Density functional theory was used to study the potential energy surface for rotation about the carbon-carbon bonds in a variety of guanosine, adenosine, and inosine fleximers, which are modified purines with the imidazole and pyrimidine rings separated by a single carbon-carbon bond. Various connectivities between C4 or C5 of the imidazole ring and C5' or C6' of the pyrimidine ring were considered. Calculations on fleximer nucleobases in the absence of the ribose moiety suggest that a planar relative arrangement of the imidazole and pyrimidine rings is favored, and that all fleximers are indeed very flexible with regards to rotation about the carbon-carbon bond, where calculated barriers are generally less than 40 kJ mol(-1). Furthermore, calculated binding energies of fleximer-pyrimidine pairs indicate that the hydrogen-bonding properties of these modified nucleobases mimic those of the corresponding natural purine. Inclusion of the sugar moiety often leads to a favored nonplanar orientation of the two rings, and either a reduction in the rotational barrier height or small changes in the rotational surface depending on the connectivity and nucleobase considered. It is concluded that several connectivities may have favorable properties for biochemical applications where flexible nucleobases would be beneficial.