Porphyrin derivatives have sensitive selectivity to molecuar intercalation and aggregation in DNA helix. On the basis of the study of tetrapyridylporphine, in order to investigate the effects of meso- (p- methylpyridyl) to the structure of N - protonated porphyring, the geometries of meso - tetrakis (p - methylpyridyl) porphyrin (TmPyPH2^4+) and its diacid (TmPyPH4^6+) were calculated by using AM1 MO method under certain symmetry restriction. The results show that in the protonation process, because of the effect of meso - (p - methylpyridyl), the changes of the structure and bond charge population are obvious; the frontier molecular orbitals have opposite change trend with the case of meso - yridyl, the rapid exchanges of inner hydrogens of the protonated diacid with solvent protons become more difficult.

Key words: PYRIDINE P, PORPHYRIN, MOLECULAR ORBITAL THEORY, CONE PACKING MODEL, PROTONATION, CONFIGURATION, SUBSTITUENT EFFECT