  We extend previous work on homogeneous bilayers to calculate the barriers to
fusion of planar bilayers which contain two different amphiphiles, a
lamellae-former and a hexagonal former, with different compositions of the
twoin each leaf. Self-consistent field theory is employed, and both standard
and alternative pathways are explored. We first calculate these barriers as the
amount of hexagonal former is increased equally in both leaves to levels
appropriate to the plasma membrane of human red blood cells. We follow these
barriers as the composition of hexagonal-formers is then increased in the cis
layer and decreased in the trans layer, again to an extent comparable to the
biological system. We find that, while the fusion pathway exhibits two barriers
in both the standard and alternative pathways, in both cases the magnitudes of
these barriers are comparable to one another, and small, on the order of 13 kT.
As a consequence, one expects that once the bilayers are brought sufficiently
close to one another to initiate the process, fusion should occur rapidly.
