Characterization of Titratable Amphiphiles in Lipid Membranes by Fluorescence Spectroscopy.
Understanding the ionization behavior of lipid membranes is a key parameter for successful development of lipid-based drug delivery systems. Accurate determination of the ionization state of a titratable species incorporated in a lipid bilayer however requires special care. Herein we investigated the behavior of titratable lipids in liposomes by fluorescence spectroscopy and determined which extrinsic parameters-i.e., besides those directly related to their molecular structure-determine their ionization state. Two fluorescent dyes, TNS and R18, have been used to investigate basic and acidic titratable lipids, respectively. Our results suggest that the titration behavior of the ionizable lipid in the membrane is more sensitive to the composition of the membrane and to its physical state than to the presence of solutes in the aqueous phase. Essentially overlooked in earlier studies on ionizable lipid assemblies, the concentration of the titratable lipid in the membrane was found to have a major effect on the ionization state of the lipid polar head. This may result in a shift in the apparent pKa value which may be as large as two pKa units and cannot be satisfactorily predicted.
Laboratoire de Conception et Application de Molécules Bioactives, Faculté de Pharmacie, UMR7199 CNRS - Université de Strasbourg , 74 route du Rhin - BP 60024, 67401 Illkirch, France.
Philippe
P
Pierrat
Laboratoire de Conception et Application de Molécules Bioactives, Faculté de Pharmacie, UMR7199 CNRS - Université de Strasbourg , 74 route du Rhin - BP 60024, 67401 Illkirch, France.
Luc
L
Lebeau
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000031:analogs & derivatives* / Q000737:chemistry; D016503:Drug Delivery Systems; D005456:Fluorescent Dyes / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008051:Lipid Bilayers / Q000737:chemistry*; D008081:Liposomes / Q000737:chemistry*; D015394:Molecular Structure; D009282:Naphthalenesulfonates / Q000737:chemistry; D010713:Phosphatidylcholines / Q000737:chemistry*; D010714:Phosphatidylethanolamines / Q000737:chemistry*; D011092:Polyethylene Glycols / Q000737:chemistry*; D013050:Spectrometry, Fluorescence
C519184:1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-methoxy-poly(ethylene glycol 2000); D005456:Fluorescent Dyes; D008051:Lipid Bilayers; D008081:Liposomes; D009282:Naphthalenesulfonates; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D015060:1,2-Dipalmitoylphosphatidylcholine; C081581:colfosceril palmitate; D011092:Polyethylene Glycols; C009497:2-(4-toluidino)-6-naphthalenesulfonic acid; C017251:1,2-oleoylphosphatidylcholine