As a model study of the intramolecular interaction between the adenine and pyridinium rings of nicotinamide adenine dinucleotide (NAD⁺), conformational analysis of 1-[3-(adenin-9-yl) propyl]-3-carbamoylpyridinium cation was carried out by means of semiempirical energy calculation. The extended conformation of the molecule, which was observed in the crystal structure, was found to be energetically a metastable one. The most energetically stable conformations seem to be folded forms, especially forms in which the two aromatic rings are stacked ; four kinds of stacking modes were observed.