well with a thermodynamic model accounting for the presence of an additional equilibrium system in which 1a is partially transformed to a borate ester (Fig. , see the SI for details and equations). Although borate esters of ribose are known to persist in dilute aqueous solution, these stable esters generally involve the anomeric hydroxyl group, which is absent in 1a. Nevertheless, Kim et al. observed borate esters of the nucleoside-based cofactor NAD + and the natural trinucleotides by mass spectrometry, indicating that analogous species might be formed by recruitment of the 2′and/or 3′-hydroxyl groups. In addition, structurally similar nucleoside boronate esters have been reported by Smietana and others. Based on these precedents, we hypothesized that the cyclic borate ester 1a* would be the predominant species causing the observed equilibrium shift (Fig. ).