SAXS measurements of azobenzene lipid vesicles reveal buffer-dependent photoswitching and quantitative Z->E isomerisation by X-rays

Photoresponsive materials feature properties that can be adjusted near- instantaneously, reversibly, and with high spatiotemporal precision, by light. There is considerable interest in maximising the degree of switching, and in measuring this degree during illumination in complex environments. We study the switching of photoresponsive lipid membranes that allow precise and reversible manipulation of membrane shape, permeability, and fluidity. Though these macroscopic photoresponses are clear, it is unclear how large the changes of trans/cis ratio are, and whether they can be improved. Here, we used small-angle X-ray scattering to measure the thickness of photoswitchable lipid membranes, and correlate thickness to trans/cis ratios. This revealed an unexpected dependency of photoswitching upon aqueous phase composition, highlighting smaller-than-expected photoswitching with deionized water, and showed thickness variations twice as large as previously observed. Soft X-rays can quantitatively isomerise photolipid membranes to the all- trans state: both enabling more powerful X-ray-based membrane control and underlining the role of high energy X-rays for observation-only soft matter experiments.