Speaker
Description
Lyotropic liquid crystals of 1-, 2-, or 3-dimensional periodicity spontaneously assemble when lipids are mixed with aqueous solvent under various conditions of temperature, pressure and hydration. The most relevant non-lamellar phases from a biological perspective are the inverse hexagonal HII phase, and the inverse cubic phases. There are two quite distinct types of inverse cubic phase: bicontinuous ones based on underlying periodic minimal surfaces, and discontinuous ones based on simple or more complex packings of discreet inverse micelles.
In this lecture I will briefly review lipid self-assembly, interfacial curvature and phase diagrams. I will then go on to describe how the phase behaviour can be controlled, and the structure of lyotropic phases can be tuned, by various parameters such as temperature, hydrostatic pressure, or the addition of amphiphilic molecules such as fatty acids, diacylglycerols, and cholesterol. For potential medical applications, bulk lipid phases can be dispersed into lipid nanoparticles of the order of 100 – 200 nm in diameter. These are named hexosomes when formed from the HII phase, cubosomes when based on inverse bicontinuous cubic phases, and micellosomes when based on discontinuous cubic phases. It is important to consider whether the internal structure has been disrupted or modified upon converting bulk phases into lipid nanoparticles.
By incorporation of charged phospholipids, we have been able to swell inverse bicontinuous cubic phases to lattice parameters of approx. 500 Å, with water channels of approx. 220 Å diameter, po-tentially expanding the range of usefulness of such phases for applications such as drug delivery or encapsulation of enzymes [1, 2]. We have shown that cubosomes formed from these lipids can be swollen by charged lipids and also by cholesterol, and can show increased incorporation of the lec-tin PHA-L, a tetrameric protein of 120 kDa upon swelling [3].
We have previously shown that by addition of weakly-polar amphiphiles such as diacylglycerols to phospholipids, we can tune the interfacial curvature to be strongly inverse, leading to the formation of a discontinuous cubic phase of spacegroup Fd3m, with a structure based upon a complex close packing of two types of quasi-spherical inverse micelles. We investigated the effect of hydrostatic pressure on the structure and stability of this phase, and discovered a number of novel effects [4].
We have dispersed this bulk Fd3m phase into ‘micellosomes’ by sonication in the presence of the amphiphilic block copolymer F127, and have used x-ray diffraction to compare their structure to that of the bulk Fd3m cubic phase (A.M. Sartor et al., unpublished data). We have recently demonstrated that Fd3m micellosomes can be formed in buffer at pH 7.4 by mixtures of monoolein and oleyl alcohol, containing a small amount of an ionizable lipid. By lowering the pH to below pH 6, the zwitterionic lipid becomes cationic, triggering a phase transition within the lipid nanoparticle from an internally-confined Fd3m structure (micellosome), to a more porous inverse hexagonal HII phase (hexosome), favouring release of any encapsulated contents. We have used a combination of small-angle x-ray scattering and cryo-TEM to determine the detailed internal structure within the Fd3m micellosomes [5].
We have developed a microfluidic hydrodynamic focussing technology for the production of cubosomes and hexosomes, whose size is relatively monodisperse and can be controlled by varying the flow rate ratio between the aqueous buffer and ethanolic streams [6].
Some time ago [7] we discovered a lyotropic phase of space group P63/mmc, whose structure is based upon a 3-D hexagonal packing of quasi-spherical inverse micelles, in a hydrated mixture of dioleoyl phosphatidylcholine, dioleoyl glycerol, and cholesterol. This phase is expected to have a greater chain packing frustration than the Fd3m cubic phase, and it appears that the cholesterol is able to relieve the chain packing frustration within the hydrophobic region of this phase, allowing the P63/mmc phase to form.
References
[1] A.I.I. Tyler, H.M.G. Barriga, E.S. Parsons, N.L.C. McCarthy, O. Ces, R.V. Law, J.M. Seddon, and N.J. Brooks, Soft Matter 1, 3279 (2015).
[2] H.M.G. Barrriga, A.I.I. Tyler, N.L.C. McCarthy, E.S. Parsons, O. Ces, R.V. Law, J.M. Seddon, and N.J. Brooks, Soft Matter 11, 600 (2015).
[3] H.M.G. Barriga, O. Ces, R.V. Law, J.M. Seddon, and N.J. Brooks, Langmuir (2019) 35, 16521-16527.
[4] A.I.I. Tyler, G.C. Shearman, N. J. Brooks, H. Delacroix, R. V. Law, R.H. Templer, O. Ces, and J. M. Seddon, PCCP 13, 3033 (2011).
[5] Z. Xu, Zexi, J.M. Seddon, P.A. Beales, M. Rappolt, and A.I.I. Tyler, JACS (2021), 143, 40, 16556-16565.
[6] C.P. Pilkington, C. Contini, J.D. Barritt, P.A. Simpson, J.M. Seddon, and Y. Elani, Scientific Reports (2023) 13:12684.
[7] G.C. Shearman, A.I.I. Tyler, N.J. Brooks, R.H. Templer, O. Ces, R.V. Law, and J.M. Seddon, J. Am. Chem. Soc. 131, 1678 (2009).