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The description of the reorientational dynamics of flexible molecules is a challenging task, in particular when the rates of internal and global motions are comparable. The commonly used simple mode-decoupling models are based on the assumption of statistical independence between these motions. This assumption is not valid when the time scale separation between their rates is small, a situation that was found to arise in oligosaccharides in the context of certain internal motions. To make possible the interpretation of NMR spin relaxation data from such molecules, we developed a comprehensive approach generally applicable to flexible rotators with one internal degree of freedom. This approach integrates a stochastic description of coupled global tumbling and internal torsional motion, quantum chemical calculations of the local potential and the local geometry at the site of the restricted torsion, and hydrodynamics-based calculations of the diffusive properties. The method is applied to the disaccharide β-D-Glcp-(1→6)-α-D-[6-13C]-Manp-OMe dissolved in a DMSO-d6/D2O cryosolvent. The experimental NMR relaxation parameters, associated with the 13CH2 probe residing at the glycosidic linkage, include 13C T1 and T2 and 13C-{1H} nuclear Overhauser enhancement (NOE) as well as longitudinal and transverse dipole-dipole cross-correlated relaxation rates, acquired in the temperature range of 253–293 K. These data are predicted successfully by the new theory with only the H–C–H angle allowed to vary. Previous attempts to fit these data using mode-decoupling models failed. [ABSTRACT FROM AUTHOR]

An interpretation of the nuclear spin relaxation rates of the [super 13]C and [super 14]N nuclei is employed to investigate the anisotropic reorientation of 9-methylpurine and 7-methylpurine molecules in methanol solutions. The various quantum chemical calculations performed on the basis of the studies show that the observed molecules reorient anisotropically and despite their structural similarity, exhibit different solvations.

A computational stochastic approach is applied to thedescriptionof flexible molecules. By combining (i) molecular dynamics simulations,(ii) hydrodynamics approaches, and (iii) a multidimensional diffusivedescription for internal and global dynamics, it is possible to buildan efficient integrated approach to the interpretation of relaxationprocesses in flexible systems. In particular, the model is appliedto the interpretation of nuclear magnetic relaxation measurementsof linear oligosaccharides, namely a mannose-containing trisaccharideand the pentasaccharide LNF-1. Experimental data are reproduced withsufficient accuracy without free model parameters. [ABSTRACT FROM AUTHOR]

In this work, we address the description of the dynamicsof cyclodextrinsin relation with nuclear magnetic resonance (NMR) relaxation datacollected for hydroxymethyl groups. We define an integrated computationalapproach based on the definition and parametrization of a stochasticequation able to describe the relevant degrees of freedom affectingthe NMR observables. The computational protocol merges molecular dynamicssimulations and hydrodynamics approaches for the evaluation of mostof the molecular parameters entering the stochastic description ofthe system. We apply the method to the interpretation of the 13C NMR relaxation of the −CH2OH group ofcyclodextrins. We use γ-cyclodextrin as a case study. Resultsare in agreement with quantitative and qualitative analyses performedin the past with simpler models and molecular dynamics simulations.The element of novelty in our approach is in the treatment of thecoupling of the relevant internal (glucopyranose ring twisting/tiltingand hydroxymethyl group jumps) and global (molecular tumbling) degreesof freedom. [ABSTRACT FROM AUTHOR]