Exploring long-range proton conduction, the conduction mechanism and inner hydration state of protein biopolymers

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.

Scheme 1. The different chemical modifications of the BSA mat used in this study, from top to bottom: BSA-OMe, BSA-NMe₂ and BSA-Hex.

Fig. 1. Proton conduction across the BSA mat as measured with EIS at (a) different water : ACN mixture, and upon (b) different chemically-functionalized BSA mats.

Fig. 2. Protonic FET measurement, source–drain current (I_DS) as a function of the source–drain voltage (V_DS) at different values of gate voltage V_GS for unmodified BSA mat. The inset shows the transfer line characteristics of the device.

Fig. 3. Normalized steady state fluorescence spectra of (a) BSA mat in different solvents, and (b) in different water : ACN mixtures.

Fig. 4. The dependence of (a) E(F) and E_T(30), (b) E_T(30) and resistivity, (c) E(F) and resistivity, as a function of the water fraction (within ACN), χ_w, in the mat (for E(F) or resistivity) or in bulk solution (for E_T(30)). The insets of (a and b) show the non-linear dependence of E(F) or resistivity vs. E_T(30), and the one of (c) shows the linear dependence of the resistivity vs. E(F).

Fig. 5. (a) Normalized steady state fluorescence spectra, and (b) time resolved fluorescence decay spectra of the different BSA mats used here compared to the native fluorescence of the BSA protein. The insets show a zoom-in of the Trp peak position and the first ns decay for (a) and (b), respectively.

Fig. 6. TRANES graphs of the different BSA mats used here for the first ns of Trp emission decay.

Scheme 2. Schematic representation of individual PT steps and general PC pathway in (a) unmodified BSA mat, where protons are mediated via the formation of hydrogen bonds between amino acid side chains and water molecules; and (b) in modified BSA, where the hydrogen bond network is disrupted and protons are less interacting with the side groups of the protein.

Fig. 7. (Top) Temperature dependence studies of the PC across the various BSA mats. The insets show the Arrhenius plot of the logarithmic of conductivity (averaged of at least 3 replicas) as a function of inverse temperature along with the calculated activation energy (E_a). (Bottom) Kinetic isotope effect of the PC across the various BSA mats upon deuterating the sample along with the calculated decrease in measured conductivity upon deuteration.