Fluorescence probe properties of N-octadecamido compounds of tryptophan

Two new long chain tryptophan compounds viz [3-(indolyl-30)-2-(N-octadecamido)]-methylpropionate (4) and sodium-[3-(indolyl-30)-2(N-octadecamido)]-propionate (5) have been prepared and their ̄uorescence properties ( max, f, f, rss, 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 ̄uorescence as compared to tryptophan. The steady state anisotropy values show that the molecules are immobilised in the membrane. While both the ̄uorophores show single exponential ̄uorescence decay pro®le in homogeneous media, a triple exponential ̄uorescence decay is observed in vesicular medium. The ̄uorescence 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 ̄uorescence data for 4 and 5 with those for retinalbinding 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.