The Nature of Conformational Correlations in α- and β-Anomers of Nucleic Acid Components in Aqueous Solution by Nuclear Magnetic Resonance

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Abstract
The solution distribution of combinations of the sugar ring puckering domains, C2′endo(S), C3′endo(N), and C4′-C5′ rotamers, +sc(g+), ap(t), -sc(g−), in α and β-anomers in ribo- and deoxyribo- pyrimidine nucleic acid components can be determined from vicinal coupling constants (M. Remin, J. Biomol. Str. Dyn. 2, 211 (1984). A general correlation pattern with a conformational constant λ, reflecting an intrinsic physical property of the sugar - side chain ensemble, is developed and expressed in terms of four principles: I) The +sc rotamer contributes to the C3′endo population to a higher extent (1 - Yt) than to C2′endo,(l-Yt-Yg-/Xs). II) The ap rotamer contributes to both C2′endo and C3′endo populations to the same extent (Yt). III) The—sc rotamer contributes only to the C2′endo population, (Yg-/Xs). IV) The molar fractions Xs, Yt and Yg- of conformations C2′endo, ap and—sc, respectively, are strongly correlated, λ = (Yg-/Xs)/Yt ≈ 0.5, and therefore Yt is a basic variable parameter which determines all others in the correlation pattern. In α-anomers, regardless of the type and conformation of the sugar ring and base, the molar fraction Yt = 0.37 ± 0.02. This finding means that different α-anomers show one correlation pattern free of the influence of the base. In β-anomers, structure and conformation of the base are important factors which modulate (through Yt) the correlation pattern, conserving its fundamental features. Yt is considerably increased by a syn-oriented pyrimidine base, but decreases when the base is anti. The transition from anti to syn orientation of the base is followed by destabilization of (C2′endo, +sc) in favor of (C3′endo, ap). The principles of conformational correlations rationalize a variety of correlations observed in the past.