Interactions of Laurdan with phosphatidylcholine liposomes: a high pressure FTIR study

The interactions of 6-lauroyl-2-dimethylaminonaphthalene (Laurdan) with L-alpha-dimyristoylphosphatidylcholine (DMPC) have been studied isothermally at 28 degrees C by Fourier-transform infrared spectroscopy (FTIR) at two pH values (6.8 and 3.0) and over the pressure range of 0.001-25 kbar. The results obtained with Laurdan are compared with those previously obtained with 6-propionyl-2-dimethylaminonaphthalene (Prodan) (Chong et al. (1989) Biochemistry 28, 8358-8363). The objective of this study is to delineate the differential interactions of Prodan and Laurdan with lipid membranes. The Laurdan carbonyl and naphthalene vibrational bands as well as the correlation field splitting of the methylene scissoring mode all indicate that in phospholipid model membrane systems, Laurdan behaves differently from Prodan. The data suggest that the chromophore of Laurdan is embedded somewhat deeper in the membrane than that of Prodan. The correlation field splitting pressure suggests that Laurdan causes more perturbation to DMPC vesicles than Prodan. Instead of being relocated to the exterior of the membrane as is the case of Prodan, Laurdan is found to remain in the membrane even when it is partially positively charged at pH 3. Apparently the stabilizing forces come from the strong van der Waals and hydrophobic interactions between the lauroyl chain and its neighboring lipid molecules. Laurdan seems to remain in the membrane at high pressures (up to 25 kbar). Using deuterated DMPC (d-DMPC) and deuterated L-alpha-dipalmitoylphosphatidylcholine (d-DPPC), we have demonstrated that, at 1 atm, there is a void space between the lauroyl chain of Laurdan and the acyl chain of the matrix lipid, regardless of the physical state of the matrix lipid. This void space, probably caused by the bulky naphthalene ring, is eventually diminished by elevated pressures.