Infrared spectra of 6-thioguanine tautomers. An experimental and theoretical approach

Both amino-thiol N9H and amino-thiol N7H tautomeric forms of 6-thioguanine have been identified in approximately equal abundance in infrared studies of these molecules isolated in the hydrophobic environment of an argon matrix at 12 K. The relative concentrations of the amino-thiol N9H and amino-thiol N7H ([SH, N9H]/[SH, N7H] = K(N9H-N7H) = 1.00 +/- 0.02) are estimated from the observed relative infrared absorbances. From these relative concentrations, the difference in the Gibbs free energy of these two tautomers (deltaG500(N9H-N7H) = -0.012 +/- 0.005 kJ mol(-1) have been estimated. The infrared and Raman spectra of 6-thioguanine in solid state are also discussed in terms of hydrogen bonding and stacking interactions in the crystal which are not considered in the calculation. In an effort to interpret the experimental results, ab initio calculation of the infrared spectrum has been made for the amino-thione N7H tautomer at 3-21G level. Comparison with experimental spectra is of some help in the assignment of the infrared and Raman spectra for 6-thioguanine in the solid state.