Table 1. Comparison of the bond distances and angles characterizing the steric conditions for the in-line attack of O3’ at the next phosphate in the crystal structure of 3’,5’ cGMP and 3’,5’ cAMP [10,11].

Structural parametera	3’,5’ cGMP	3’,5’ cAMP
d(P1…P2), Å	7.38	5.82
d(O3’1…P2), Å	6.34	5.49
<(O3’1-P-O3’2), degrees	125.3	93.9

^ad: interatomic distance, <: angle. 1 and 2 in the subscripts refer to two adjacent nucleotides in the crystal structure.