TWISTED INTRAMOLECULAR CHARGE-TRANSFER EMISSIONS OF FLUORESCENCE PROBES IN DIDODECYLDIMETHYLAMMONIUM BROMIDE, DIOCTADECYLDIMETHYLAMMONIUM BROMIDE, AND DIDODECYL PHOSPHATE VESICLES UNDERGOING FUSION

Twisted intramolecular charge-transfer (TICT) 4-(dimethylamino)benzonitrile (DMABN) and 1,1,4,4-tetraphenylbutadiene (TPB) fluorescence probes have been shown to sensitively report changes of their microenvironments (polarity and/or free volume) in didodecyldimethylammonium bromide (DDAB), dioctadecyldimethylammonium bromide (DODAB), and didodecyl phosphate (DDP) surfactant vesicles. Dynamic light scattering established the hydrodynamic diameters (D(H)) of DDAB, DODAB, and DDP vesicles to be 115, 105, and 85 nm, respectively. Fluorescence anisotropy measurements of vesicle-incorporated, 1,6-diphenyl-1,3,5-hexatriene as a function of temperature led to phase transition temperatures of approximately 17, 35, and 28-degrees-C for DDAB, DODAB, and DDP vesicles. The observed emission maxima of DMABN at 21-degrees-C indicated a more tightly packed and, hence, less polar environment for the probe in DODAB vesicles (489 nm) in their gel states than that in DDAB vesicles (504 nm) in their fluid states. Identical emission maxima of DMABN (488 nm) were observed, however, in DDAB and DODAB vesicles in their gel states at 5-degrees-C. No difference in the emission spectra of TPB was observed between that in DDAB and that in DODAB vesicles at any temperature. Incubation of DODAB vesicles in Na2SO4 at 20-degrees-C resulted in fusion, as demonstrated by fluorescence energy-transfer experiments from N-(7-nitrobenz-2-oxa-1,3-diazol-4-oyl)phosphatidylethanolamine (D) to N-(lissamine rhodamine B sulfonyl)phosphatidylethanolamine (A). During fusion, D(H) values of DODAB vesicles increased to 740 nm over a 5-day period. A substantial blue shift of DMABN (from 489 to 463 nm) was observed in DODAB vesicles undergoing fusion, indicating decreased micropolarities and/or free volumes of the probe. Similarly, the emission maximum of TPB in DODAB vesicles was found to blue shift from 455 to 488 and 440 nm upon incubation in Na2SO4 at 20-degrees-C for 2 and 7 days, respectively. Fluorescence maximum of DMABN in DDP vesicles also blue shifted (from 489 to 455 nm) upon the addition of calcium chloride, which indicated decreased micropolarities and/or free volumes of the probe upon fusion from D(H) = 85 nm to D(H) = 1060 nm vesicles. Fluorescence maximum of TPB in DDP vesicles changed from 453 to 457 nm following the addition of CaCl2, which, upon complexing the calcium ions by excess EDTA, showed a pronounced blue shift to 444 nm. These results are explicable in terms of the reported formation of tubular structures upon the addition of calcium ions to DDP vesicles and in terms of the subsequent reformation of spherical structures by the introduction of EDTA.