Before revision:  The model also demonstrates both the A to B and the B to A transitions. The number of degrees of freedom is reduced to six grains per nucleotide. We show that this is the minimal number sufficient for this purpose. The key features of the model are (1) simulation of sugarrepuckering between C2'-endo and C3'-endo by the use of one nonharmonic potential and one three-particle potential, (2 ) explicit representation of ions in solution around the DNA, (3) implicit solventapproach and (4 ) sequence dependence.
After revision:  The model also demonstrates both the A to B and the A-form during interactions with biological molecules. We propose a CG representation of the ribose conformational flexibility. We substantiate the choice of the CG sites (6 per nucleotide) needed for the "sugar" GC DNA model, and obtain the potentials of the CG interactions between the sites by the "bottom-up" approach using the all-atom AMBER force field. We show that the representation of the ribose flexibility requires one non-harmonic and one three-particle potential, (2 ) explicit representation of ions in solution around the DNA, (3) implicit solventapproach and (4 ) sequence dependence.