Position of cyclization in cyclic ADP-ribose.

Cyclic adenosine diphosphoribose (cADPR) is a putative second messenger of calcium homeostasis synthesized from NAD by cleavage of the nicotinamide-ribose bond and cyclization of the ribose to the adenine ring. In this study, the ultraviolet absorption spectra of cADPR have been studied as a function of pH and compared to other compounds containing an adenine ring with substitutions at known positions. The results support a structure for cADPR in which cyclization is to position 1 of the adenine ring, rather than to N6, as has been previously proposed.