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Rabe, M. Understanding protein adsorption phenomena on solid surfaces. 2009, University of Zurich, Faculty of Science.

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Abstract

Protein adsorption at solid surfaces plays a key role in many natural processes and has therefore promoted a widespread interest in many research areas. Despite considerable progress in this field there are still widely differing and even contradictive opinions on how to explain the phenomena that are frequently observed. The present dissertation aims to advance the understanding of protein adsorption and to systematically unravel the underlying molecular mechanism. This is achieved by acquiring and evaluating comprehensive experimental data sets using fluorescence sensing and imaging methods. Experiments are conducted on model systems comprising the proteins BSA, Fibrinogen, β-Lactoglobulin, and α-Synuclein, hydrophilic and hydrophobic surfaces and varying pH and ionic strength conditions. One of the comprehensively studied adsorption phenomena is cooperativity which refers to the effect that the adsorption of proteins is enhanced by the presence of pre-adsorbed proteins. Contradicting a widespread opinion it is shown that cooperativity is not necessarily associated with the growth of tight surface aggregates. Instead, a macroscopic model description is suggested that simply assumes the overlap of two parallel adsorption pathways, one for the adsorption at isolated surface positions and one for the adsorption near other surface-bound proteins. The proposed mechanism implies increasing adsorption rates plus a specific distribution of proteins over the surface. Both properties are experimentally confirmed. Further, a microscopic treatment of the mechanism behind cooperativity is realized through Monte-Carlo simulations which reproduce the experimental data accurately and thereby confirm the suggestion that approaching proteins can be tracked to favorable binding sites near other pre-adsorbed proteins. In addition, phenomena related to protein adsorption include exchange mechanisms between adsorbing and pre-adsorbed proteins, conformational and orientational rearrangements, as well as overshooting adsorption kinetics. Another model combining these effects is developed and tested with a strong fundament of experimental data. The primary accomplishment related to this model is a consistent and comprehensible explanation of the overshooting effect. Finally, the behavior of protein aggregates or clusters on surfaces is explored. Protein clusters can form spontaneously in the solution and subsequently deposit onto the surface. For the first time it was shown that induced by protein-surface interactions freshly deposited protein clusters start to spread in order to maximize the contact area between proteins and surface. The spreading rate is considerably faster on hydrophobic surfaces as compared to hydrophilic surfaces which correlates with the lateral mobility of the protein monomers on theses surfaces. Interestingly, on a hydrophobic surface a spreading protein cluster can even rupture a pre-adsorbed protein monolayer by displacing the monomers from the area that it is about to occupy. Inversely to protein aggregation in solution, the direct growth of aggregates on the surface can also be observed using the protein α-Synuclein which is the pathological component of Parkinson’s disease. Whereas the on-surface growth mechanism is the typically proposed one when protein aggregates are detected on a surface, the discovery that protein aggregates can also come from the solution and spread on the surface opens a completely new perspective on this topic. Experimental strategies to distinguish between these two different mechanisms are comprehensively discussed. The significance of this work results from the successful combination of substantial experimental investigations with efficient theoretical methods giving access to a clear and illustrative view on some exciting adsorption phenomena. Novel ideas shed light on the diversely discussed topics of cooperative adsorption, overshooting adsorption kinetics, and protein aggregation.

Additional indexing

Item Type:Dissertation
Referees:Seeger S, Hamm P, Robinson J A
Communities & Collections:07 Faculty of Science > Department of Chemistry
Dewey Decimal Classification:540 Chemistry
Language:English
Date:2009
Deposited On:30 Jan 2010 18:13
Last Modified:05 Jun 2014 13:50
Number of Pages:153

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