Bottom-up synthetic biology aims at building structures and functions of living cells from non-living building blocks by leveraging tools from biology, chemistry, and materials science. The field has progressed from recreating simple biological functions to integrating complex systems that combine both biological and synthetic functions. For this reason, there is a pressing need to develop new tools to functionalise artificial cells to achieve higher levels of complexity. One approach to functionalise artificial cells is through their membranes, particularly through the incorporation of functional phospholipids in the membrane. Thus, there is a need to synthesise artificial lipids with functional headgroups to enable their incorporation into the membrane of vesicles.

In this research, a phosphatidyl-histidinol lipid carrying an imidazole moiety was synthesised as a tool for reversibly binding his-tagged compounds to the membrane of vesicles and facilitating vesicle-vesicle interactions.
The lipid was synthesised by enzyme-assisted synthesis using phospholipase D, wherein the headgroup of a phospholipid was transphosphatidylated with L-histidinol. We validated the lipid-binding functionality with metal ions, both in a coated system and in a vesicular system. For proof of concept, his-mEGFP was utilized. Additionally, the lipid exhibited catalytic properties attributed to its imidazole moiety, with a metal dependency as well as a lipid presentation dependency. Unexpectedly, the incorporation of PtdHisOH into a vesicular system promoted membrane asymmetry with the synthetic lipid being predominantly located in the outer leaflet of the bilayer. Finally, we demonstrated lipid-lipid interactions mediated by zinc ions, resulting in hemi-fusion of vesicles lipid membrane mixing.

The PtdHisOH lipid offers a new tool for facilitating vesicle aggregation and promoting cell-cell interaction, thereby enabling the formation of tissue-like structures. Its potential applications extend to biomedical fields, including targeted nanodelivery, leveraging the lipid's metal selectivity. Moreover, the lipid holds promise in the development of micromachines, facilitating in situ specific reactions.