Partitioning of amphiphiles between coexisting ordered and disordered phases in two-phase lipid bilayer membranes

The partition coefficients (K-P) of a series of single-chain and double-cha in fluorescent amphiphiles, between solid ordered (P-beta, and L-beta) and liquid disordered (L-alpha of the type I-d) lipid phases coexisting in the same lipid bilayer, was studied using steady-state fluorescence emission an isotropy. The single-chain amphiphiles were N-(7-nitrobenzoxa-2, 3-diazol-4 -yl)-alkylamines, and the double-chain amphiphiles were N-(7-nitrobenzoxa-2 , 3-diazol-4-yl)-phosphatidylethanolamines with chain lengths of 12-18 carb on atoms. Saturated 18-carbon alkyl/acyl chain compounds were also compared with Delta(9)-cis unsaturated chains of the same chain length. The fluores cence anisotropy of the probes was examined in lipid bilayers (multilamella r vesicles) prepared from an equimolar mixture of dilauroylphosphatidylchol ine and distearoylphosphatidylcholine and studied as a function of temperat ure through the entire temperature range of coexistence of ordered gel phas es and a disordered fluid phase in this system. The unsaturated chain amphi philes partitioned exclusively into the fluid phase whenever this phase was present, as did the saturated chain amphiphiles with the shortest chains ( C-12:0), while K-P ranges between 1 and 2, in favor of the L-beta solid pha se, for the amphiphiles with long saturated (C-18:0) alkyl/acyl chains, wit h intermediate behavior for the intermediate chain lengths. All probes appe ared to be totally excluded from P-beta, solid (gel) phases. The technique was also used to determine partitioning of some of the probes between coexi sting liquid ordered (cholesterol-containing) (I-o) and liquid disordered ( I-d) L-alpha phases. In this case the ratio of signal amplitude to noise al lowed us to obtain a qualitative, but not quantitative, measure of the phas e partitioning of the probes. We conclude that the partitioning behavior of the probes examined between coexisting I-o and I-d phases is qualitatively similar to that observed between solid ordered and liquid disordered phase s.