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Abstract
Proteins are the main proton mediators in various biological proton circuits. Using proteins for the formation of long-range proton conductors is offering a bioinspired approach for proton conductive polymers. One of the main challenges in the field of proton conductors is to explore the local environment within the polymers, along with deciphering the conduction mechanism. Here, we show that the protonic conductivity across a protein-based biopolymer can be hindered using straightforward chemical modifications, targeting carboxylate- or amine-terminated residues of the protein, as well as exploring the effect of surface hydrophobicity on proton conduction. We further use the natural tryptophan residue as a local fluorescent probe for the inner local hydration state of the protein surface and its tendency to form hydrogen bonds with nearby water molecules, along with the dynamicity of the process. Our electrical and spectroscopic measurements of the different chemically-modified protein materials as well as the material at different water–aprotic solvent mixtures result in our fundamental understanding of the proton mediators within the material and gaining important insights on the proton conduction mechanism. Our biopolymer can be used as an attractive platform for the study of bio-related protonic circuits as well as a proton conducting biopolymer for various applications, such as protonic transistors, ionic transducers and fuel cells.
Highlights
Directional proton transport (PT) is one of the most fundamental processes in biology, which takes place within proteins and usually across the two sides of a membrane, such as in light- or voltage-activated channels or the protein complexes participating in photosynthesis and aerobic respiration.[1,2,3,4,5] Several proton pathways in natural proton mediating proteins have been identi ed, all involving structural water molecules and speci c amino acids participating in the formation of a hydrogen bond network spanning in the direction of the PT pathway
We have used two strategies in order to explore the role of the inner hydration layer within the bovine serum albumin (BSA) mat on its ability to support long-range (2.5 mm) proton conduction (PC)
As suggested previously for the latter materials, we suggest here that the PC across the native mat is due to proton hopping between water molecules and functional group on the surface of the BSA mat that can participate in an H-bond
Summary
Directional proton transport (PT) is one of the most fundamental processes in biology, which takes place within proteins and usually across the two sides of a membrane, such as in light- or voltage-activated channels or the protein complexes participating in photosynthesis and aerobic respiration.[1,2,3,4,5] Several proton pathways in natural proton mediating proteins have been identi ed, all involving structural water molecules and speci c amino acids (mostly the oxo-amino acids of Glu and Asp) participating in the formation of a hydrogen bond network spanning in the direction of the PT pathway. The ability of proteins to support short-range (nm-scaled) PT in nature has resulted in several recent works showing the ability of proteins to support macroscopic proton conduction (PC).[6,7,8,9] As shown by Ordinario et al, one way to manipulate PC across the protein material is by mutagenesis, where the authors replaced all the Glu and Asp in the protein to alanine This mutagenesis resulted in a dramatic order of magnitude decrease in measured current density, and further proved the importance of these amino acids in supporting long-range PC. Our study pinpoints the importance of natural oxo-amino-acids and amine groups as proton-hopping sites as well as their interactions with water molecules in supporting long-range PC across proteins
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