6-Thioguanine luminescence probe to study DNA and low-molecular-weight systems

6-Thioguanine, an antitumor drug, has been tested as a luminescence probe to study DNA and cryoprotector solutions at temperatures between 4.2 and 273 K. The electronic structure of the tautomeric and ionic forms of 6-thioguanine is studied comprehensively both theoretically and experimentally. An excited-state diagram of 6-thioguanine N9H tautomer is proposed. The temperature behavior of 6-thioguanosine is examined in different cryoprotector solutions and with different aggregate states of solvents. Structure and phase transitions in low-molecular-weight cryoprotectors (glycerol, ethanol, propanediol, DMSO) and their water solution are investigated in the 4.2-273 K temperature range. New structural transitions in propanediol-water solutions are found in the temperature interval 10-180 K. DNA solutions are investigated by using 6-thioguanine incorporated in DNA by the method of biosynthesis. Phosphorescence intensity curves for 6-thioguanosine in native DNA manifest peculiarities at 21, 64, 87, 140, 180, and 268 K.