Self-Assembly Mechanism of pH-Responsive Glycolipids: Micelles, Fibers, Vesicles, and Bilayers

Abstract

A set of four structurally related glycolipids are described: two of them have one glucose unit connected to either stearic or oleic acid, and two other ones have a diglucose headgroup (sophorose) similarly connected to either stearic or oleic acid. The self-assembly properties of these compounds, poorly known, are important to know due to their use in various fields of application from cleaning to cosmetics to medical. At basic pH, they all form mainly small micellar aggregates. At acidic pH, the oleic and stearic derivatives of the monoglucose form, respectively, vesicles and bilayer, while the same derivatives of the sophorose headgroup form micelles and twisted ribbons. We use pH-resolved in situ small angle X-ray scattering (SAXS) under synchrotron radiation to characterize the pH-dependent mechanism of evolution from micelles to the more complex aggregates at acidic pH. By pointing out the importance of the COO − /COOH ratio, the melting temperature, T m , of the lipid moieties, hydration of the glycosidic headgroup, the packing parameter, membrane rigidity, and edge stabilization, we are now able to draw a precise picture of the full self-assembly mechanism. This work is a didactical illustration of the complexity of the self-assembly process of a stimuli-responsive amphiphile during which many concomitant parameters play a key role at different stages of the process.

pH- and Time-Resolved in Situ SAXS Study of Self-Assembled Twisted Ribbons Formed by Elaidic Acid Sophorolipids

“…At pH below 6, mild turbidity appears in the range of about 20 min, and it strongly increases after about 30 min to 1 h. At pH above 9, the turbidity appears only after 30 min and one needs to wait ~ 1 h for a highly turbid solution. This behaviour is very different than SASL, in which the micellar phase (observed at pH >7.4) 18 does not undergo any phase change. As can be seen, these qualitative observations in fact suggest that pH has a less drastic influence on the self-assembly properties of EASL in solution.…”

“…Similarly as in the acidic medium, the SAXS signal of the ribbons always contains a non-negligible fraction of the micelles. Micelles are commonly found in sophorolipids systems and we have previously modeled the SAXS data of both OASL and SASL 18 as a function of pH. On the basis of that work, we employ here the same methodology and we apply a core-shell ellipsoid form factor Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 13 model to fit the time-resolved in-situ SAXS data.…”

“…pH changes have been obtained by using a 0.1 M HCl solution introduced via a motor-controlled syringe pump. 18 Data have been acquired on the high brilliance ID02…”

“…10,18 The entire series of time-resolved experiments has been modelled between 0.42 < q [nm -1 ] < 1.84 so to leave out the upcoming diffraction peak, but modelling on a larger q-scale (0.42 < q [nm -1 ] < 4.50) was also successfully tested (see supporting information, modelling of the time-resolved in-situ SAXS data section).…”

Surface-induced assembly of sophorolipids

“…[55][56][57] It has also been established that the structure of the molecules greatly influenced the self-assembly. 58 Acidic monounsaturated sophorolipids form ellipsoidal micelles in water in a very broad pH range (from pH 11 to pH 3), 59 although at basic pH, formation of nanoplatelets is also observed. 60 We also reported the micellar structure and their surface charge induced by pH.…”