On the Morphology and Dynamics of Purple Membranes at the Solid-Liquid Junction

Im Rahmen dieser Arbeit konnte gezeigt werden, dass das an der fest-flüssig Grenzfläche adsorbierte und in der Purpurmembran eingebettete Bacteriorhodopsin in Abhängigkeit von der Substratoberfläche zu einer mehr oder weniger stark ausgeprägten Dynamik neigt. Dieser dynamische Wandel von BR wurde...

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Bibliographic Details
Main Author: Baumann, Roelf-Peter
Contributors: Hampp, Norbert (Prof. Dr.) (Thesis advisor)
Format: Dissertation
Published: Philipps-Universität Marburg 2011
Online Access:PDF Full Text
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Table of Contents: 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.