Solvation of a tetrapeptide, NAc-Ser-Phe-Val-Gly-OMe (1), in water and in water/alcohol mixtures with 2,2,2-trifluoroethanol (TFE)/water or ethanol (ETH)/water has been studied by diffusion NMR and intermolecular NOE measurements. The experimental results were compared with those obtained from detailed Molecular Dynamics (MD) calculations. Independently, all three methods revealed preferential solvation on the surface of the peptide by TFE in the water/TFE mixtures, but not by ETH in the water/ETH mixtures. The MD calculations show that the TFE concentration coating the peptide is higher than that in the bulk, while for ethanol, the concentration is nearly equal to that in the bulk. Calculated site-specific preferential solvation data between TFE, ETH, and water with the different peptide groups have been compared with the NMR data and shown to be in general agreement with the experimental facts.

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Residual dipolar couplings were successfully used to distinguish between the two diastereotopic protons on C-20 of strychnine dissolved in an organic liquid crystal (PBLG/CDCl(3)). The results presented here strongly suggest that this method will be of help in organic structure determination, making the determination of relative stereochemistry in the absence of NOE data possible.

The use of tetramethylsilane (TMS) as an internal diffusion reference for DOSY measurements related to molecular association forced by H-bond in solution is proposed. By using such a reference, it is possible to determine the extent to which the diffusion of the solutes is affected by changes in viscosity or H-bonding, after modifications in the composition of the solution under study. Therefore, changes in hydrodynamic radii due to H-bonding can be calculated. Three examples of the use of this reference in diffusion studies related to H-bonding are presented: the addition of an H-bond acceptor to a mixture of two compounds with very similar hydrodynamic radii and molecular weights, but where only one can be involved in H-bonding, the addition of an H-bond acceptor to a mixture of alcohols with different H-bond acidities and the addition of an H-bond acceptor to a mixture of two isomeric alcohols with different steric hindrances.

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