Density functional theory study of N–H⋯O, O–H⋯O and C–H⋯O hydrogen-bonding effects on the 14N and 2H nuclear quadrupole coupling tensors of N-acetyl-valine

Nuclear relaxation rate enhancement by a 14N quadrupole nucleus in a fluctuating electric-field gradient

We consider the longitudinal quadrupole relaxation rate enhancement (QRE) of a ¹H nucleus due to the time fluctuations of the local dipolar magnetic field created by a close quadrupole ¹⁴N nucleus, the electric-field gradient (EFG) Hamiltonian of which changes with time because of vibrations/distortions of its chemical environment. The QRE is analytically expressed as a linear combination of the cosine Fourier transforms of the three quantum time auto-correlation functions G_AA(t) of the ¹⁴N spin components along the principal axes A = X, Y, and Z of the mean (time-averaged) EFG Hamiltonian. Denoting the three transition frequencies between the energy levels of this mean Hamiltonian by ν_A, the functions G_AA(t) oscillate at frequencies ν_A + s_A/(2π) with mono-exponential decays of relaxation times τ_A, where the frequency dynamic shifts s_A and the relaxation times τ_A are closed expressions of the magnitude of the fluctuations of the instantaneous EFG Hamiltonian about its mean and of the characteristic fluctuation time. Thus, the theoretical QRE is the sum of three Lorentzian peaks centered at ν_A + s_A/(2π) with full widths at half maxima 1/(πτ_A). The predicted peak widths are nearly equal. The predicted dynamic shifts of the peaks are much smaller than their widths and amazingly keep proportional to the transition frequencies ν_A for reasonably fast EFG fluctuations. The theory is further improved by correcting the transition frequencies by the ¹⁴N Zeeman effects of second order. It is successfully applied to reinterpret the QRE pattern measured by Broche, Ashcroft, and Lurie [Magn. Reson. Med. 68, 358 (2012)] in normal cartilage.

Quantitative analysis of 14N quadrupolar coupling using 1H detected 14N solid-state NMR

Magic-angle spinning solid-state NMR is increasingly utilized to study the naturally abundant, spin-1 nucleus 14N, providing insights into the structure and dynamics of biological and organic molecules. In particular, the characterisation of 14N sites using indirect detection has proven useful for complex molecules, where the 'spy' nucleus provides enhanced sensitivity and resolution. Here we exploit the sensitivity of proton detection, to indirectly characterise 14N sites using a moderate rf field to generate coherence between the 1H and 14N at moderate and fast-magic-angle spinning frequencies. Efficient numerical simulations have been developed that have allowed us to quantitatively analyse the resulting 14N lineshapes to determine both the size and asymmetry of the quadrupolar interaction. Exploiting only naturally occurring abundant isotopes will aid the analysis of materials with the need to resort to isotope labelling, whilst providing additional insights into the structure and dynamics that the characterisation of the quadrupolar interaction affords.

Theoretical investigation of NMR–NQR tensors of hallucinogenic harmine in monomeric and cluster states

In the present study, density functional theory was employed to analyze the structure and nuclear magnetic resonance (NMR) — nuclear quadrupole resonance (NQR) spectra of hallucinogenic harmine in monomeric, dimeric, trimeric, and tetrameric states in the gas phase. Furthermore, the effects of hydrogen and resonance interactions on the values of NMR and NQR parameters of nitrogen nuclei in the four states mentioned above were investigated. The computations at the B3LYP/6-311[Formula: see text]G** level of theory indicated that NQR — NMR parameters of nitrogen nuclei varied for each of the four states and were strongly affected by chemical environment, molecular cluster size and molecular interactions. Accordingly, by increasing the participation of lone pair electrons in resonance interactions and aromaticity development, the values of NMR chemical shielding around them increased, whereas their NQR parameters ([Formula: see text] and [Formula: see text] decreased. In contrast, it could be observed that resonance interaction was not the only effective factor influencing changes in values and trends of NMR — NQR parameters by passing from monomeric state to other ones. Moreover, the negative charge on nitrogen atoms and the possibility of hydrogen bond formation were other important factors influencing NMR — NQR parameters.

Cooperative interaction between hydrogen bond and N···Y interactions (Y = H, Li, F, Cl, and Br): a comparative study

A comparative ab initio study is performed to investigate the cooperativity between the N···H hydrogen bond and the N···Y interactions in XCN···HCN···YCN complexes, where X = H, F, and Y = H, Li, F, Cl, and Br. To understand the properties of the systems better, the corresponding dimers are also studied. It is found that the lithium bond has a larger influence on the hydrogen bond than vice versa. The shortening of the N···H distances in the trimers is dependent on the strength of the H···Y interactions and they become larger in the order lithium bond > hydrogen bond > halogen bond. The estimated values of cooperative energy Ecoop are all negative with much larger Ecoop in absolute value for the systems including lithium.

Cooperative effects in hydrogen bond and pnicogen bond: a comparative study

A comparative study of the cooperative effects of hydrogen and pnicogen bonding on open-chain clusters of (PH2CN)n=2–7 and (HCN)n=2–7 is performed at the MP2/6-311++G(d,p) level of theory. These effects are studied in terms of geometric and energetic properties, electron density analysis, and 15N chemical shielding parameters of the clusters at the MP2/6-311++G** level. The intermolecular distances observed in the (HCN)n clusters exhibit quite larger bond contractions than those found in the (PH2CN)n clusters. Our results strongly suggest that cooperative effects induced by pnicogen and hydrogen bonds are significant in both linear PH2CN and HCN clusters, respectively. They also provide some evidence that these effects seem to reach a limit for a relatively small number of monomers. The n-dependent variation in the 15N chemical shielding tensor should serve as a useful signature of cooperativity effects in the PH2CN and HCN clusters.

Mechanism of 1H-14N cross-relaxation in immobilized proteins

A resonant enhancement of the water-1H relaxation rate at three distinct frequencies in the range 0.5-3 MHz has been observed in a variety of aqueous biological systems. These so-called quadrupole (Q) peaks have been linked to a dipolar flip-flop polarization transfer from 1H nuclei to rapidly relaxing amide 14N nuclei in rotationally immobilized proteins. While the Q-peak frequencies conform to the known amide 14N quadrupole coupling parameters, a molecular model that accounts for the intensity and shape of the Q peaks has not been available. Here, we present such a model and test it against an extensive set of Q-peak data from two fully hydrated crosslinked proteins under conditions of variable temperature, pH and H/D isotope composition. We propose that polarization transfer from bulk water to amide 14N occurs in three steps: from bulk water to a so-called intermediary proton via material diffusion/exchange, from intermediary to amide proton by cross-relaxation driven by exchange-mediated orientational randomization of their mutual dipole coupling, and from amide proton to 14N by resonant dipolar relaxation 'of the second kind', driven by 14N spin fluctuations, which, in turn, are induced by restricted rigid-body motions of the protein. An essentially equivalent description of the last step can be formulated in terms of coherent 1H-->14N polarization transfer followed by fast 14N relaxation. Using independent structural and kinetic information, we show that the Q peaks from these two proteins involve approximately 7 intermediary protons in internal water molecules and side-chain hydroxyl groups with residence times of order 10(-5) s. The model not only accounts quantitatively for the extensive data set, but also explains why Q peaks are hardly observed from gelatin gels.