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On the Morphology and Dynamics of Purple Membranes at the Solid-Liquid Junction
In dependence on the substrate bacteriorhodopsin within purple membrane at the solid-liquid junction is prone to dynamical transitions which were observed for the first time and found to be responsible for, in part, large scale morphological changes of the adsorbed purple membrane patches. Extrin...
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| Format: | Doctoral Thesis |
| Language: | English |
| Published: |
Philipps-Universität Marburg
2011
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| Subjects: | |
| Online Access: | PDF Full Text |
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| Summary: | In dependence on the substrate bacteriorhodopsin within purple membrane at the
solid-liquid junction is prone to dynamical transitions which were observed for the
first time and found to be responsible for, in part, large scale morphological changes
of the adsorbed purple membrane patches. Extrinsic and intrinsic factors could be
identified to govern the interplay of morphology and dynamics, which lead to the
deduction of a general, substrate independent stability criterion applicable to purple
membranes comprising BR-wild-type and variants alike. Surface free energy
and substrate roughness are key extrinsic parameters in this context that enable or
slow down dynamical transitions within solid-supported PMs. Conversion of BR
into a chloride pump by a single-point mutation seriously affects its crystallization
tendency and reveals how intrinsic factors figure into the observed interplay. Interestingly,
upon stabilization of the tertiary structure of BR-D85T, intrinsically mediated
by chloride binding in the retinal binding pocket, the ability of PM-D85T to form coherent
crystalline domains can be restored. PM-D85T also allowed for purple membrane bending of various photocycle intermediates to be studied in thermal equilibrium
at the solid-liquid junction. Side discrimination via SMFS and EFM showed
that PM-D85T is bent cytoplasmic side out in the M2 photointermediate and passes
through a flat topology in the N intermediate in order to bend extracellular side out
in the final O photointermediate. The different bending modes observed illustrate a
reoccurring motif in nature of how the form of BR follows its function which is that
of a vectorial proton pump transfering protons from the cytoplasmic to the extracellular
medium. Membrane curvature was further analyzed to reveal that smaller
membranes are strongly influenced by extrinsic constraints imposed by the substrate
surface. Larger membranes on the contrary retain a characteristic curvature which
is dependent on the physicochemical conditions and the corresponding shape of BR,
thereby demonstrating that solely the intrinsic shape changes of the embedded proteins
are responsible for the macroscopically bent nature of the membranes.
From a nanobiotechnology perspective, PM represents a supramolecular actuator set
in motion by intrinsic and extrinsic influences, which opens a wide field of possible
applications. One example would be as chemomechanical transducer, which upon
a pH-shift changes shape and thereby interacts with its environment. These interaction
capabilities place purple membrane in the context of other molecular machines
and supramolecular switches. The observed and tentatively tunable dynamical transitions
and the ability to reversibly control PM crystallinity might allow for the preparation
of larger artificial membranes, which might be used for data storage or photovoltaic
applications or serve as template in the construction of novel nanobiomaterials
such as photonic crystals. |
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| DOI: | 10.17192/z2011.0608 |