NEAR-INFRARED SURFACE-ENHANCED RAMAN-SPECTROSCOPIC STUDY OF ANTIRETROVIRALY DRUGS HYPERICIN AND EMODIN IN AQUEOUS SILVER COLLOIDS

The near infrared surface-enhanced Raman spectra (NIR SERS) of antiret roviraly drugs hypericin and its analogs emodin and bianthrone were st udied at different drug concentration, dimethylsulfoxide concentration , pH and time. The differences observed in the SERS spectra when varyi ng some of these parameters are attributed to changes in the adsorbate coverage and orientation of these molecules on the silver colloids, a nd can be related to the monomeric drug concentration existing in the solution. Two different drug orientations on the metal surface can be deduced: perpendicular and planar, which can be characterized by two d ifferent SERS profiles. The drug reorientation on the surface is indic ative of a change in the oligomer-monomer equilibrium in the solution, what implies that the SERS technique can be used as an indirect metho d to monitor the intermolecular interactions of these molecules in wat er. In addition the UV-visible absorption spectra of these drugs also reveals the existence of intermolecular interactions in water and the temperature dependence of these interactions. (C) 1997 Elsevier Scienc e B.V.