Adenosine, AMP, S-adenosylhomocysteine, S-adenosylmethionine, aristeromycin and 25 other synthetic adenosine analogs modified in the 4' or 5' positions show certain groups of different circular dichroism (CD) spectra. Both positive and negative Cotton effects can occur in the long-wavelength part (250-270 nm) of the spectra. Molar ellipticities [O] range from -6000 (in adenosine 5'-carboxylate) to + 4000 deg. cm2 dmol-l (in 5'-deoxy-5'iodoadenosine), including some compounds with small, polar 5'-substituents in which low-intensity bands are found in signed pairs. Most of these adenosine derivatives that have the same adenine chromophore and a ribofuranose moiety unsubstituted in the 2' and 3' positions prefer an anti-conformation of the adenine base, as evidenced by proton magnetic resonance spectroscopy. In the majority of cases, electronic perturbations of the chromophore or major alterations of the assymmetric sugar residue can be excluded as sources of the C D variations. Therefore a correlation of the long-wavelength C D bands with the glycosyl torsion angle bCN is suggested, where the gauche, gauchelanti combination which is typical of AMP in the crystal and in solution (&.N zz -40 ', [el negative) is one reference point and a region for &N = 0 'is assigned to compounds with space-filling substituents such as S-adenosylmethionine. Both negative and positive Cotton effects can be associated with the anti conformation range. Within this series, the base conformation of novel nucleoside structures could be predicted from CD measurements. The C D spectrum gives no indication, however, of whether a certain torsion angle is the result of a rigid structure (as in AMP) or the average value of a molecule with high rotational freedom (as in 5'-deoxyadenosine). The conformations of aristeromycin and 4'-thioadenosine are discussed in relation to adenosine, and a structure-determining effect of the 4' bridge atom is noted. Circular dichroism (CD) and, in earlier years, optical rotatory dispersion (ORD) spectroscopy have become useful tools for the study of conformation changes in nucleic acids, pol ynucleotides, and nucleosides or mononucleotides [l]. Like nuclear magnetic resonance, these techniques are valuable because they provide information about the state of the molecules in dilute solutions resembling physiological conditions of pH, ionic strength, and concentration. Several authors have described correlations of the sign of Cotton effects in pyrimidine and purine nucleosides with the sugar-base torsion angle &N [2] about the glycosidic bond, based upon empirical assignments important result of these studies is the reversion of long-wavelength Cotton effects found in compounds in which the base is forced out of its regular anticonformation by bulky substituents at the 6 or 8 position of a pyrimidine or purine ring, respectively [9]. The ORD and C D spectra of anomeric pentofuranosides have also been well documented [lo, 111. Studies of magnetic circular dichroism (the Faraday effect) have provided precise resolu...

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