Adenine Nucleosides in Solution: Circular Dichroism Studies and Base Conformation

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Abstract
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 [theta] range from -6000 (in adenosine 5'-carboxylate) to +4000 deg. cm2 dmol-1 (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 CD variations. Therefore a correlation of the long-wavelength CD bands with the glycosyl torsion angle phiCN is suggested, where the gauche, gauche/anti combination which is typical of AMP in the crystal and in solution (phiCN approximately -40degrees, [theta] negative) is one reference point and a region for phiCN = 0degrees ([theta] positive) 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 CD 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.

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