Conserved molecular superlattices in a series of homologous synthetic mycobacterial cell-wall lipids forming interdigitated bilayers

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Synthetic analogues of the cell-wall lipid monomycoloyl glycerol (MMG) are promising as next-generation vaccine adjuvants. In the present study, the thermotropic phase behaviour of an array of synthetic MMG analogues was examined using simultaneous small- and wide-angle X-ray scattering under excess water conditions. The MMG analogues differed in the alkyl chain lengths and in the stereochemistry of the polar glycerol headgroup or of the lipid tails (native-like versus alternative compounds). All MMG analogues formed poorly hydrated lamellar phases at low temperatures and inverse hexagonal (H2) phases at higher temperatures before melting. MMG analogues with a native-like lipid acid configuration self-assembled into non-interdigitated bilayers whereas the analogues displaying an alternative lipid acid configuration formed interdigitated bilayers in a subgel (Lc′) state. This is in contrast to previously described interdigitated phases for other lipids, which are usually in a gel (Lβ) state. All investigated MMG analogues displayed an abrupt direct temperature-induced phase transition from Lc′ to H2. This transition is ultimately driven by the lipid chain melting and the accompanying molecular shape change. No intermediate structures were found, but the entire array of MMG analogues displayed phase coexistence during the lamellae-H2 transition. The structural data also showed that the headgroups of the MMG analogues adopting the alternative lipid acid configuration were ordered and formed a two-dimensional molecular superlattice, which was conserved regardless of the lipid tail length. To our knowledge, the MMG analogues with an alternative lipid acid configuration represent the first example of a lipid system showing both interdigitation and superlattice formation and as such could serve as an interesting model system for future studies. The MMG analogues are also relevant from a subunit vaccine perspective because they are well-tolerated and display promising immunopotentiating activity. The structural characterization described here will serve as a prerequisite for the rational design of nanoparticulate adjuvants with specific and tailored structural features.
Original languageEnglish
Issue number48
Pages (from-to)12693–12701
Number of pages9
Publication statusPublished - 7 Nov 2016

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