It is generally acknowledged that geometrical and conformational properties of biopolymers have an important effect on their biochemical behaviour. It is less easily recognized that these properties depend also on their macromolecular electronic characteristics.The aim of this review is to demonstrate the significance of such macromolecular electronic effects. Particularly useful for this sake is the recently much developed concept of ‘molecular electrostatic potential’ (MEP) (Scrocco & Tomasi, 1973, 1978) by which is defined the electrostatic (Coulomb) potential created in the neighbouring space by the nuclear charges and the eletronic distribution of a molecule.

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During the last twenty-five years the development of quantum mechanical calculations and experimental measurements of chemical shifts of the different type of nuclei present in nucleic acids have run parallel in close relation to each other. The first calculations dealt with intramolecular effects on base proton shifts (Veillard, 1962) but the real breakthrough of the theory occurred with the advent of computations of intermolecular shielding due to the ring current effect of the nucleic acid bases (Giessner-Prettre & Pullman, 1970).

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SynopsisThe spatial dependence of the ring-current magnetic anisotropy of nucleic acid bases is presented in a series of graphs in cylindrical coordinates. The curves may be used to calculate the ring-current shift a t a point in a cylinder of radius 10 A extending 8 A above and below each ring of the base. These distance effects are found to influence considerably the predicted chemical shifts of nucleic acid protons, particularly in RNA duplexes. The contribution of polarization (electric field) effects and the diamagnetic anisotropy of individual atoms (local A x ) are briefly discussed.

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structure, led Srivastava19 to assign a planar structure to anthrone. Therefore, the enthalpy of formation (Table III) and resonance energy (Table IV) of anthrone were estimated and compared with those of xanthone. The resonance stabilization energy of xanthone falls between those of anthraquinone and anthrone, both of which are known to be planar molecules. These findings suggest that xanthone may also be planar. The somewhat greater resonance energy of xanthone, when compared with that of anthrone, may be due to the contribution of the unshared electrons of the ether oxygen atom to the resonance conjugation. This is consistent with the reported results20-22 that the xanthone molecules, to which the 7-pyrone ring is coupled, are aromatic.