title: H-aggregation of azobenzene-substituted amphiphiles in vesicular membranes.
abstract: Photochemical switching has been studied of double-tailed phosphate amphiphiles containing azobenzene units in both tails in aqueous vesicular dispersions and in mixed vesicular systems with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Since the ease of switching depends on the strength of the bilayer packing, particular emphasis has been placed on the occurrence of H-aggregation in the hydrophobic core of the vesicles. UV-vis spectrometry was employed to monitor H-aggregation and showed how this process depends on the ionic strength and on the mode of preparation of the vesicles. Two types of H-aggregates were observed in mixed DOPC vesicles with 5 mol % of azobenzene phosphate: one with lambda(max) at around 300 nm and one with lambda(max) at 305-320 nm. Those with lambda(max) at 300 nm could not be trans-cis photoisomerized, whereas those with lambda(max) at 305-320 nm are more loosely packed and can be photochemically switched. The permeability of the vesicular bilayers, as probed with leakage experiments using calcein as a fluorescent probe, was examined as another measure for the strength of bilayer packing. Leakage occurred only for DOPC vesicles containing more than 20 mol % of azobenzenephosphate, irradiated with UV light to induce trans-cis photoisomerization. We contend that detailed information on bilayer packing will be of crucial importance for fine-tuning the lateral pressure in vesicular membranes with the ultimate aim to steer the opening and closing of mechanosensitive protein channels of large conductance.
authors.affiliation: Stratingh Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
authors.forename: Johanna M
authors.initials: JM
authors.lastname: Kuiper
authors.forename: Jan B F N
authors.initials: JB
authors.lastname: Engberts
mesh_terms: D001391:Azo Compounds / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning; D029968:Escherichia coli Proteins / Q000737:chemistry; D005452:Fluoresceins / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D007473:Ion Channels / Q000737:chemistry; D008027:Light; D008051:Lipid Bilayers; D008055:Lipids / Q000737:chemistry; D008081:Liposomes; D008560:Membrane Fluidity; D008563:Membrane Lipids; D008567:Membranes, Artificial; D046529:Microscopy, Electron, Transmission; D008956:Models, Chemical; D010710:Phosphates / Q000737:chemistry; D010713:Phosphatidylcholines / Q000737:chemistry; D010743:Phospholipids; D010777:Photochemistry; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature; D014466:Ultraviolet Rays
chemical_list: D001391:Azo Compounds; D029968:Escherichia coli Proteins; D005452:Fluoresceins; D007473:Ion Channels; D008051:Lipid Bilayers; D008055:Lipids; D008081:Liposomes; D008563:Membrane Lipids; D008567:Membranes, Artificial; C086596:MscL protein, E coli; D010710:Phosphates; D010713:Phosphatidylcholines; D010743:Phospholipids; D006859:Hydrogen; C017251:1,2-oleoylphosphatidylcholine; C009850:azobenzene; C007740:fluorexon