Figure 2. Schematic representation of c-di-GMP-I riboswitch (A) and cdi-GMP-II riboswitch (B). c-di-GMP is represented in red. Adapted with permission from ref 99.

Spin diffusion in NMR occurs for small- and medium-sized molecules when their tumbling rate reduces in solution so that magnetization exchange by longitudinal cross relaxation becomes highly efficient. Composite DMSO-water viscous solvents were used for the first time to access the individual NMR spectra of a mixture components in spin diffusion conditions. The easy handling and high dissolution power of [D ]DMSO/H O offers a wide range of potential applications for polar and moderately apolar mixture analysis. In addition to 2D H- H NOESY and H- C HSQC-NOESY, H- N HSQC-NOESY, 1D and 2D H- F heteronuclear NOESY (HOESY) experiments were set up to offer new ways to individualize molecules within a mixture. This article reports the analysis of a polar mixture of four dipeptides dissolved in [D ]DMSO/H O (7:3 v/v) and that of a medium-polarity fluorinated dinucleotide dissolved in [D ]DMSO/H O (8:2 v/v) by means of spin diffusion in NOESY, HOESY, and HSQC-NOESY experiments.

The di-2'-α-fluoro analogue of thymidylyl(3',5')thymidine, synthesized to probe the effect of a minimum amount of S conformer on the photoreactivity of dinucleotides, is endowed with only 3% and 8% of S sugar conformation at its 5'- and 3'-end, respectively. This analogue gives rise to the (6-4) photoproduct as efficiently as the dithymine dinucleotide (74% and 66% at the 5'- and 3'-end, respectively) under 254 nm. Our results suggest that the 5'-N, 3'-S conformer gives rise to the (6-4) photoproduct.