Variable-Temperature NMR Spectroscopy, Conformational Analysis, and Thermodynamic Parameters of Cyclic Adenosine 5'-Diphosphate Ribose Agonists and Antagonists.

Cyclic adenosine 5'-diphosphate ribose (cADPR) is a ubiquitous Ca ²⁺ -releasing second messenger. Knowledge of its conformational landscape is an essential tool for unraveling the structure-activity relationship (SAR) in cADPR. Variable-temperature ¹ H NMR spectroscopy, in conjunction with PSEUROT and population analyses, allowed us to determine the conformations and thermodynamic parameters of the furanose rings, γ-bonds (C4'-C5'), and β-bonds (C5'-O5') in the cADPR analogues 2'-deoxy-cADPR, 7-deaza-cADPR, and 8-bromo-cADPR. A significant finding was that, although the analogues are similar to each other and to cADPR itself in terms of overall conformation and population (ΔG°), there were subtle yet important differences in some of thermodynamic properties (ΔH°, ΔS°) associated with each of the conformational equilibria. These differences prompted us to propose a model for cADPR in which the interactions between the A2'-N3, A5″-N3, and H2-R5' atoms serve to fine-tune the N-glycosidic torsion angles (χ).