Shangwu Ding

Breaking the T1 constraint for quantitative measurement in magic angle spinning solid-state NMR spectroscopy.

Quantitative solid-state NMR experimental schemes that break the conventional T(1) constraint are described. The combination of broad-band homonuclear recoupling techniques and the conventional single pulse or cross-polarization (CP) schemes (referred as QUSP or QUCP) render the long T(1) of low-gamma spins no longer a constraint for obtaining quantitative NMR spectra. During the mixing time when dipolar recoupling occurs, the nonuniformly CP enhanced or recovered spin magnetization is redistributed under the reintroduced homonuclear dipole-dipole interactions so that uniformly enhanced or recovered magnetization is achieved when the system reaches the quasi-equilibrium state. It is shown that quantitative NMR spectra can be obtained for the recycle delays substantially shorter than the conventionally required 5T(1). In addition, the high efficiency gain can be achieved in QUSP and QUCP experiments with a relatively short recycle delay.

Towards uniform enhancement in solid-state cross polarization magnetic angle spinning NMR: A scheme incorporating cross polarization with rotational resonance

A recently proposed experimental scheme for achieving uniform cross polarization enhancement of low-gamma nuclear species in solids under magic angle spinning, termed quantitative cross polarization (QUCP) [Hou , Chem. Phys. Lett. 421, 356 (2006)], is described, supported with comprehensive theoretical analysis, numerical simulation, and experimental investigation with both uniformly labeled and naturally abundant solids. This method combines cross polarization with dipolar-assisted rotational resonance (DARR) [Takegoshi , Chem. Phys. Lett. 344, 631 (2001)] broadband homonuclear recoupling technique to achieve quantitative CP spectra under fast magic angle spinning. In addition to the correct and systematical interpretation on the phenomenon we reported in the previous Letter, a number of general guidelines for performing QUCP experiments are presented in this work. It is firmly established that while the enhancement factor in QUCP depends on the CP contact time, uniform enhancement can nevertheless be realized for all types of carbon group. For natural abundance samples, the polarization transfer rate is generally slower than that in labeled samples, but quasiequilibrium among dilute spins in the mixing period can always be reached and uniform enhancement can be achieved albeit the DARR irradiation time needed can be much longer. For labeled samples, the time gain of QUCP experiment is almost the same as that of conventional CP. For natural abundance samples, it is generally much better than single-pulse experiment. Various representative systems, including uniformly C-13-labeled DL-alanine and C-13, N-15 labeled L-tyrosine, as well as naturally abundant alanine, tyrosine, and monoethyl fumarate, are used to verify the validity of our theoretical analysis and numerical simulation and to demonstrate the utility and advantages of the present approach. (c) 2006 American Institute of Physics.

A computer simulation study of water drying at the interface of protein chains

This study investigated the water drying (cavitation) in the interfacial region of two chains of a dimeric protein by nanosecond molecular dynamics simulations using explicit water representation. Separation-induced cavity of water was directly observed in the region. We evaluated the separation length scale of two chains on which the drying transition occurs, and the average number of water molecules that are expelled from the interfacial region during the transition. The obtained values can be rationalized by Kelvin equation for finite lateral size of confinement [K. Lum and A. Luzar, Phys. Rev. E 56, R6283 (1997)]. Also, we found that the drying transition is accompanied by an exponential reduction in the average hydrogen-bond number per interfacial water molecule. The results of this study may deepen the understanding of how hydrophobic interaction drives the assembly of protein chains.

Effects of pulse strength, width, and sample spinning speed on the spectral spin diffusion of multiquantum coherences of spin-3/2 quadrupolar nuclei

The effects of radio-frequency pulse strength, width, and sample spinning speed on the spin-diffusion spectrum of half-integer quadrupolar spins in solids have been studied by theoretical, numerical, and experimental investigations. It is revealed that the line shape of the cross peaks changes nonmonotonically with respect to the change of pulse strength, pulse width, or sample spinning speed. It is also found that the sample spinning speed has much more pronounced influence on the spin diffusion spectral line shape. In many cases of practical importance, the effect of sample spinning must be included in spectral analysis, in contrast to the practice of previous studies. Moreover, this effect can be exploited to further improve the precision in the determination of relative orientation of the electric-field gradient tensors of the exchange partners.

A realistic potential model for N–H vector diffusion in proteins

A realistic model for the potential energy for the diffusion of N-H vectors in a protein is proposed, massively modifying the simplistic models currently used in the literature. In particular, a quantitative and analytical connection between the order parameter of the N-H vector diffusion in a protein and the number of potential minima is established, offering a significant insight into the longstanding question of how protein dynamics is affected by the potential-energy landscape. The largest number of potential minima in a protein is estimated to be no more than around 25. In addition, the conformational entropies derived from classical statistical mechanics and quantum statistical mechanics are proved to be identical. Based on the presented theoretical formula, the number of potential minima for each residue of five representative proteins is evaluated and shows a good correlation between local structural flexibility and the number of potential minima.

Long time tail effect in liquid state NMR spectroscopy

Abstract Experimental evidence of long time tail effect in liquid state NMR spectroscopy has been observed, characterized and its potential applications proposed. It is found that the physical mechanism for the long time tail of the spin auto-correlation function in the liquid state is similar to, but not identical with, that in the solid state. The net effect is shown that the decaying function t − d ′ /2 where d ′ d (the dimension of the spin network), which is clearly more desirable for its applications. Particularly significant to the applications to the enhancement of resolution is that decaying slower than t − d /2 can be achieved, indicating better resolution improvement can be achieved than in the solid state. Extensive exposure of the characteristics of this phenomenon is provided with a number of model compounds and tentative theoretical analysis is given.