Spectral properties of thymine and its interaction with tryptophan and bioinspired membranes

The interaction between the nitrogenous base thymine (Thy) and the amino acid tryptophan (Trp) plays a role in DNA-protein cross-linking and has been associated with the inhibition of urothelial carcinogenesis. In this work, we investigate these molecules through their fluorescence emission and electronic absorption properties in the presence of bioinspired membrane models, such as ionic surfactant micelles and neutral liposomes. Photophysical parameters, such as single-photon cross section, fluorescence quantum yield, and molar absorption coefficient, were determined for Thy in different solvents. The low partitioning of the target molecules into highly hydrophobic environments, such as the micelle core, was overcome by changing the medium pH, thereby exploiting the protonation/deprotonation states of Trp and Thy. Under these conditions, the molecules were electrostatically attracted to the micelle surfaces. The optical absorption spectrum of Thy is highly dependent on the medium pH, whereas for Trp, the greatest pH sensitivity lies in its fluorescent emission. The results demonstrate that Trp and Thy interact with micelle surfaces when their charges are opposite to those of the micelle. Steady-state anisotropy of Trp, combined with the spectral position of its emission, enabled accurate monitoring of the critical micelle concentration (CMC) of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) micelles. In contrast, in the presence of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DMPC) liposomes, neither Trp nor Thy caused changes in the bilayer phase behavior, as monitored by the fluorescence anisotropy of two distinct probes inserted into different bilayer environments.