Novel 8-substituted base and sugar-modified analogues of the Ca2+ mobilizing second messenger cyclic adenosine 5′-diphosphate ribose (cADPR) were synthesized using a chemoenzymatic approach and evaluated for activity in sea urchin egg homogenate (SUH) and in Jurkat T-lymphocytes; conformational analysis investigated by 1H NMR spectroscopy revealed that a C2′ endo/syn conformation of the “southern” ribose is crucial for agonist or antagonist activity at the SUH-, but not at the T cell-cADPR receptor.