  Bending rigidity, k, is classically measured for lipid membranes to
characterize their nanoscale mechanical properties as a function of
composition. Widely employed as a comparative tool, it helps understanding the
relationship between the lipid's molecular structure and the elastic properties
of its corresponding bilayer. Widely measured for phospholipid membranes in the
shape of giant unilamellar vesicles (GUVs), bending rigidity is determined here
for three self-assembled structures formed by a new biobased glucolipid
bioamphiphile, rather associated to the family of glycolipid biosurfactants
than phospholipids. In its oleyl form, glucolipid G-C18:1 can assemble into
vesicles or crystalline fibers, while in its stearyl form, glucolipid G-C18:0
can assemble into lamellar gels. Neutron spin-echo (NSE) is employed 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 gel, Fiber gel) solely
composed of a single glucolipid. The low (k= 0.30 $\pm$ 0.04 kbT) values found
for the Vesicle suspension and high values found for the Lamellar (k= 130 $\pm$
40 kbT) and Fiber gels (k= 900 $\pm$ 500 kbT) are unusual when compared to most
phospholipid membranes. 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.
