Time-gated fluorescence spectroscopy of the tumor-localizing fraction of HpD in the presence of cationic surfactant

Time-gated fluorescence spectroscopy was performed on the tumor localizing fraction (TLF) of HpD in buffer at different concentrations of cationic surfactant. This technique obtains emission spectra with programmable delay relative to the excitation pulse. According to the measured fluorescence decay-time constants (0.7, 3 and 15 ns) three gates were considered, delayed by 0, 5 and 18 ns, respectively, to evaluate the contribution of the emitting molecular species to the spectra. Simultaneous to these measurements, fluoresence decay waveforms and time-integrated spectra were also detected. In buffer and in detergent micelles the fluorescence spectra are given by the superposition of the emission of the different molecular species present in the solution, and no appreciable interaction among the chromophores is observed. On the contrary, in the pre-micellar range of the surfactant, evidence for the existence of an energy transfer mechanism was found. This effect seems to be related to the conflgurational state of the TLF polymeric chains and depends on the relative TLF/surfactant concentration.