Association of Cu2+ with uracil and its thio derivatives: a theoretical study.
The structures and relative stabilities of the complexes between Cu2+ and uracil, 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil were investigated by B3LYP/6-311+G(2df,2p)//B3LYP/6-31G* DFT calculations. In those systems in which both types of basic centers, that is, a carbonyl and a thiocarbonyl group, are present, association of Cu2+ with the oxygen atom is systematically favored, in contrast to what was found for the corresponding Cu+ complexes. This can be understood by considering that association of Cu2+ is immediately followed by oxidation of the base, which accumulates the negative charge at the oxygen atoms. Similarly, for 2,4-dithiouracil the most basic site for Cu+ attachment is the sulfur atom at the 4-position, while for association of Cu2+ it is sulfur at the 2-position. In contrast, differences between uracil-Cu+ and uracil-Cu2+ complexes are very small, and in both cases the oxygen atom at the 4-position is the most basic. Cu2+ binding energies are about 4 and 1.2 times larger than Cu+ binding energies and proton affinities, respectively. Uracil- and thiouracil-Cu2+ complexes are thermodynamically unstable but kinetically stable with respect to their dissociation into uracil*+ + Cu+ or thiouracil*+ + Cu+. The Cu2+ binding energies vary with the difference between the second ionization potential of the metal and the first ionization potential of the base. regardless of the reference acid (H+, Cu+, Cu2+) the basicity trend is 2,4-dithiouracil > 4-thiouracil > 2-thiouracil > uracil.
Lamsabhi
Al Mokhtar
AM
Departamento de Química C-9, Facultad de Ciencias Universidad Autónoma de Madrid Cantoblanco, 28049 Madrid, Spain.
Alcamí
Manuel
M
Mó
Otilia
O
Yáñez
Manuel
M
Tortajada
Jeanine
J
D002412:Cations; D003300:Copper / Q000737:chemistry*; D005609:Free Radicals; D008956:Models, Chemical; D015394:Molecular Structure; D013816:Thermodynamics; D013889:Thiouracil / Q000737:chemistry*; D014498:Uracil / Q000737:chemistry*
D002412:Cations; D005609:Free Radicals; D014498:Uracil; D013889:Thiouracil; D003300:Copper