A. Walter et al., DIACYLGLYCEROL AND HEXADECANE INCREASE DIVALENT CATION-INDUCED LIPID MIXING RATES BETWEEN PHOSPHATIDYLSERINE LARGE UNILAMELLAR VESICLES, Biophysical journal, 66(2), 1994, pp. 366-376
Bovine brain phosphatidylserine (BBPS) vesicles were prepared with tra
ces of dioleoylglycerol (18:1,18:1 DAG) or hexadecane (HD) to determin
e the influence of changes in headgroup or acyl chain packing on dival
ent cation-induced lipid mixing rates. A stopped-flow apparatus was us
ed to combine vesicles with 3 mM Ca2+ or Ba2+. Aggregation was monitor
ed by light scattering and lipid mixing by lipid probe dilution. Neith
er 3-6 mol % 18:1,18:1 DAG nor up to 10 mol % HD significantly altered
the BBPS chain melting temperature, vesicle diameter, or vesicle aggr
egation rates. Lipid mixing rates doubled by adding either 3 mol % 18:
1,18:1 DAG or 6 mot % HD to BBPS with no change in the Ca2+ concentrat
ion threshold. The Arrhenius slopes of the lipid mixing rates for cont
rol, 3 mol % 18:1,18:1 DAG, and 6 mol % HD vesicles were identical. H-
2-nuclear magnetic resonance spectra of perdeuterated dipalmitoylglyce
rol and HD in BBPS in the absence and presence of Ca2+ and Ba2+ showed
that the solutes occupied different time-averaged positions in the bi
layer under each condition. These data suggest that: 1) the enhanced l
ipid mixing rate is related to the volume of the added alkyl chains; 2
) 18:1,18:1 DAG and HD may alter the activation entropy or the attempt
frequency at one or more steps in the lipid mixing process; 3) 18:1,1
8:1 DAG and MD are likely to act at a different spatial or temporal po
int than the divalent cation; and 4) it is unlikely that the effect of
these solutes on lipid mixing is due to their equilibrium time-averag
ed positions in the bilayer. Others have shown that apolar lipids acce
lerate fusion in nonbilayer phase-forming systems, but BBPS does not f
orm these phases under these conditions. Therefore, we propose that th
e effect of very small amounts of apolar substances may be very genera
l, e,g., stabilizing the hydrophobic interstices associated with a var
iety of proposed intermediate structures.