Using the semiempirical potential function, conformational energies of the model compounds, d(pCp), d(pGp), and d(pCpGpCp) are calculated, and the B $\rightarrow$ Z transition is discussed along the pseudorotational path of the sugar ring. For the sugar ring without the base attached, the minimum energies for each sugar puckering form are calculated along the pseudorotational path. The energy barrier of the interconversion between the C(3``)-endo form and the C(2``)-endo form is calculated to be about 2.0 kcal/mol. From the conformational energy calculations of the interconversions of mononucleotide diphosphates, d(pCp) and d(pGp), between the C(2``)-endo and the C(3``)-endo conformers, the purine sugar component is known to be more convertible than the pyrimidine sugar segment. Based on the results of conformational studies of DMP. d(pCp), and d(pGp), a topological transition of the handedness of the model compound, d(pCpGpCp), is studied. The left-handed Z-form is found to be less stable by about 8.5 kcal/mol than is the right-handed B-form. The energy barrier of the Z$\rightarrow$B transition is calculated to be about 17.4 kcal/mol. The contributions of the electrostatic and nonbonded energies to the energy barrier are discussed in connection with the conformational changes of the model compound, d(pCpGpCp). Conformational changes in the RNA-protein interaction are studied by calculating the intramolecular interaction energy of a model complex with the use of potential functions. The model complex has hydrogenbonded water moleculas bridging polypeptide NH groups to 2``-hydroxyl groups and sugar ring oxygen atoms. Since the sugar ring is constrained by the bridging water, the preliminary calculations with the model compounds, the sugar ring, mononucleoside diphosphates, pCp and pGp, are practiced. And the flexibility and the energetics of the sugar ring are compared with the previous results of DNA. The shift of the phase angle of the sugar ring is occurred i...