Dynamic properties of the lipid-water interface of model membranes as revealed by lifetime-resolved fluorescence emission spectra

Abstract

We examined the dynamic properties of the lipid-water interface region of model membranes, on the nanosecond time scale, by using the fluorescent probe 2-p-toluidinylnaphthalene-6-sulfonic acid (TNS). In particular, we examined the steady-state emission spectra of TNS as its average lifetime was decreased by oxygen quenching. Under these quenching conditions the centers of gravity (Vcg) of the emission spectra shift ot shorter wavelengths. The lifetime dependence of these shifts reveals the time dependence of membrane relaxation around the excited-state dipole moment of TNS. The lipids investigated include dioleoyl-, dimyristoyl-, and dipalmitoyl-L-alpha-phosphatidylcholines, bilayers containing cholesterol, and an ether analogue of dipalmitoyl-L-alpha-phosphatidylcholine. For these lipids, the spectral relaxation times and the temperature dependence of the relaxations are similar in magnitude. Most relaxation times fall in the range of 0.6-6 ns, and except for the ether analogue, the inactivation energies for spectral relaxation are 10 plus or minus 2 kcal/mol. The average energy loss during spectral relaxation was 1000 cm(-1). However, for the saturated phosphatidylcholines at temperatures below their transition temperatures, smaller relaxation losses were observed (approximately 600 cm(-1)). We attribute these smaller losses to ordering of the polar head groups around the ground-state dipole moment of TNS. Overall, these results indicate that the dynamic properties of the lipid-water interface region are similar among the phosphatidylcholines and depend only slightly on the chemical composition and phase state of the acyl side chains.

Time-resolved fluorescence emission spectra of labeled phospholipid vesicles, as observed using multi-frequency phase-modulation fluorometry

“…The time-resolved spectral properties of fluorophores may be easily and reliably obtained from the frequency-domain measurements. The rates of spectral relaxation are in agreement with those estimated previously using the time-resolved method [11,16,23] and phase-sensitive detection of fluorescence [11] for TNS-labeled vesicles. Also, in the case of Patman, the results are in agreement with earlier studies in which the spectral relaxation was examined by phase-sensitive detection of fluorescence [17].…”

“…At each wavelength, the phase angle and modulation were measured using approximately ten frequencies. Patmanlabeled and TNS-labeled vesicles were prepared as described previously [17,23].…”

“…The center of gravity of the emission is defined by: (15) where I'(F, t) represent the number of photons per wavenumber interval. Our data were collected at equally spaced wavelength intervals [23], and using an emission monochromator with a constant bandpass in nanometers [24]. For these conditions, the center of gravity in kK (= 103 cm -I) is given by:…”

Fluorescence Studies of Red Blood Cell Membranes from Individuals with Huntington's Disease

“…where I (hi) is the intensity at wavelength hi (Lakowicz and Hogen, 1981). Emission spectra were not corrected for instrumental response.…”

Influence of Morphology on Polar Solvation Dynamics in Lecithin Reverse Micelles

“…6,16 In addition to perturbing the structure of intramicellar water, the reverse micellar environment can significantly alter its dynamical properties. While a range of measurements have shown that the mobility of water at the lecithin interface is significantly lower than that in bulk water, 12,19,21,36,39,43 the techniques used could not follow the ultrafast motion of water. Hence, these studies do not distinguish water interacting with the interface unless its dynamical response is slowed by orders of magnitude.…”