The interaction of 6-thioguanine and human serum albumin was investigated by fluorescence, ultraviolet-visible absorption, and surface-enhanced Raman scattering. The fluorescence of human serum albumin decreased with the concentration of 6-thioguanine, and the fluorescence quenching Downloaded by [New York University] at 10:14 29 May 20152 of human serum albumin by 6-thioguanine was static. Molecular modeling showed that 6-thioguanine was located in the hydrophobic cavity in subdomain IIA of human serum albumin.Surface-enhanced Raman scattering was combined with density function theory to characterize the orientation of 6-thioguanine on gold and the 6-thioguanine functional groups bonded to human serum albumin. 6-Thioguanine was shown to be tilted on the gold surface by a N-C=S moiety. The binding sites of 6-thioguanine to human serum albumin were the NH and amino groups of the pyrimidine ring of 6-thioguanine. This study may provide information regarding the metabolism of anticancer pharmaceuticals in the human body and assist in the development of efffective compounds.

In this study, surface-enhanced Raman scattering (SERS) spectra of 6-propylthiouracil (6-PTU) on Au surface and the interaction between 6-PTU and human serum albumin (HSA) were studied. The Raman bands were assigned by the density functional theory calculations at the B3LYP/6-311++g(d,p) level. Furthermore, the effects of concentration on the SERS spectra of 6-PTU were analyzed. It shows that with the changes of the concentrations of 6-PTU, the SERS spectra of 6-PTU present significant changes, and it can be concluded that with the changing of concentrations, the orientation of 6-PTU on Au surface also changes. In addition, the SERS spectra of the interaction between 6-PTU and HSA show that the binding sites of 6-PTU to HSA are the functional groups N3H and C¼O. The information will not only be references to the study of the mechanism of the interaction between drugs and blood plasma or serum albumin but also a guidance to understand the metabolism of drugs in human body.