Letters On What Time Scale Does Solvent Relaxation in Phospholipid Bilayers Happen? J. Sy ´ kora, † P. Kapusta, ‡ V. Fidler, ‡ and M. Hof* ,† J. Heyrovsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, and Center for Complex Molecular Systems and Biomolecules, 18223 Prague 8, Czech Republic, and Department of Physical Electronics, Czech Technical University in Prague, 18000 Prague 8, Czech Republic Received August 21, 2001. In Final Form: November 19, 2001 Time-resolved emission spectra of seven fluorescent probes in egg-phosphatidylcholine bilayers have been investigated. About 90% of the solvent relaxation monitored by the headgroup labels Prodan, Laurdan, and Patman and by the backbone label 2-AS can be captured with an instrument providing subnanosecond time resolution. In comparison to 2-AS, the transient red-shift of 9-AS is characterized by a larger contribu- tion of a picosecond process and by slower nanosecond dynamics. The major contribution to solvent relaxation probed by C17DiFU and Dauda is faster than the ultimate time resolution of the experiment; those chromophores appear to be located within the external interface of the bilayer. Introduction During the past years, solvent relaxation monitored by time-resolved fluorescence measurements has become an extremely useful method in membrane research. 1 It has been shown that suitable fluorescent dyes allow for direct observation of viscosity and polarity changes in the vicinity of the probe molecule which can be intentionally located in the hydrophobic backbone or in the hydrophilic head- group region of the phospholipid bilayer. The benefit of this approach in biomembrane research has been dem- onstrated in several publications. 1-5 The development of ultrafast spectroscopic methods has led to an accurate description of the solvent relaxation process for a large variety of isotropic polar solvents. At ambient tempera- tures, a typical solvent relaxation process in solution starts with a fast inertial (librational) motion on the 50-500 fs time range, followed by rotational and translational diffusion occurring on the pico- to subnanosecond time scale. 6,7 On the other hand, it has been demonstrated 1-5,8,9 that a substantial part of solvent relaxation monitored by dyes associated with phospholipid bilayers occurs on the nanosecond (ns) time scale. A quantitative description of the solvent relaxation in bilayers, however, is still missing, which represents a limitation for more frequent applica- tions of this approach in membrane studies. Thus, we * To whom correspondence should be addressed. † J. Heyrovsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, and Center for Complex Molecular Systems and Biomolecules. ‡ Department of Physical Electronics, Czech Technical University in Prague. (1) Hof, M. In Solvent Relaxation in Biomembranes (Applied Fluo- rescence in Chemistry, Biology, and Medicine); Rettig, W., Ed.; Springer- Verlag: Berlin, 1999; p 439. (2) Hutterer, R.; Schneider, F. W.; Sprinz, H.; Hof, M. Biophys. Chem. 1996, 61, 151-160. (3) Hutterer, R.; Schneider, F. W.; Hermens, W. T.; Wagenvoord, R.; Hof, M. Biochim. Biophys. Acta 1998, 1414, 155-164. (4) Hutterer, R.; Schneider, F. W.; Lanig, H.; Hof, M. Biochim. Biophys. Acta 1997, 1323, 195-207. (5) Hutterer, R.; Schneider, F. W.; Hof, M. J. Fluoresc. 1997, 7, 27- 33. (6) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. J. Phys. Chem. 1995, 99, 17311-17337. (7) Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. Nature 1994, 369, 471-473. (8) Krishna, M. M. G. J. Phys. Chem. A 1999, 103, 3589-3595. (9) Kumar Pal, S.; Sukul, D.; Mandla, D.; Bhattacharyya, K. J. Phys. Chem. B 2000, 104, 4529-4531. © Copyright 2002 American Chemical Society FEBRUARY 5, 2002 VOLUME 18, NUMBER 3 10.1021/la011337x CCC: $22.00 © 2002 American Chemical Society Published on Web 01/10/2002