Spectroscopy of model-membrane liposome-protein systems: complementarity of linear dichroism, circular dichroism, fluorescence and SERS

Abstract

A range of membrane models have been developed to study components of cellular systems. Lipid vesicles or liposomes are one such artificial membrane model which mimics many properties of the biological system: they are lipid bilayers composed of one or more lipids to which other molecules can associate. Liposomes are thus ideal to study the roles of cellular lipids and their interactions with other membrane components to understand a wide range of cellular processes including membrane disruption, membrane transport and catalytic activity. Although liposomes are much simpler than cellular membranes, they are still challenging to study and a variety of complementary techniques are needed. In this review article, we consider several currently used analytical methods for spectroscopic measurements of unilamellar liposomes and their interaction with proteins and peptides. Among the variety of spectroscopic techniques seeing increasing application, we have chosen to discuss: fluorescence based techniques such as FRET (fluorescence resonance energy transfer) and FRAP (fluorescence recovery after photobleaching), that are used to identify localisation and dynamics of molecules in the membrane; circular dichroism (CD) and linear dichroism (LD) for conformational and orientation changes of proteins on membrane binding; and SERS (Surface Enhanced Raman Spectroscopy) as a rapidly developing ultrasensitive technique for site-selective molecular characterisation. The review contains brief theoretical basics of the listed techniques and recent examples of their successful applications for membrane studies.

Couette flow fluorescence detected linear dichroism for analytes in lipid bilayers

“…To bridge this gap, a range of complementary methods that probe membrane proteins in environments that at least mimic a biological system are required. Flow linear dichroism (LD) 3–7 has been successfully used to determine the orientation of protein secondary structures with respect to the membrane normal of a range of liposomes 4,5,7–12 . However, scattering from the liposomes is larger than the LD signals and correcting for the scattering component at lower wavelengths is challenging 13 .…”

Determination of Protein Amount in Nanosized Synthetic Liposomes by Surface Effect Raman Spectroscopy (SERS)

“…The author found that the signal from the lipid bilayer could be differentiated from the total liposome signal with protein with 1% and 10% concentration. As demonstrated before (Tukova et al, 2021), SERS analysis is sensitive in investigating liposome-protein systems. Here, the author presents a workflow to fully characterize nano-vesicles via SERS-tweezers, which paves the way towards label-free integrated extracellular vesicle analysis, which is important for cancer research.…”

Quantification of lung surfactant lipid (dipalmitoylphosphatidylcholine/sphingomyelin) ratio in binary liposomes using Raman spectroscopy

“…Liposomes are small artificial vesicles made from amphiphilic lipids, such as PC and SM. They can be used as model systems 8 to study cell membrane dynamics in the laboratory. Several experimental techniques such as differential scanning calorimetry, 9 fluorescent imaging, 10 atomic force microscopy, and Raman imaging are used to study liposomes to understand the domain separation, domain co‐existence, lateral separation, and phase transition in biological membranes.…”