Carbon-13 NMR in Conformational Analysis of Nucleic Acid Fragments 2. A Reparametrization of the Karplus Equation for Vicinal NMR Coupling Constants in CCOP and HCOP Fragments

13C-31P coupling constants of 10 oligoribonucleoside phosphates, measured at a number of temperatures, are presented. The combination of these data with 1H-31P couplings of the same compounds leads to the derivation of two new and mutually consistent sets of Karplus parameters: J(CCOP) = 6.9cos2 phi--3.4cos phi + 0.7 J(HCOP) = 15.3cos2 phi--6.1cos phi + 1.6 At the same time new values for the base sequence dependent magnitude of the trans conformer of the backbone angle epsilon (C4'-C3'-O3'-P) are calculated. The present results show that the magnitude of epsilon(t) in right-handed ribo helices is confined to the range 214 degrees-226 degrees (average 219 degrees), which is in much better agreement with single crystal X-ray studies (average 218 degrees) than were previous deductions from NMR spectroscopic results (average 208 degrees).