Experiments and Modeling on an Air-water Interface Populated by Biological Molecules
Date Issued
2009
Date
2009
Author(s)
Huang, Yu-Lin
Abstract
This dissertation is the first stage of a research program devoted to the development of a new hydrodynamic process in which the object is to facilitate the recognition of certain important proteins structures (such as membrane protein) in the field of molecular biology. The major objective is to find hydrodynamic conditions which favor the growth of a two-dimensional (2-D) crystal of proteins at a chemically-functionnalized air-water interface. It includes researching the best hydrodynamic conditions for growing 2-D crystal protein sufficiently large and regular, e.g. a single (mono) crystal. Obtaining such a single (mono) crystal would helpful for X-ray diffraction technique to identify the primary structure of a protein rapidly.
The state of the art today is based upon a water surface at rest, functionalized (covered) by a monolayer of lipids thus obtained for Langmuir film (nanometer thick) at equilibrium. Under the effect of chemical diffusion within water subphase, solubilize
dproteins in the subphase adsorb to the lipids specifically designed to trap them (consider for instance the molecular complex [biotinylated lipid–avidin protein]). A protein crystal obtained at 2-D interface limited to its 2-D diffusion in the lipid monolayer is more like a crystalline powder. The irregularity of the molecular self-assembly in the crystalline powder is particularly ill-suited to the X-ray diffraction technique to identify its structure.
The aim of this dissertation is to control a recirculating flow in the subphase to:
- accelerate the capture of proteins to lipids presented in the liquid surface,
- densification of 2-D complexes [lipid–protein] after adsorption,
- ultimately, should logically lead to a 2-D single crystal assembly.
This dissertation contributes to the experimental and modeling tools needed to develop a enhanced 2-D single crystal protein assembly.
This dissertation contributes to the experimental and modeling tools needed to develop a enhanced 2-D single crystal protein assembly.
A first part of this dissertation is devoted to the experimental set-up which is based upon an annular channel whose floor is put in rotation whilst its two vertical (cylindrical) side walls are maintained stationary. The channel is filled with a supporting subphase of acidified ultra-pure water. In order to confirm the feasibility of a flow-induced molecular densification, the user-friendly pentadecanoic acid (PDA) is chosen to mimic the response of a lipidic Langmuir monolayer when it is put out of equilibrium. A monolayer of PDA is therefore submitted to an annular shear flow. we have studied the behavior of a monolayer of pentadecanoic acid (PDA) simultaneously subject to two types of shear, one is in-plane shear, the other one is subphase shear valued at the surface. It is worthy to note that transition between liquid-expanded and liquid-condensed phases is conserved even in conditions far from thermodynamical equilibrium. Brewster angle microscopy (BAM) is used to image selectively the mesoscopic morphology of the subsequent two-phase PDA film. The area fraction of the condensed phase is carefully investigated after a permanent regime is established. The distribution of the area fraction demonstrates radially-inwards packing along the liquid surface which is induced by a centripetal surface flow originating from centrifugation of subphase along the rotating floor. For a growing level of centrifugation, a circular segregation front arises along the liquid surface. For a high enough level of centrifugation, the Langmuir film even experiences a strong morphological transition driven by a balance between surface shearing and reduced line tension. As a result, a shear-induced melting of the condensed phase generates a new patterning which can be described as a 2-D monodispersed matrix of tiny condensed droplets.
The last part of this dissertation is devoted to modelling the previous annular shear flow. The liquid surface at the top of the channel is again supposed to be covered by a layer of chemically-functionnalised hydrophobic molecules. The flow is considered as permanent, axisymmetric and creeping. The ratio of the liquid depth to the outer radius of the channel is small enough (shallow flow) so that it is possible to develop a matched asymptotic technique. In the rotating subphase, a core flow is therefore distinguished from the boundary layers along side walls. The modeling includes the possibility to take into account the impact of the radially-inwards molecular packing induced by centrifugation of the underlying bulk. More particularly, radial stratification of surface viscosity is taken into account via the jump momentum balance at the liquid surface (Boussinesq-Scriven balance).
Subjects
Annular flow
shear
surfactants
Langmuir monolayer
surface viscosity
Stokes lubrication
Type
thesis
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