Quadrupole-enhanced proton spin relaxation for a slow reorienting spin pair: (I)–(S). A stochastic Liouville approach

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

“…An alternative, essentially equivalent picture of the resonant dipolar relaxation of the amide proton is a coherent polarization transfer from the amide proton to the rapidly relaxing 14 N nucleus. In contrast to our model, previous authors have argued that the relaxation of the amide proton is driven by restricted fluctuations in the polypeptide backbone [9,11,13,14] or by slow isotropic protein tumbling [21,23]. However, at least in the model systems investigated here, those processes do not occur on the required microsecond time scale.…”

“…Previous authors have either ignored the magnetization transfer from bulk water to the amide proton [21,23] or assumed fast-exchange conditions [13,15]. Whereas Kimmich et al did not specify the mechanism for this exchange [13], Koenig proposed a model similar to ours but with the important difference that he identified the intermediary protons as water molecules at the protein surface with a residence time of 0.3 ns [15].…”

“…Although the QP phenomenon has been analyzed theoretically from different perspectives [3,7,9,13,15,[21][22][23], the detailed molecular mechanism of the ubiquitous 1 H-14 N Q peaks in biological systems has not been established. The positions of the Q peaks can be accurately predicted from the principal components of the 14 N residual quadrupole tensor, but a rigorous theory that can also predict the intensities (absolute and relative) and the width and shape of the Q peaks has not been available.…”

Nuclear quadrupole resonance lineshape analysis for different motional models: Stochastic Liouville approach

“…This treatment has been followed by further work for S = 1. [34][35][36] Only just recently the SLE approach to nuclear spin-lattice relaxation has been extended to the case of quadrupolar nuclei with S > 1. 37 The treatment presented in Ref.…”

“…37 The treatment presented in Ref. 37 is a generalization of the work 35,36 to an arbitrary quadrupole spin quantum number. In this context one must mention a somewhat different version of the slow motion theory based on the stochastic Liouville equation in Langevin form, formulated by Åman and Westlund.…”