Ultrafast vibrational dynamics of water confined in phospholipid reverse micelles

Abstract

Abstract. We study the ultrafast dynamics of OH stretching and bending vibrations of water inside dioleoylphosphatidylcholine (DOPC) reverse micelles in a wide range of hydration. A strong hydration level dependence for the spectral diffusion rates is found and explained by the distinctly different environment for single water molecules bound to the anionic phosphate group. We show that the energy relaxation pathway of the OH stretching vibration at low hydration level involves the OH bending.Phospholipids, building blocks of biological membranes, typically self-assemble into bilayers in aqueous solution. The amphiphilic nature of phospholipids allows the formation of a variety of other structures, including reverse micelles. We recently introduced reverse micelles composed of dioleoylphosphatidylcholine (DOPC), a phospholipid with hydrophobic tails and a hydrophilic head consisting of a phosphate (PO 4 -) unit and a covalently linked choline group (cf. Figure 1). Such systems form when DOPC is dissolved in nonpolar solvents resulting in hydrophilic head groups pointing to the interior. Addition of water leads to hydration of the head groups as well as the buildup of nanoscopic water pools inside the reverse micelles (the water content is characterized by the parameter w 0 =[H 2 O]/[DOPC]). Therefore, reverse micelles allow for detailed studies of the fundamental hydration interactions of phospholipids.

Water in Contact with a Cationic Lipid Exhibits Bulklike Vibrational Dynamics

“…Water that is hydrogen-bonded to the lipid is found to be more isolated from bulk water, thereby displaying a slower relaxation time. 31 Water molecules in contact with zwitterionic lipids show predominantly O−D/O−H groups pointing to the surfactant/lipid, 24,26,51 making the dynamics similar to that of the negatively charged case as observed by Costard et al 52 For the positively charged lipids, the O−D/O−H vibration of the water molecules points away from the lipids (with the O atom pointing toward the positive charge) and are thus in direct contact with bulk water. The dynamics may then be expected to be mostly determined by bulk water, accounting for the faster energy transfer observed previously for water in contact with CTAB 18,41 and for water underneath DPTAP as described in the present work.…”

Water Structure at the Lipid Multibilayer Surface: Anionic Versus Cationic Head Group Effects

“…In our previous study of water hydrogen-bonding dynamics when lipid multibilayer undergoes a temperature-induced phase transition, 1 we observed the same red-shifted component and this type of the red-shifted component was assigned to phosphate-bound water. 8,9,13,29 Therefore, the comp1 with subpicosecond lifetime (0.5 ps) can be safely assigned to phosphate-bound water molecules at the DMPG lipid multibilayer surface. Because of the presence of the negatively charged phosphate group at the DMPG lipid multibilayer, it creates a negative electric field along the OD stretch, 8 which red-shifts the peak position of comp1 at the DMPG lipid multibilayer.…”

“…) stretch mode 38 Note that the asymmetric stretch frequency of the PO 2 − is at 1230 cm −1 . 1 Therefore, the vibrational density of states (VDOS) of the overtone of the asymmetric phosphate stretch mode (2460 cm −1 ) is close to the comp1 frequency in the case of the DMPG lipid multibilayer.…”

Structural Dynamics of Hydrated Phospholipid Surfaces Probed by Ultrafast 2D Spectroscopy of Phosphate Vibrations

“…Vibrational spectroscopy in the femto- to picosecond time domain allows for mapping structural dynamics of hydrated phospholipids and for characterizing phospholipid–water interactions on the time scale of molecular motions and hydrogen bond dynamics. So far, femtosecond infrared pump–probe experiments have focused on dynamics of OH or OD stretch excitations of water mainly to address water structure, reorientation, and energy dissipation. − A major drawback of this approach consists of the spatial averaging over all water environments, that is, a selective observation of interfacial water at biologically relevant high hydration levels is impossible. Nevertheless, results for low hydration levels point to a slowing down of water reorientation at the interface compared to bulk water. ,, …”