Encapsulation of Stimuli-Responsive Fusion Proteins in Silica: Thermally Responsive Metal Ion-Sensitive Hybrid Membranes
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Encapsulation of Stimuli-Responsive Fusion Proteins in Silica: Thermally Responsive Metal Ion-Sensitive Hybrid Membranes Linying Li, 1,3 Owen Im,1 Ashutosh Chilkoti1,2,3 and Gabriel P. López1,2,3 1
Department of Biomedical Engineering, Duke University, Durham NC 27708, U.S.A. Department of Mechanical Engineering and Materials Science, Duke University, Durham NC 27708, U.S.A. 3 NSF Research Triangle Materials Research Science and Engineering Center, U.S.A 2
ABSTRACT In this study, we demonstrate the fabrication of hybrid membranes that exhibit discrete and reversible changes in permeability in response to changes in calcium ion (Ca2+) concentration and temperature. Fusion proteins comprising calmodulin (CAM) and elastin-like polypeptides (ELPs) were used as stimuli-responsive elements due to their ability to undergo a reversible lower critical solution temperature (LCST) phase transition, which is sensitive to Ca2+ binding. The calmodulin elastin-like polypeptides fusions (CAM-ELPs) were incorporated into polymerizing silica networks using a simple sol-gel process and spin coating. Permeation experiments with solutions of crystal violet showed that the membranes are both Ca2+-responsive and thermally responsive. Under suitable pressure drop across the membranes, in the absence of Ca2+ or below the LCST of the ELPs, the hybrid membranes are impermeable to water. After addition of Ca2+ or above the LCSTs, they become permeable to water. The permeability can be toggled back and forth by sequential addition of calcium and ethylenediamine tetraacetic acid (EDTA). These results demonstrate that CAMELP/silica hybrid membranes can serve as tunable molecular filters whose permeability can be switched on and off in response to Ca2+ and temperature. INTRODUCTION Stimuli-responsive materials can exhibit switchable microstructure and properties in response to changes in environmental factors such as temperature, pH, light, magnetic and electric fields, ionic concentration, and the presence of biomolecules. A widely studied class of stimuli-responsive materials are those that experience reversible changes in their microstructure alternating between hydrophilic and hydrophobic states depending on specific stimuli [1]. Such materials have found interest because of their potential for applicability in bioseparations, [2] biosensing systems [3], anti-fouling surfaces [4] and drug delivery systems [5]. Elastin-like polypeptides (ELPs) are a versatile class of stimuli-resonsive polymers and consist of repeated pentapeptide sequences derived from elastin (i.e., Val-Pro-Gly-XaaGly, where Xaa is a “guest residue” that can be any amino acid except proline), and typically exhibit a characteristic lower critical solution temperature (LCST) in water [6]. Below this temperature, these polymers are soluble in water and exist in a hydrophilic, extended
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conformational state. At the LCST, a reversible phase change occurs such that at higher temperatures the polymers exist in a hydrophobic, collapsed and aggregated structure [7]. Changing t
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