Rmrs. Mesquita et al., Partitioning of amphiphiles between coexisting ordered and disordered phases in two-phase lipid bilayer membranes, BIOPHYS J, 78(6), 2000, pp. 3019-3025
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.