Ultrafast Dynamics at Lipid-Water Interfaces

Abstract

Lipid membranes are more than just barriers between cell compartments; they provide molecular environments with a finely tuned balance between hydrophilic and hydrophobic interactions that enable proteins to dynamically fold and self-assemble to regulate biological function. Characterizing dynamics at the lipid-water interface is essential to understanding molecular complexities from the thermodynamics of liquid-liquid phase separation down to picosecond-scale reorganization of interfacial hydrogen-bond networks.Ultrafast vibrational spectroscopy, including two-dimensional infrared (2D IR) and vibrational sum-frequency generation (VSFG) spectroscopies, is a powerful tool to examine picosecond interfacial dynamics. Two-dimensional IR spectroscopy provides a bond-centered view of dynamics with subpicosecond time resolutions, as vibrational frequencies are highly sensitive to the local environment. Recently, 2D IR spectroscopy has been applied to carbonyl and phosphate vibrations intrinsically located at the lipid-water interface. Interface-specific VSFG spectroscopy probes the water vibrational modes directly, accessing H-bond strength and water organization at lipid headgroup positions. Signals in VSFG arise from the interfacial dipole contributions, directly probing headgroup ordering and water orientation to provide a structural view of the interface.In this Account we discuss novel applications of ultrafast spectroscopy to lipid membranes, a field that has experienced significant growth over the past decade. In particular, ultrafast experiments now offer a molecular perspective on increasingly complex membranes. The powerful combination of ultrafast, interface-selective spectroscopy and simulations opens up new routes to understanding multicomponent membranes and their function. This Account highlights key prevailing views that have emerged from recent experiments: (1) Water dynamics at the lipid-water interface are slow compared to those of bulk water as a result of disrupted H-bond networks near the headgroups. (2) Peptides, ions, osmolytes, and cosolvents perturb interfacial dynamics, indicating that dynamics at the interface are affected by bulk solvent dynamics and vice versa. (3) The interfacial environment is generally dictated by the headgroup structure and orientation, but hydrophobic interactions within the acyl chains also modulate interfacial dynamics. Ultrafast spectroscopy has been essential to characterizing the biophysical chemistry of the lipid-water interface; however, challenges remain in interpreting congested spectra as well as designing appropriate model systems to capture the complexity of a membrane environment.

Figure 1.

Diagram of functional groups commonly used as vibrational reporters of lipid membranes. Labels indicate the approximate region of the spectrum where each functional group is most often probed.

Figure 2.

Cartoon representation of spectral diffusion. Diagonally elongated 2D peaks (middle panel) become round at long pump–probe waiting times due to population exchange. Analogous steady-state spectra (top) show no line shape change upon spectral diffusion. Points overlaid on the 2D spectra represent the assigned peak maxima at each pump frequency. The white lines indicate the diagonal elongation of each 2D peak. The slope of this line is plotted versus the waiting time (bottom). Time constants from an exponential fit to this data represent the ensemble-averaged frequency–frequency correlation lifetime, from which dynamics such as H-bond lifetimes are extracted. The loss of correlation at longer waiting times is referred to as “spectral diffusion”.

Figure 3.

Distribution of functional group locations along the membrane normal (Z-coordinate). The distributions are extracted from a molecular dynamics (MD) trajectory of a dimyristoylphos-phatidylcholine bilayer. Histograms computed from MD trajectories are published in ref .

Figure 4.

Location and identity of vibrational probes in a lipid bilayer for 2D IR (left) and at the air–water interface for VSFG (right) experiments. The lipids are shown as gray sticks, with the headgroups (dark gray) and the carbonyl groups (red) highlighted for clarity. The configurations were sampled from a MD trajectory of dipalmitoylphosphatidylcholine (DPMC) published in ref

Figure 5.

Dynamics of lipids with differing tail lengths and unsaturation observed via spectral diffusion. (a) Two-dimensional IR sequence spectra of dipalmitoylphosphatidylcholine (DPPC) in the ester C=O region at various waiting times, showing a loss of diagonal elongation due to spectral diffusion. (b) Spectral diffusion analyzed by a center line slope (CLS) analysis of the saturated lipid DPPC (blood red) and the unsaturated lipid dipalmitoylphosphatidylcholine (DOPC) (teal). Experimental methods are described in the Supporting Information.

Figure 6.

Transmembrane crowding effects on ultrafast dynamics. (upper) Top-view of representative “pure lipid” and peptide-crowded membranes with a 1:10 peptide:lipid (P:L) ratio. (lower) Center line slope relaxation time constants (left) and computed frequency autocorrelations of the carbonyls, as a function of the peptide:lipid ratio. Reproduced from ref . Copyright 2020 American Chemical Society.