Phenanthrenyl-indole as a fluorescent probe for peptides and lipid membranes

Abstract

A 3-(phenanthren-9-yl)-1H-indole-2-carboxylic acid (2) obtained from the cleavage of the methyl ester of the methyl 3-(phenanthren-9-yl)-1H-indole-2-carboxylate (1) was inserted into a peptide containing the RGD sequence. The GGRGDG peptide sequence was prepared by solid phase synthesis and coupled to compound (2), using diisopropylcarbodiimide (DIC) and 1-hydroxybenzotriazole (HOBt) in DMF. The peptide (3) labelled with the phenanthrenylindole moiety was obtained in 31% yield. The photophysical properties of the phenanthrenyl-indole derivatives were studied in several solvents of different polarity. Compounds 1 and 2 have reasonably high fluorescence quantum yields (between 27% and 85%) in non-protic solvents, the methyl phenanthrenyl-indole-2-carboxylate 1 being the more fluorescent compound. The fluorescence emission of both compounds is sensitive to solvent, indicating that they are good candidates for fluorescent probes. Fluorescence emission measurements of the labelled peptide in solution showed a strong decrease of fluorescence quantum yield value caused by the attachment of the Gly-Gly-Arg-Gly-Asp-Gly chain. The phenanthrenyl-indoles 1 and 2 and the labelled peptide 3 were incorporated in liposomes of dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidylglycerol (DPPG) and mixtures of both lipids. Steady-state anisotropy measurements showed that compounds 1 and 2 are located inside the lipid bilayers and are able to report the transition between the gel and liquid-crystalline phases. The RGD labelled peptide locates mainly in the outer part of the vesicle interface. These results indicate that the phenanthrenyl-indole moiety may be used as a fluorescent probe for peptides and lipid membranes.