The researchers used the OffSpec instrument at ISIS to look at the biophysical changes that take place in the membranes of human immune cells during HIV infection, in particular the fusion process mediated by the viral glycoprotein gp41. HIV enters host cells by fusing its membrane with the target cell, a process initiated when gp41 embeds its hydrophobic segment into the host plasma membrane, prior to exchange of viral genetic material.
One of the central questions in the study was how electrostatic interactions in charged membranes influence fusion, an aspect that has remained poorly understood. The researchers found that before fusion can occur, water molecules must be expelled from between the membranes. This dehydration step forces part of the target membrane into a saddle-like shape - technically called negative Gaussian curvature, similar to the curve of a Pringles chip. Creating this saddle geometry is the crucial structural change that allows the viral and host membranes to merge.
The researchers found that zwitterionic membranes formed tighter bonds with water molecules, suggesting that membrane composition significantly influences fusion dynamics. The most striking finding was that gp41 activity increased in phase-separated membranes, highlighting the importance of membrane heterogeneity in viral fusion.
Unlike other techniques, which may not capture dynamic biological conditions, neutron reflectometry allows experiments under realistic physiological environments in a controlled model system.
“Many biological processes involve some kind of interface. Not many other techniques allow you to look at the detailed structure of a biological interface under realistic conditions," explains ISIS instrument scientist Dr. Maximilian Skoda. “Neutron reflectometry provides a molecular level view, and establishing a well-characterised model system as a platform enables further research, for example with other peptides or membranes."
In particular, all enveloped virus including HIV and SARS-COV enters host cell through a process of fusion between the viral and the host membrane, and the fusion peptides play a key role in enabling this process. Disabling such a process through some external molecules could be a very efficient anti-viral drug development strategy. Thus understanding at a molecular level what enables or enhances the fusion process could be critical towards development of effective therapies against ongoing and future viral infections. In this regard neutron scattering in general and neutron reflectometry in particular, with its unique ability to probe molecular interfaces under physiological conditions, is a very effective technique to unravel the microscopic mechanism of membrane fusion mediated by peptides.
The experimental setup involved creating bio-mimetic bilayers (a simplified version of a cell membrane) on a silicon substrate, roughly the size of a deck of cards. The approach opens the door to studying other similarly enveloped viruses, potentially leading to broader antiviral strategies.
Related publication: Microscopic insight into HIV fusion peptide-mediated dehydration and packing regulation in membranes. S Swarnakar, A Chaudhury (Indian Institute of Science), M W A Skoda (ISIS Neutron and Muon Source), Hirak Chakraborty (Sambalpur University), Jaydeep K. Basu ((Indian Institute of Science). Biophysical Journal. 124 (15), 5 August 2025, Pages 2465-2475. https://doi.org/10.1016/j.bpj.2025.06.023
