New Insights on the Behavior of Prodan in Homogeneous Media and in Large Unilamellar Vesicles.

MOYANO F.; BIASUTTI M.A.; SILBER J.J.; CORREA N M

JOURNAL OF PHYSICAL CHEMISTRY B

American Chemical Society

Lugar: Washington DC, USA; Año: 2006 vol. 110 p. 11838 - 11846

1089-5647

Abstract The behaviour of PRODAN was studied in homogeneous media and in large unilamellar vesicles (LUV) of the phospholipid 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC), using absorption, emission, depolarization and time resolved spectroscopies. In homogeneous media, the Kamlet and Taft´s solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission PRODAN bands. These studies demonstrate that the absorption band is sensitive to the polarity-polarizability (p*) and the hydrogen bond donor ability (a) parameters of the media. PRODAN in the excited state is even more sensitive to these parameters and to the hydrogen bond acceptor ability (b) of the media. The transition energy (expressed in kcal/mol) for both absorption and emission bands, gives a linear correlation with the well known polarity parameter ET(30). The results from the absorption and emission bands also reveal that PRODAN aggregates in water. The monomer has two fluorescence lifetimes: 2.27 and 0.65 ns while the aggregate has a lifetime of 14.6 ns. Using steady state anisotropy measurements, the volume of the aggregate and the monomer calculated give a value of 5590 mL mol-1 and 222 mL mol-1, respectively.             In DOPC LUV, PRODAN undergoes a partition process between the water bulk and the DOPC bilayer. We show that the partition constant, Kp, value is large enough that only at [DOPC] below 0.15 mgr/ml PRODAN in water can be detected. PRODAN dissolved in LUV at [DOPC] > 1 mgr/mL, exists completely incorporated in its monomer form and senses two different microenvironments within the bilayer: a polar region in the interface near the water and a less polar and also less viscous environment, between the phospholipids tails. These environments were characterized by their fluorescence lifetimes (t) showing that, PRODAN in the polar microenvironment has a t of approximately 4ns while in the less polar region gives a value of 1.2 ns. Moreover, this probe also senses the micropolarity of these two different regions of the bilayer and yield values similar to that of methanol and tetrahydrofuran