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Proton magnetic resonance melting curves are reported for the base and ribose 1' nonexchangeable protons of CCGGp, CCGGAp, ACCGGp, CCGGUp, and ACCGGUp. In general, the double helices melt in a cooperative two-state transition. For CCGGp, CCGGAp, CCGGUp, and ACCGGUp, the 3' ribose exhibits conformational flexibility at lower temperatures than the other ribose residues. For ACCGGp, the 5'-C ribose is the first to exhibit flexibility. The conformations of the 3' terminal bases in CCGGAp and CCGGUp change cooperatively with the double- to single-strand transition, whereas the 5' A of ACCGGp appears to have more conformational freedom.
Petersheim et al. (Tue,) studied this question.