Theoretical Studies on the Binding of Cd^(2+) to Uracil and Its Thio-derivatives

The interaction of Cd2＋ with uracil,2-thiouracil,4-thiouracil and 2,4-dithiouracil have been investigated by the density functional theory（DFT） calculations.For uracil and 2,4-dithiouracil,where the two basic sites are the same,Cd2＋ attachment to the heteroatom at position 4 is preferred.However,for the systems where both types of basic centers,a carbonyl and a thiocarbonyl groups,are present,Cd2＋ association with sulfur is favorable.The enhanced stability of these enolic and thiol forms comes from Cd2＋ interaction with two basic sites simultaneously,which thereby triggers a significant aromatization of the ring.More significantly,the Cd2＋ binding energy with uracil and its thio-derivatives is larger than the tautomerization barriers connecting the diketo-like forms with the corresponding enolic-like tautomers.Consequently,when associated with Cd2＋,all tautomers are energetically accessible and should be observed in the gas phase.