3,888 research outputs found
New mechanism of membrane fusion
We have carried out Monte Carlo simulation of the fusion of bilayers of
single chain amphiphiles which show phase behavior similar to that of
biological lipids. The fusion mechanism we observe is very different from the
``stalk'' hypothesis. Stalks do form on the first stage of fusion, but they do
not grow radially to form a hemifused state. Instead, stalk formation
destabilizes the membranes and results in hole formation in the vicinity of the
stalks. When holes in each bilayer nucleate spontaneously next to the same
stalk, an incomplete fusion pore is formed. The fusion process is completed by
propagation of the initial connection, the stalk, along the edges of the
aligned holes.Comment: 4 pages, 3 figure
Fluctuations in mixtures of lamellar- and nonlamellar-forming lipids
We consider the role of nonlamellar-forming lipids in biological membranes by
examining fluctuations, within the random phase approximation, of a model
mixture of two lipids, one of which forms lamellar phases while the other forms
inverted hexagonal phases. To determine the extent to which nonlamellar-forming
lipids facilitiate the formation of nonlamellar structures in lipid mixtures,
we examine the fluctuation modes and various correlation functions in the
lamellar phase of the mixture. To highlight the role fluctuations can play, we
focus on the lamellar phase near its limit of stability. Our results indicate
that in the initial stages of the transition, undulations appear in the
lamellae occupied by the tails, and that the nonlamellar-forming lipid
dominates these undulations. The lamellae occupied by the head groups pinch off
to make the tubes of the hexagonal phase. Examination of different correlations
and susceptibilities makes quantitative the dominant role of the
nonlamellar-forming lipids.Comment: 7 figures (better but larger in byte figures are available upon
resuest), submitte
Phase Separation of Saturated and Mono-unsaturated Lipids as determined from a Microscopic Model
A molecular model is proposed of a bilayer consisting of fully saturated DPPC
and mono unsaturated DOPC. The model not only encompasses the constant density
within the hydrophobic core of the bilayer, but also the tendency of chain
segments to align. It is solved within self-consistent field theory. A model
bilayer of DPPC undergoes a main chain transition to a gel phase, while a
bilayer of DOPC does not do so above zero degrees centigrade because of the
double bond which disrupts order. We examine structural and thermodynamic
properties of these membranes and find our results in reasonable accord with
experiment. In particular, order-parameter profiles are in good agreement with
NMR experiments. A phase diagram is obtained for mixtures of these lipids in a
membrane at zero tension. The system undergoes phase separation below the
main-chain transition temperature of the saturated lipid. Extensions to the
ternary DPPC, DOPC, and cholesterol system are outlined.Comment: 29 pages, 4 figures, 1 table. revised versio
Membrane Heterogeneity: Manifestation of a Curvature-Induced Microemulsion
To explain the appearance of heterogeneities in the plasma membrane, I
propose a hypothesis which begins with the observation that fluctuations in the
membrane curvature are coupled to the difference between compositions in one
leaf and the other. Because of this coupling, the most easily excited
fluctuations can occur at non-zero wavenumbers. When the coupling is
sufficiently strong, it is well-known that it leads to microphase separation
and modulated phases. I note that when the coupling is less strong, the
tendency towards modulation remains manifest in a liquid phase that exhibits
transient structure of a characteristic size; that is, it is a microemulsion.
The characteristic size of the fluctuating domains is estimated to be on the
order of 100 nm, and experiments to verify this hypothesis are proposed.Comment: 4 page
The strong Novikov conjecture for low degree cohomology
We show that for each discrete group G, the rational assembly map
K_*(BG) \otimes Q \to K_*(C*_{max} G) \otimes \Q is injective on classes dual
to the subring generated by cohomology classes of degree at most 2 (identifying
rational K-homology and homology via the Chern character). Our result implies
homotopy invariance of higher signatures associated to these cohomology
classes. This consequence was first established by Connes-Gromov-Moscovici and
Mathai.
Our approach is based on the construction of flat twisting bundles out of
sequences of almost flat bundles as first described in our previous work. In
contrast to the argument of Mathai, our approach is independent of (and indeed
gives a new proof of) the result of Hilsum-Skandalis on the homotopy invariance
of the index of the signature operator twisted with bundles of small curvature.Comment: 11 page
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