Fluorescence probe properties of N-octadecamido compounds of tryptophan

Abstract

Two new long chain tryptophan compounds viz [3-(indolyl-3')-2-(N-octadecamido)]-methylpropionate (4) and sodium-[3-(indolyl-3')-2-(N-octadecamido)]-propionate (5) have been prepared and their fluorescence properties (λ_max, φ_f, τ_f, r_ss, quenching) have been investigated in homogeneous media and in phosphatidyl choline vesicles. Both 4 and 5 in a membrane mimicking system of phosphatidylcholine vesicles show blue-shifted emission maxima but higher quantum yields of fluorescence as compared to tryptophan. The steady state anisotropy values show that the molecules are immobilised in the membrane. While both the fluorophores show single exponential fluorescence decay profile in homogeneous media, a triple exponential fluorescence decay is observed in vesicular medium. The fluorescence of both the compounds are quenched by retinyl polyenes viz. retinal and retinyl acetate. The extent of quenching is more in organised media of vesicles as compared to organic solvents. A comparison of the fluorescence data for 4 and 5 with those for retinal-binding protein bacteriorhodopsin and the apoprotein bacterioopsin reveals some similarities in the nature of interaction between retinylidene polyenes and 4 and 5, and between retinylidene Schiff base chromophore and tryptophan residues in the protein.