TAUTOMERISM OF THIOGUANINE - FROM GAS-PHASE TO DNA

High-level ab initio calculations were performed on five of the most s table tautomers of 6-thioguanine (6TG) at the second- and fourth-order levels of correlated Moller-Plesset theory using Pople's type of vale nce double-zeta basis sets with additional sets of polarization and (i n some cases) diffusion functions. At the highest applied level of the ory -31++G(d,p)/MP2/6-31G(d,p)+0.9(HF/6-31G(d,p)ZPE)), the order of st ability of the tautomers was found to be N9(H)trans > N9(H)cis > N1,7( H) > N1,9(H) > N7(H). When the ZPE corrections were not employed, all calculations utilizing structures minimized at the correlated MP2 leve l returned a stability order of N1,7(H) > N9(H)trans > N9(H)cis > N1,9 (H) much greater than N7(H). Calculations carried out using the self-c onsistent reaction field (SCRF) model to simulate aqueous solution beh avior yielded a stability order (with correction for ZPE) of NB(H)tran s > N1,9(H) > N9(H)cis > N1,7(H) much greater than N7(H). CNDO/S calcu lations on the excited states of 6TG tautomers showed higher oscillato r strengths and lower excited-state energies for thiones than for thio ls; the latter was confirmed experimentally via UV absorbance and lumi nescence spectroscopic studies on 6TG and two model compounds for spec ific tautomers. These experimental spectra confirmed the existence of at least two types of 6TG thione chromophores.