INFRARED-SPECTRA AND THE STRUCTURE OF 1-METHYLADENINE IN AN ARGON MATRIX AND SOLUTIONS

The spectral characteristics of the so-called ''rare'' imine form of m ethylated nucleotide purine base adenine is investigated. A study is m ade of 1-methyladenine (1-mAde) having generally the three tautomeric forms: amine, imino-N9H, and imino-N7H. IR spectra of 1-mAde isolated in Ar matrix at 12 K are obtained in the range 4000-400 cm(-1). The no rmal-coordinate analysis is employed to interpret the experimental spe ctra. It is shown that isolated 1-mAde molecules can only exist as imi ne. The set of fundamental vibrational frequencies and force constants of the imine tautomer is found. The relative energies of the 1-mAde t automers are calculated by the SCF MO LCAO method in the AM1 approxima tion with complete geometry optimization. This method predicts the sta bility of the tautomers in decreasing order: imine-N9H, imine-N7H, ami ne. An IR spectroscopic study of solutions of 1-mAde, and also adenine , 9-methyladenine, 1-methyladenosine, 2-aminopyrimidine, 2- and 4-oxop yrimidine in polar solvents in the range 1750-1500 cm(-1) is carried o ut. The results show the presence of a considerable amount of the ''ra re'' imine tautomer in water, as well in non-aqueous solutions.