Molecular mechanics studies have been carried out on "B-DNA-like" structures of [d(C-G-C-G-A-A-T-T-C-G-C-G)](2) and [d(A)](12).[d(T)](12). Each of the backbone torsion angles (psi, phi, omega, omega', phi') has been "forced" to alternative values from the normal B-DNA values (g(+), t, g(-), g(-), t conformations). Compensating torsion angle changes preserve most of the base stacking energy in the double helix. In a second part of the study, one purine N3-pyrimidine N1 distance at a time has been forced to a value of 6 A in an attempt to simulate the base opening motions required to rationalize proton exchange data for DNA. When the 6-A constraint is removed, many of the structures revert to the normal Watson-Crick hydrogen-bonded structure, but a number are trapped in structures approximately 5 kcal/mol higher in energy than the starting B-DNA structure. The relative energy of these structures, some of which involve a non-Watson-Crick thymine C2(carbonyl)[unk]adenine 6NH(2) hydrogen bond, are qualitatively consistent with the DeltaH for a "base pair-open state" suggested by Mandal et al. of 4-6 kcal/mol [Mandal, C., Kallenbach, N. R. & Englander, S. W. (1979) J. Mol. Biol. 135, 391-411]. The picture of DNA flexibility emerging from this study depicts the backbone as undergoing rapid motion between local torsional minima on a nanosecond time scale. Backbone motion is mainly localized within a dinucleoside segment and generally not conformationally coupled along the chain or across the base pairs. Base motions are much smaller in magnitude than backbone motions. Base sliding allows imino N-H exchange, but it is localized, and only a small fraction of the N-H groups is exposed at any one time. Stacking and hydrogen bonding cause a rigid core of bases in the center of the molecule accounting for the hydrodynamic properties of DNA.