Energy profiles and molecular binding number (MBN) were calculated for purine and its mono-protonated forms by the CNDO/2 algorithm. Total energy indicates that the 3,7-species is the most stable among the protonated isomers, being insignificantly more stable than the 1,7-analogue. Electronic energy suggests that proton may preferentially be attached to position 3. Charge densities are presented by both Mulliien's population method and CNDO/2.Attention has recently been devoted to the structure of the protonated forms of heterocyclic compounds possessing more than one hetero atom [ I ] . Purine embodies four nitrogen atoms to each of which a proton can be linked. It seemed of particular interest to study the protonation of this compound, since, in this case, one could compare the calculations with results obtained by NMR measurements [2 -5 ) .While searching for a suitable theoretical method, the following points were considered: a) u and II electrons must be simultaneously calculated, if one wishes to rely on b) Core-core interactions (nuclear repulsion energies) must be explicitly computed. The CNDO/2 method [ 6 ] , which embraces both of these features, was chosen for the energy profiles. present investigation.* Although this treatment is known [8] to overestimate energy profiles, we are interested here in relative changes only. Also, this method apparently gives reliable information in charge distributions (cf. A different approach to protonation of heterocycles, based on mapping of the potential energies, has recently been developed [ 101 , and successfully been used by Pullmann for adenine and guanine [ 111.
METHOD OF CALCULATIONAccurate geometry was available only for crystalline purine in its 7-NH tautomeric form [ 121 . For the 9-NH isomer we have assumed a "reverse" arrangement of the bond * Attempts to use the modified INDO (MINDO) [7 1 method failed because of difficulties in obtaining convergence in the SCF procedure.
