title: Determination of 13C CSA tensors: extension of the model-independent approach to an RNA kissing complex undergoing anisotropic rotational diffusion in solution.
abstract: Chemical shift anisotropy (CSA) tensor parameters have been determined for the protonated carbons of the purine bases in an RNA kissing complex in solution by extending the model-independent approach [Fushman, D., Cowburn, D. (1998) J. Am. Chem. Soc. 120, 7109-7110]. A strategy for determining CSA tensor parameters of heteronuclei in isolated X-H two-spin systems (X = 13C or 15N) in molecules undergoing anisotropic rotational diffusion is presented. The original method relies on the fact that the ratio kappa2=R2 auto/R2 cross 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 molecular motion, provided rotational diffusion is isotropic. However, if the overall motion is anisotropic kappa2 depends on the anisotropy D(parallel)/D (perpendicular) of rotational diffusion. In this paper, the field dependence of both kappa2 and its longitudinal counterpart kappa1=R1 auto/R1 cross are determined. For anisotropic rotational diffusion, our calculations show that the average kappa(av) = 1/2 (kappa1+kappa2), of the ratios is largely independent of the anisotropy parameter D(parallel)/D (perpendicular). The field dependence of the average ratio kappa(av) may thus be utilized to determine CSA tensor parameters by a generalized model-independent approach in the case of molecules with an overall motion described by an axially symmetric rotational diffusion tensor.
authors.affiliation: Central NMR Facility, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune, 411008, India. s.ravindranathan@.ncl.res.in
authors.forename: Sapna
authors.initials: S
authors.lastname: Ravindranathan
authors.forename: Chul-Hyun
authors.initials: CH
authors.lastname: Kim
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Bodenhausen
mesh_terms: D016880:Anisotropy; D002244:Carbon; D002247:Carbon Isotopes; D041961:Deuterium Exchange Measurement; D004058:Diffusion; D009682:Magnetic Resonance Spectroscopy; D011487:Protein Conformation; D012313:RNA / Q000737:chemistry*; D012996:Solutions
chemical_list: D002247:Carbon Isotopes; D012996:Solutions; D012313:RNA; D002244:Carbon