Ultraviolet absorption, circular dichroism and magnetic circular dichroism studies of sulfur-containing nucleic acid bases and their nucleosides

Spectroscopic studies on electronic structures were carried out for sulfur-containing nucleic acid bases, 2-thiouracil, 4-thiouracil, 2,4-dithiouracil and 2-thiocytosine along with their nucleosides. The absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra were interpreted in terms of tautomerism to suggest that 4-thiouracil is in a 2-keto-4-thione form and the other bases exist as an equilibrium mixture of thione and thiol forms in solution. The π-π★ transition energies of the thione structures of 2-thiouracil, 2-thiocytosine and their nucleosides are almost the same as those of unsubstituted pyrimidine bases and nucleosides. The n-π★ transition from a 2-thione function was clearly detected as a negative CD band at about 320 nm. On the other hand, from the absorption, CD and MCD spectra, the transitions of the 4-thiouracil derivatives were found to take place in an energy region lower than those of the corresponding parent bases.