Calculations of the circular dichroism of adenosine derivatives constrained in the syn form

The rotational strengths of the four longer wavelength transitions, B(2u), B(1u), and the two E(1u), of adenosine derivatives constrained in the syn form have been investigated theoretically and experimentally. The theory combines a complete neglect of differential overlap version S (CNDO/S) description of the base with a generalized bond exciton method by means of a matrix method. Rotational strength wheels for these transitions showing the rotational strength as a function of the glycosidic rotational angles are presented and sector rules discussed. Similar sector rules are predicted for purine nucleoside derivatives containing the 6-amino substituent, regardless of heteroatom content at positions 1 or 3 of the base. The sector rules for the B(2u) rotational strength of purine nucleosides lacking the 6-amino substituent are strongly influenced by aza substitution. The circular dichroism spectra of 3-deazaadenosine, 8-(alpha-hydroxyisopropyl)-adenosine, and other purine nucleosides having a strong preference for the syn conformation are presented and compared with the theoretical results.