THE INTERACTION OF THE 2'-OH GROUP WITH THE VICINAL PHOSPHATE IN RIBONUCLEOSIDE 3'-ETHYLPHOSPHATE DRIVES THE SUGAR-PHOSPHATE BACKBONE INTO UNIQUE (S,EPSILON(-)) CONFORMATIONAL STATE

The analysis of temperature-dependent vicinal proton-proton coupling c onstants has shown that the North (N) reversible arrow South (S) pseud orotational equilibria of ribonucleoside 3'-ethylphosphates [ApEt (21) , GpEt (22), CpEt (23), rTpEt (24) and UpEt (25)], modelling simple di ribonucleoside(3'-->5')monophosphate without any intramolecular base-b ase slacking, are driven more towards the South-type sugar (S) by Delt a Delta H degrees approximate to -2.5 kJ mol(-1) in the case of purine and by approximate to -3.8 kJ mol(-1) in the case of pyrimidine nucle otides compared to the corresponding parent ribonucleosides 1 - 5. In contrast, the S-type sugar conformation in 2'-deaxyribonucleoside 3'-e thylphosphates (ref. 3d) is stabilized by Delta Delta H degrees approx imate to -1.9 kJ mol(-1) in both purine and pyrimidine nucleotides com pared to the parent 2'-deoxyribonucleosides. The total energetic effec t of 2'-OH group due to its interaction with the vicinal phosphate in ribonucleotides in contrast with the corresponding 2'-deoxynucleotide counterparts can be assessed by subtracting the free-energies of N rev ersible arrow S pseudorotational equilibria in the ribonucleotide anal ogs 21 - 25 from tile corresponding 2'-deoxynitcleotide counterparts 1 6 - 20 : Delta Delta G(298) approximate to +0.3 kJ mol(-1) in ApEt (21 ), +0.6 kJ mol(-1) in GpEt (22), +2.1 kJ mol(-1) CpEt (23), +1.1 kJ mo l(-1) in rTpEt (24) and +1.3 kJ mol(-1) in UpEt (25). The additional s tabilization of the S-type pseudorotamers in ribonucleoside 3'-ethylph osphates 21 - 25 compared to ribonucleoside 3'-monophosphates 11 - 15 by Delta Delta H degrees approximate to -2.0 kJ mol(-1) is attributed to the influence of the 2'-OH group in the former. The population of e psilon(t) rotamers increases by 7-13% from 278 to 358K, which correspo nds also, with an equal increase of he population of N-type pseudorota mers, suggesting a unique cooperativity in the two-state (N,epsilon(t) reversible arrow (S,epsilon) conformational equilibria in 21 - 25. Th ese cooperative conformational transitions of (N,epsilon(t)) reversibl e arrow (S,epsilon) equilibrium in 21 - 25 have been found to De orche strated by the interaction of 2'-hydroxyl group with the vicinal phosp hate as evident by the non-equivalent methylene protons of the 3'-ethy lester function up to 348K in 21 - 25 compared to the 2'-deoxynucleoti de counterparts 16 - 20 (ref. 3d). The intramolecular interaction of t he 2'-OH function with the vicinal phosphodiester stabilizes the S and E. conformers (''On'' switch), whereas 2'-OH in a non-interacting sta te stabilizes the N and epsilon(t) conformers (''Off'' switch) in 21 - 25. The strengths of this ''On-Off'' molecular switch for the prefere nce of(S,epsilon) conformational stale over (N,epsilon(t)) state in 21 - 25 are as follows: Delta G(298) approximate to -2.8 kJ mol(-1) for adenosine 3'-ethylphosphate (21), 2.1 kJ mol(-1) for guanosine 3'-ethy lphosphate (22), approximate to 0.1 kJ mol(-1) for cytidine 3'-ethylph osphate (23), -0.9 kJ mol(-1) for ribothymidine 3'-ethylphosphate (24) and -0.7 kJ mol(-1) for uridine 3'-ethylphosphate (25).