# Membrane Proteins in Action Monitored by pH-Responsive Liquid Crystal Biosensors

## TL;DR

Researchers developed a pH-sensitive liquid crystal biosensor to monitor the activity of membrane proteins, specifically bacteriorhodopsin, by detecting changes in local pH.

## Contribution

A novel pH-responsive liquid crystal biosensor was created to study membrane protein function by integrating bacteriorhodopsin with a liquid crystal interface.

## Key findings

- Bacteriorhodopsin's proton-pumping activity was successfully monitored using a pH-sensitive liquid crystal thin film.
- Local pH changes induced by membrane proteins were visually detectable via color changes in the liquid crystal film.
- The system enables the study of membrane protein biofunctions and their environmental effects in solution.

## Abstract

Liquid crystal (LC) biosensors have received significant
attention
for their potential applications for point-of-care devices due to
their sensitivity, low cost, and easy read-out. They have been employed
to detect a wide range of important biological molecules. However,
detecting the function of membrane proteins has been extremely challenging
due to the difficulty of integrating membrane proteins, lipid membranes,
and LCs into one system. In this study, we addressed this challenge
by monitoring the proton-pumping function of bacteriorhodopsin (bR)
using a pH-sensitive LC thin film biosensor. To achieve this, we deposited
purple membranes (PMs) containing a 2D crystal form of bRs onto an
LC-aqueous interface. Under light, the PM patches changed the local
pH at the LC-aqueous interface, causing a color change in the LC thin
film that is observable through a polarizing microscope with crossed
polarizers. These findings open up new opportunities to study the
biofunctions of membrane proteins and their induced local environmental
changes in a solution using LC biosensors.