To simulate the types of coordination and solution structures of the active site of haloperoxidases, the interaction systems between diperoxovanadate complexes [OV(O2)2L]n-(n=1 or 3, L=oxalate or H2O) and a series of histidine-like ligands in solution have been studied by using 1D multinuclear (1H, 13C, and 51V) NMR, 2D diffusion ordered spectroscopy, and variable-temperature NMR in 0.15 mol/L NaCl ionic medium, representing the physiological conditions of human blood. Some direct NMR data are given for the first time. The reactivity among the histidinelike ligands is imidazole>2-methylimidazole>carnosine≈4-methylimidazole>histidine. Competitive coordinationinteractions result in a series of new peroxovanadate species [OV(O2)2L']- (L'=histidine-like ligands). When theligands are 4-methylimidazole, histidine, and carnosine, a pair of isomers have been observed, which are attributedto different types of coordination between vanadium atom and ligands. The results of density functional theorycalculations provided a reasonable explanation on the relative reactivity of the histidine-like ligands and the molarratios of isomers. Theoretical results signify the importance of the solvation effect for the reactivity and stability of the interaction systems.

To date, both the intermolecular multiple-quantum coherence (MQC) and demagnetizing field models have led to fully quantitative predictions of NMR signals in a highly polarized system using the CRAZED and similar sequences. In this paper, measurements of apparent MQC diffusion rates, Dn app, for a specific apparent coherence order, n, were used to investigate the equivalent between the intermolecular MQC and demagnetizing field treatments. A number of physical effects were analyzed both theoretically and experimentally. These effects include molecular diffusion, variation in dipolar correlation distance, radiation damping, inhomogeneous broadening, and spin relaxation, all of which may alter the NMR signal. Two variations of a two-pulse CRAZED sequence, where the signal attenuation is almost entirely caused by the diffusion weighting, were designed to accurately measure and characterize Dn app during the evolution period. Apparent diffusion rates were extracted from a least-squares fitting of a series of 1H spectra, measured with varying diffusion weighting factors. Complete theoretical formations were explicitly derived from both the intermolecular MQC and demagnetizing field treatments. Numerical simulations based on the demagnetizing field treatment were performed and it was found that the model can be used to predict the apparent diffusion rates. A novel diffusion model for intermolecular MQC is proposed in which the phase shift of each individual spin on different molecules is considered to be uncorrelated. This model successfully predicts the unconventional diffusion behaviors of intermolecular MQCs, specifically for differences of apparent diffusion rates between inter-and intramolecular MQCs. Our theoretical predictions and experimental confirmation demonstrate, for the first time, that Dn app for intermolecular MQCs of order n are characterized by Dn app5nDT for n>2 and D0 app52DT for n50, where DT is the translational molecular diffusion rate of the single quantum coherences. These results do not coincide with Dn5n2DT for n>0 which is a general relationship for an intramolecular n-quantum coherence. These works about the apparent diffusion rates during the evolution period of the CRAZED sequences provide additional evidence to support the argument of the equivalence between the intermolecular MQC and demagnetizing field models. The general results derived from both intermolecular MQC and demagnetizing field treatments in this report can reasonably explain new observations of diffusion phenomena in nonlinear spin echoes by Kimmich and co-workers. Even though the theoretical prediction about intermolecular MQC diffusion is verified only with specific experiments using tailor-made pulse sequences, it is demonstrated that the function dependence of diffusion rate on coherence order is general. These results provide independent evidence to support the intermolecular MQC theory proposed by Warren and co-workers.

Although the theories and potential applications of intermolecular multiple-quantum coherences iMQCs have been under active investigations for over a decade, discussion of iMQC NMR signal formation was mainly confined in the time domain. In this paper, a full line-shape theory was developed to describe iMQC signals in the frequency domain. Relevant features of the line shape, such as peak height, linewidth, and phase, were investigated in detail. Predictions based on the theory agree well with experimental and simulated results. Since radiation-damping effects always couple with iMQCs in highly polarized liquid-state NMR systems, and strongly radiation-damped signals have many spectral characteristics similar to those of iMQCs, a detailed comparison was also made between them from different spectral aspects. With detailed comparison of peak height, linewidth, and phase, this work demonstrates that the iMQC and radiation-damping phenomena result from two completely different physical mechanisms despite that both present similar signal features and coexist in highly polarized liquid-state NMR systems.

In this paper, signals originating from a pure specific coherence of intermolecular three-spin orders were separated and characterized experimentally in highly polarized two-component spin systems. A modified CRAZED sequence with selective radio-frequency excitation was designed to separate the small signals from the strong conventional single-spin single-quantum signals. General theoretical expressions of the pulse sequence with arbitrary flip angle pulses were derived using dipolar field treatment. The expressions were used to predict the relaxation and diffusion properties and optimal xperimental parameters such as flip angles. For the first time, relaxation and diffusion properties of pure intermolecular single-quantum, double-quantum, and triple-quantum coherences of three-spin orders were characterized and analyzed in one-dimensional experiments. All experimental observations are in excellent agreement with the theoretical predictions. The theoretical results show that the quantum-mechanical treatment leads to exactly the same predictions as the dipolar field treatment. The quantitative study of intermolecular multiple-quantum coherences of three-spin orders presented herein provides a better nderstanding of their mechanisms.