Quenching of tryptophan fluorescence by brominated phospholipid

Bromolipids [1-palmitoyl-2-(dibromostearoyl)phosphatidylcholine] with bromines at the 4,5-, 6,7-, 9,10-, 11,12-, and 15,16-positions were used to examine the fluorescence quenching of a synthetic, membrane-spanning peptide (Lys2-Gly-Leu8-Trp-Leu8-Lys-Ala-amide) incorporated into both small and large unilamellar vesicles. The peptide-lipid vesicles were analyzed to show that at least 75% of the peptide was in a transbilayer configuration, placing the single tryptophan in its predicted place in the center of the bilayer. Quenching profiles of the peptide in bromolipid showed maximal (90%) quenching by the 15,16-bromolipid, indicating that the bromolipids can accurately locate the position of a tryptophan in the bilayer. The quenching by the other bromolipids decreased with an r6 dependence and an apparent R0 of 9 A. In addition, indole in methanolic solution was subjected to quenching by a variety of mono- and dibrominated hydrocarbons. The quenching was analyzed, by using a modified Stern-Volmer equation, and found to be greatly dependent upon the number and positioning of the bromines. Monobromobutanes were found to have a quenching efficiency of only 7% while dibromobutanes, with bromines on adjacent carbon atoms, had efficiencies of over 80%. In addition, the dibromobutanes exhibited significant "static" quenching whereas the monobrominated butanes did not. These data suggest that the bromolipids are more appropriately defined as short-range quenchers rather than strictly contact quenchers.