Comparison of the Transition-Metal Dihydride Ions CrH,' and MoH,'

change is the greatest. Indeed, in that orientation the C3 hydroxyl group is directed toward the acetamido group (rather than away from it), and the intramolecular interactions stabilizing such an orientation would become more significant as the solvent polarity is lowered. Evidence supporting such a hypothesis, independent of the present theoretical model, is presented in Table 11, which displays a strong correlation between the incremental negative change observed in [O],, and the solvent dielectric constant. The proposal that the conformational preferences of substituent groups in these compounds depend on solvent is unexceptional. Determining the precise nature of those preferences, however, is not straightforward. The present work shows that CD, within limits, will reflect differences in the orientation of substituent groups in acetamido sugars. The calculations reported here, although necessarily approximate, direct attention to specific conformational features which are amenable to study by independent means such as NMR. Conclusions The theoretical model presented here displays the dependence of acetamido sugar n-7r* CD on specific conformational features. It provides an interpretive tool for relating the nr* CD of glycosaminoglycans to the orientation of constituent acetamido groups. It also allows a qualitative rationalization of several features of the observed solvent dependence of the CD of aGlcNAc-OMe and 8-GlcNAc-OMe, including (1) the stronger solvent dependence of the a-anomer, (2) the positive CD of the a-anomer in strong hydrogen-bond-donor solvents, (3) the negative CD of the a-anomer in other solvents, and (4) the particularly weak CD of the &anomer in fluorinated alcohols.