Supported Lipid Bilayer Formation from Phospholipid-Fatty Acid Bicellar Mixtures

Versatile formation of supported lipid bilayers from bicellar mixtures of phospholipids and capric acid

“…Experimentally, the QCM-D protocol included the following steps: (1) establish a stable baseline signal in buffer for 5 min; (2) inject bicelles under continuous flow; and (3) wash with buffer for at least 10 min. For each q-ratio, the injection time in step (2) was defined based on the initial results obtained at the highest tested lipid concentration (0.5 mM DOPC); the time was 15 min in cases when only monotonic adsorption without rupture occurred (our previous studies indicated that bicelle rupture typically commenced within at least 15 min in cases of SLB formation 43,44 ) and as long as necessary for the signals to stabilize in cases of SLB formation. The exact injection time in the latter case depended on the total lipid concentration in the particular experiment because the rate of bicelle adsorption onto the silicon dioxide surface is diffusion-limited and the corresponding diffusion flux is inversely proportional to the total lipid concentration.…”

“…This difference can be attributed to the ionic nature of CA molecules that limits hydrogen bonding capacity with phospholipids and therefore shortens the residence time in SLBs, which minimizes membrane intercalation and potential membrane-disruptive effects 54 . On the other hand, the SLB conditions of DOPC/CA bicelles are wider-ranging than those of DOPC/LA bicelles 43 (q = 0.25 with 0.031-0.016 mM DOPC & q = 2.5 with 0.5-0.016 mM DOPC). Nevertheless, the SLBs fabricated from both DOPC/CA and DOPC/LA bicelles have similar phase uniformity owing to similar headgroup properties of the fatty acid molecules (i.e., lack of hydrogen bonding capacity) that limits membrane intercalation and favors return of these molecules to the bulk solution.…”

“…In particular, 1,2-dihexanoyl- sn -glycero-3-phosphocholine (DHPC 6 ) 29 – 32 and 1,2-diheptanoyl- sn -glycero-3-phosphocholine (DHPC 7 ) 28 , 30 are the two main short-chain phospholipids that have been examined in most SLB fabrication-related studies despite the practical need to find natural substitutes with high abundance. Since it has been possible to form solution-phase bicelles by replacing short-chain phospholipids with detergents and detergent-like molecules 39 – 42 , we recently demonstrated the ability to fabricate SLBs using bicellar mixtures composed of long-chain 1,2-dioleoyl- sn -glycero-3-phosphocholine (DOPC) lipid and detergent-like molecules consistent of a saturated fatty acid called lauric acid (LA) with a 12-carbon long chain 43 or a saturated monoglyceride called monocaprin (MC) with a 10-carbon long chain 44 . At a q-ratio of 2.5, DOPC/LA bicelles were able to form SLBs across a wide range of lipid concentrations, however, SLB formation was less successful at a q-ratio of 0.25 and only worked at very low lipid concentrations 43 .…”

“…Since it has been possible to form solution-phase bicelles by replacing short-chain phospholipids with detergents and detergent-like molecules 39 – 42 , we recently demonstrated the ability to fabricate SLBs using bicellar mixtures composed of long-chain 1,2-dioleoyl- sn -glycero-3-phosphocholine (DOPC) lipid and detergent-like molecules consistent of a saturated fatty acid called lauric acid (LA) with a 12-carbon long chain 43 or a saturated monoglyceride called monocaprin (MC) with a 10-carbon long chain 44 . At a q-ratio of 2.5, DOPC/LA bicelles were able to form SLBs across a wide range of lipid concentrations, however, SLB formation was less successful at a q-ratio of 0.25 and only worked at very low lipid concentrations 43 . On the other hand, DOPC/MC bicelles were able to form uniform-phase SLBs across a wider range of q-ratios, albeit with stricter lipid concentration-dependent requirements and other biophysical effects 44 .…”

“…In particular, DOPC/MC bicelles at the q-ratio of 0.25 were effective to form SLBs across all tested lipid concentrations, which led us to reason that using a fatty acid with equivalent chain length to that of MC would increase the range of effective lipid concentrations for SLB fabrication beyond that observed in the DOPC/LA bicelle case. In addition, since there are more surfactant-like fatty acids than DOPC phospholipids at the q-ratio of 0.25, the fatty acids would likely enhance bicelle deformation via membrane softening and hence promote stronger bicelle-substrate interactions 43 , as opposed to in the high q-ratio regime where DOPC phospholipids are the predominant species. Accordingly, these molecular properties prompted us to investigate the SLB formation potential of bicellar mixtures composed of long-chain phospholipid and capric acid (CA), which is a 10-carbon long, saturated fatty acid which has similar headgroup and chain properties to LA and MC, respectively.…”