An ab initio study on nucleic acid bases aromaticities

Aromatic character of nucleic acid bases was characterized by means of magnetic and geometric properties. Optimized geometries at DFT/6-311+G** level were used for calculation of nucleus independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) values. Aromaticities of both pyrimidine and imidazole rings were analyzed. Solvent impact on HOMA values was estimated. Predictions of both methods are in qualitative agreement and suggest more aromatic character of purines rather than pyrimidines. Aromaticity decreases in the following sequence: adenine>guanine>cytosine>thymine≈uracil. Significant impact of protonation on the aromatic character was noticed. Protonation of adenine reduced its aromatic character of both pyrimidine and imidazole rings. However, protonation of guanine and all pyrimidines leads to increase of aromaticity compared to neutral species.