Measuring the bending rigidity of microbial glucolipid (biosurfactant) bioamphiphile self-assembled structures by neutron spin-echo (NSE)

Bending rigidity, k, is classically used as a comparative tool to characterize the mechanical properties of phospholipid biomimetic membranes. [1,2] Generally measured for giant unilamellar vesicles (GUVs), k is determined here for three self-assembled structures formed by a new biobased glucolipid bioamphiphile, rather known for its surfactant properties. [3] However, recent work has shown that this class of new biobased amphiphile is able to assemble into vesicles, which can be of interest for drug delivery purposes. [4] In its oleyl form, the single-glucose (G) lipid, G-C18:1, can form either a vesicle suspension or a fibrous hydrogel, while in its stearyl form, the glucolipid G-C18:0 can assemble into lamellar hydrogels (Figure 1). Considering the morphological variety of the self-assembled forms of these two similar molecules, the range of analytical techniques estimating k is very limited. Classical approaches like flickering spectroscopy or pipette aspiration [1] must be excluded, as these require the use of giant unilamellar vesicles only. On the other hand, neutron spin-echo (NSE) is a good option, [2] as it can be applied to molecular and supramolecular objects of any shape and size. On the other hand, the data analysis and interpretation is essentially optimized for bilayers [5,6] and use of the current theoretical grounds for other shapes should be taken with care and essentially for comparative purposes.
NSE is employed here in the q-range between 0.3 nm-1 (21 nm) and 1.5 nm-1 (4.1 nm) with a spin-echo time in the range of up to 500 ns to characterize the bending rigidity of three different structures (vesicle suspension, lamellar and fiber hydrogels, Figure 1), each composed of one type of glucolipid only. Very low bending rigidity values (k<k_BT) are found for the vesicle suspension while unexpectedly high values of k are found for the lamellar (k= 130 ± 40 k_BT) and fiber (k= 900 ± 500 k_BT) hydrogels. Typical k values obtained by NSE for phospholipid bilayers are in the range between 25 and 100 k_BT, depending on the saturation and sterol content. By attempting to quantify for the first time the bending rigidity of self-assembled bioamphiphiles, this work not only contributes to the fundamental understanding of these new molecular systems, but it also opens new perspectives in their integration in the field of soft materials. However, these results should certainly be reproduced, possibly using other analytical approaches, as full understanding of the discrepancy compared to biomimetic membranes is still unclear.