The Role of Optical Projection in the Analysis of Membrane Fluctuations

TL;DR

This paper introduces a new optical microscopy analysis method that accounts for projection effects in membrane fluctuation measurements, leading to more accurate determination of membrane mechanical properties.

Contribution

It provides an analytical model for membrane fluctuation spectra considering focal depth projection, improving the accuracy of mechanical property measurements from microscopy data.

Findings

The model corrects for projection effects, yielding higher, more accurate bending rigidity values.

Results align with X-ray scattering and micromechanical measurements, resolving previous discrepancies.

The approach enhances the reliability of optical microscopy in membrane mechanics studies.

Abstract

We propose a methodology to measure the mechanical properties of membranes from their fluctuations and apply this to optical microscopy measurements of giant unilamellar vesicles of lipids. We analyze the effect of the projection of thermal shape undulations across the focal depth of the microscope. We derive an analytical expression for the mode spectrum that varies with the focal depth and accounts for the projection of fluctuations onto the equatorial plane. A comparison of our model with existing approaches, that use only the apparent equatorial fluctuations without averaging out of this plane, reveals a significant and systematic reduction in the inferred value of the bending rigidity. Our results are in full agreement with the values measured through X-ray scattering and other micromechanical manipulation techniques, resolving a long standing discrepancy with these other experimental methods.