Lipid Bilayers and Membrane Dynamics: Insight into Thickness Fluctuations

Abstract

Thickness fluctuations have long been predicted in biological membranes but never directly observed experimentally. Here, we utilize neutron spin echo spectroscopy to experimentally reveal such fluctuations in a pure, fully saturated, phosphocholine lipid bilayer system. These fluctuations appear as an excess in the dynamics of undulation fluctuations. Like the bending rigidity, the thickness fluctuations change dramatically as the lipid transition temperature is crossed, appearing to be completely suppressed below the transition. Above the transition, the relaxation rate is on the order of 100 ns and is independent of temperature. The amplitude of the thickness fluctuations is 3:7 # A AE 0:7 # A, which agrees well with theoretical calculations and molecular dynamics simulations. The dependence of the fluctuations on lipid tail lengths is also investigated and determined to be minimal in the range of 14 to 18 carbon tails.

Tuning Membrane Thickness Fluctuations in Model Lipid Bilayers

“…Membrane thickness fluctuations were only observable in the fluid phase of the bilayer, implying that such fluctuations are either completely suppressed in the gel phase of the membrane (19) or are slowed down beyond the temporal resolution of the NSE technique. The study also showed almost identical thickness fluctuation patterns (12) in the bilayers, regardless of the length of the lipid molecules used. This invariability in the dynamical behavior relative to lipid tail-length can be attributed to geometrical constraints caused by the minimalistic bilayer composition in this overly simplistic system, which favors uniform bilayer thickness and lateral membrane homogeneity.…”

“…However, the full extent of the role of fast dynamics is still poorly understood due to the experimental difficulties in probing this time regime. Interestingly, these timescales coincide with those of collective thermal fluctuations in lipid bilayers (12,13), which starts to suggest that tuning these fluctuations is critical for medical applications requiring selective macromolecule binding. Of particular interest in this context are membrane thickness fluctuations whose dynamics are on the same timescale as conformational transitions in proteins (8) and that have been associated with other membrane functions such as pore formation (14) and passive permeation (15).…”

“…In most cases, the experimental evidence for local membrane fluctuations has been rather indirect (16,17) or inferred from simulations (18,19). More recently, neutron spin-echo (NSE) spectroscopy was effectively used for direct observation of membrane fluctuations in oil-swollen surfactant bilayers (20) and unilamellar lipid vesicles (12). The NSE studies on single-component lipid bilayers with different lipid tail lengths, i.e., DMPC (dimyristoylphosphatidylcholine), DPPC (dipalmitoylphosphatidylcholine), and DSPC (distearoylphosphatidylcholine), revealed some interesting membrane dynamics.…”

Water and Lipid Bilayers

“…However, alternative explanations of this anomaly have cited internal bilayer friction as a possible explanation (Watson and Brown 2010;Woodka et al 2012).…”

Room-temperature ionic liquids meet bio-membranes: the state-of-the-art

“…Elastic and quasielastic neutron scattering can access the pico-to nanosecond dynamics (Benedetto and Kearley 2016;Magazù et al 2008Magazù et al , 2009Magazù et al , 2010Magazù et al , 2012Bee 1988;Volino 1978). Neutron spin-echo can be used to investigate slower relaxation processes (Mezei 1972) and to probe structural fluctuations (Nagao 2009;Woodka et al 2012). Finally, smallangle neutron scattering and diffraction can be used for further structural characterization.…”