Understanding the structure of lipid sponge phase nanoparticle (L3-NPs) using small angle neutron scattering

Why?

Nonlamellar lipid liquid crystalline phases have many potential applications, such as for drug delivery, protein encapsulation or crystallization. Lipid liquid crystalline sponge phase (L3) has so far not been very much considered in these applications, in spite of apparent advantages in terms of its flexibility and capacity of forming large aqueous pores able to encapsulate large bioactive molecules.

We earlier reported a mixture of mono- and diglycerides able to form colloidally stable sponge-like nanoparticles (L3-NPs) with aqueous pores up to 13 nm of diameter. This lipid system was composed of diglycerol monoleate (DGMO), glycerol monoleate (Capmul GMO-50) and polysorbate 80 (P80). However, the L3 detailed structure and the role of P80 in the stabilization of the nanoparticles were still unknown and were therefore investigated in this study.

How?

Using small angle neutron scattering (SANS) and ‘contrast variation’, which is used to increase, create or vary the contrast of the sample or parts of the sample. To understand better the L3 structure and reveal the location of P80, we:

  1. a) Increased the contrast by using deuterated P80 (dP80), where 93 % of the head group hydrogens were replaced by deuterium. In this way, we could locate dP80 within the L3-NPs, but also gave extra contrast to the L3 phase structure formed compared to the non-deuterated hP80 L3-NPs.
  2. b) Modified the solvent contrast by studying different H2O/D2O mixtures. Depending on the H/D ratio different parts or components within the lipid nanoparticles were highlighted or matched out (Δρ = 0).

SANS showed that P80 was mostly located at the particle surface at the expense of a more lipophilic core. In addition, SANS data also indicated that some P80 was in the core of nanoparticles contributing to the formation of the L3 structure and thus, being part of the L3 bicontinuous membrane.

SANS measurements were carried out on the time-of-flight SANS2D instrument at the ISIS Neutron and Muon source of the STFC Rutherford Appleton Laboratory

Figure 1.This SANS work was conducted by Maria Valldeperas and Tommy Nylander from the division of Physical Chemistry at Lund University with collaboration with Sarah Rogers and Najet Mahmoudi from ISIS Neutron and Muon source at Rutherford Appleton Laboratory. This work was financially supported by the BIBAFOODs project from the European Union's Seventh Framework Programme FP7/2007-2013...

What’s next?

The potential of these L3 phases as carriers of enzymes have been explored in our group by successfully encapsulating Aspartic protease (34 kDa) and the dimeric form of β-galactosidase (238 kDa). However, it is challenging to know exactly where the enzymes are located within the lipid L3 matrix. In further experiments SANS will be used again to reveal the location of the enzymes, since proteins and lipids have very different scattering length density (shown in Figure 1).

Who?

Further information:Valldeperas et al. Soft Matter, 2019

DOI: 10.1039/C8SM02634C

Contact:

Maria Valldeperas Badell, Lund University
maria.valldeperas_badell@fkem1.lu.se