\mathrm{u}}{}^{\mathrm{?}}\text{(}{}^3\text{Σ}{}^{\mathrm{+}}\text{)}$ by Mathieson and Rees.     

NMR spectra of androstane analogs of brassinosteroids

We have used two-dimensional NMR spectroscopy to make a complete assignment of signals from the nuclei of hydrogen and carbon atoms in the spectra of brassinosteroids in the androstane series. We have confirmed the stereochemistry of the chiral centers and the structure of the molecules. We have studied the effect of the configuration of the 2,3-diol groups in the A ring of the steroids on the chemical shift of adjacent atoms in the ¹³C and ¹H NMR spectra. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 583–590, September–October, 2007.     

Water enrichment by H2O ortho-isomer: Four-photon and NMR spectroscopy

Ratio of H₂O ortho-/para-spin-isomers in water of different treatment procedures (distilled or cavitation fountain) were studied by both Rayleigh wing four-photon spectroscopy and ¹H nuclear magnetic resonance (NMR) spectroscopy. Low-frequency gas-like rotational resonances were observed in the 0.1–1.5 cm^?1 (3–45 GHz) spectral range and NMR proton density was measured in both water samples. We established that the intensity of ortho-isomer line (6₁₆?5₂₃)0.74 cm^?1 measured by four-photon spectroscopy increases by factor of ～3.5 after cavitation treatment of distilled water. Moreover, the proton density measured by NMR spectroscopy in the same sample grows on ～17%. We have suggested that the enrichment of the distilled water by ortho-H₂O molecules was achieved due to cavitation bubbles collapse when the water passes through the supercritical state.