Determination of 13C CSA Tensors: Extension of the Model-independent Approach to an RNA Kissing Complex Undergoing Anisotropic Rotational Diffusion in Solution

Chem. shift anisotropy (CSA) tensor parameters have been detd. for the protonated carbons of the purine bases in an RNA kissing complex in soln. by extending the model-independent approach [Fushman, D., Cowburn, D. (1998) J. Am. Chem. Soc. 120, 7109-7110]. A strategy for detg. CSA tensor parameters of heteronuclei in isolated X-H two-spin systems (X = 13C or 15N) in mols. undergoing anisotropic rotational diffusion is presented. The original method relies on the fact that the ratio k2=R/R of the transverse auto- and cross-correlated relaxation rates involving the X CSA and the X-H dipolar interaction is independent of parameters related to mol. motion, provided rotational diffusion is isotropic. However, if the overall motion is anisotropic k2 depends on the anisotropy D.dblvert./D.perp. of rotational diffusion. In this paper, the field dependence of both k2 and its longitudinal counterpart k1=R/R are detd. For anisotropic rotational diffusion, our calcns. show that the av. kav = 1/2 (k1+k2), of the ratios is largely independent of the anisotropy parameter D.dblvert./D.perp.. The field dependence of the av. ratio kav may thus be utilized to det. CSA tensor parameters by a generalized model-independent approach in the case of mols. with an overall motion described by an axially sym. rotational diffusion tensor. [on SciFinder (R)]