The proposed activated state of cyclic adenosine 3,5-monophosphate (cAMP) is modelled by two nucleoside cyclic 3 ,5 Pv-TBP compounds 3 and 4. The reason for the design of compound 3, in which a probe fragment (OCH2CH2OCH3 group) is linked to phosphorus, was reflected in a conformational transmission effect, which occurs when the probe is located in the axis of a Pv-TBP. This means that the six-membered 35-dioxaphosphorinane ring predominantly remains in a diequatorial (e,e) orientation. In the absence of conformational transmission, as in compound 4, the 3,5-dioxaphosphorinane ring favors an equatorial-axial (e,a) orientation. From this we conclude that the occurrence of conformational transmission can stabilize the (e,e) orientation of the 3,5-ring. This can be of relevance to the activation of protein kinase by cAMP. In order to obtain more structural information from 3 and 4, we performed MNDO calculations on the models 8-10. These calculations revealed that the (e,e) orientation of the 3,5-ring is destabilized by 3-4 kcal/mol compared to the (e,a) orientation. For the (e,a) geometry, the 3,5-ring adopts a twist conformation, whereas the (e,e) orientated 3,5-ring shows a half-chair geometry.
Broeders, N. L. H. L., Koole, L. H., & Buck, H. M. (1991). Conformational transmission in pentacoordinated phosphorus systems, modelling the activated state of cyclic AMP. Heteroatom Chemistry, 2(1), 205-211. https://doi.org/10.1002/hc.520020123