PROGRAMMABLE PROTON CONDUCTION IN STRETCHABLE AND

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PROGRAMMABLE PROTON CONDUCTION IN STRETCHABLE AND SELF-HEALING PROTEINS Abdon Pena-Francesch, Huihun Jung, Michael A. Hickner, Madhusudan Tyagi, Benjamin D. Allen, and Melik C Demirel Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04574 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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PROGRAMMABLE PROTON CONDUCTION IN STRETCHABLE AND SELF-HEALING PROTEINS

Abdon Pena-Francesch1a,b, Huihun Jung1a,b, Michael A. Hickner1a,c, Madhusudan Tyagi2, Benjamin D. Allen1d,e, Melik C. Demirel1a,b,e 1a

Materials Research Institute, 1b Department of Engineering Science and Mechanics,1c

Department of Materials Science and Engineering, 1d Department of Biochemistry and Molecular Biology, 1e Huck Institutes of Life Sciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA 2

Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Maryland 20742, USA


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ABSTRACT Proton conduction is ubiquitous in nature and has many applications in energy and electronic technologies. Although protein based materials show bulk proton conduction ten times lower than conventional ion-conducting materials, they have unique advantages including biocompatibility, self-healing, tunable structure, and fine-grained control of material properties via amino acid sequence. Here, we studied the bulk proton conduction of tandem repeat proteins, and demonstrate that tandem repetition of sequences from squid ring teeth (SRT) proteins significantly and systematically enhances bulk proton transport properties. Inelastic neutron scattering experiments between 4K and 350K reveal that highly repetitive proteins shows enhanced conductivity. Our highly repetitive proteins achieve higher proton conductivity than state-of-the-art artificial proton conductors (with peak conductivities of 3.5 mS cm-1), as well as demonstrate unique self-healing characteristics. These proteins also exhibit exceptionally high stretching (~300%) relative to proton conductive materials while maintaining their high-strength, offering the unique possibility of dynamic responsivity to strain. Programming physical properties through tandem repetition introduces a new approach for understanding proton conductivity and enhancing the transport properties of synthetic proteins.



INTRODUCTION Proton conductivity have been extensively studied for over two hundred years,1 but over the last few decades focus has shifted towards solid-state conducting materials (polymers, metal-organic frameworks, ceramic oxides, and composite materials) due to their technological relevance in energy applications such as fuel cells, batteries, and water electrolysers.2–4 Ionomers such as Nafion and other sulfonated polymers represent the state-of-the-art proton-conducting materials with some of the highest reported proton conductivities, and have received much research attention.5 Proton conduction in protein-based materials has not been explored to such extent (e.g., the highest bulk proton conduction in protein based material achieve 1.2-2 mS/cm).6 Protein based materials exhibit lower bulk proton conduction comparable to conventional conducting materials, but has unique advantages over non-biological materials such as biocompatibility, tunable structure and tunable transport properties through amino acid sequence control. Because of this unique set of properties, protein-based conducting materials are good candidates for developing bioelectronics devices such as proton transistors. 7

Programming physical properties through genetic duplication represents a new approach to enhancing the transport properties of biomaterials. Tandem-repeat (TR) proteins, found throughout the tree of life and in all eukaryotes, feature a modular design in which a sequence motif encoding 20-40 amino acids is duplicated a handful or scores of times in a single openreading frame to yield a full-length gene.8,9 TR proteins exhibit a wide range of structures and functions, from soluble forms such as ankyrins and HEAT-repeats that serve to bind other biomolecules, to structural fibers such as collagens and silks.10–12 Tandem-repeat arrays are thought to expand and contract (typically at rates between 10-2 and 10-6 per generation13) due to polymerase slippage during genome replication, leading to variations in repeat number that are subject to natural selection.14 For soluble TR proteins, the influence of repeat number on biophysical properties such as thermodynamic stability and binding affinity has been investigated extensively.15,16,17 Although the dependence of mechanical properties on repeat number studied extensively,18 relationship between transport properties and tandem repeats remains relatively uncharted. For example, fibrous structural proteins are generated by the repetition of short peptide segments. Collagen has polyproline- and glycine-rich helices, whereas silk and elastin have β-spiral [GPGXX], linker [GP(S,Y,G)], and 310-helix [GGX] repeats.19


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These repetitions are advantageous because of the intrinsic promotion of stability through the periodic recurrence of favorable interactions in the protein structure. However, the effects of structural repetition and related transport properties have never been quantified in this class of proteins.

Our protein design relies on synthetic tandem repeat polypeptides based on cephalopod proteins. Squid proteins have inspired the design and development of various technologies including invisibility cloaking,20 self-healing materials21, synthetic nacre,22 and renewable bioplastics.23 Particularly, squid ring teeth (SRT) proteins are high strength hydrogen-bonded self-healing polymers that can be extracted from the tentacles of the squid suction cups or expressed recombinantly in bacteria.24 Here, we study the effects of tandem repetition on bulk proton conductivity in a family of highly stretchable and self-healing proteins inspired from SRT, and show that materials performance depends strongly on repetition number. Using inelastic neutronscattering, we demonstrate that proton conductivity rises due to increase in the concentration of free protons in the protein films. We also compared protein conductivity of TR proteins to other biological materials and showed that TR proteins have the highest bulk proton conductivity values among those reported to date.

RESULTS AND DISCUSSION SRT proteins are highly repetitive structural proteins that have varying molecular weights (MW) between 15 kDa – 65 kDa. Recent sequencing of SRT proteins shows a common motif across squid species that consists of alternating A,V,S,T,H-rich and G,L,Y-rich segments.25 Despite this general common motif in amino acid sequence, variations in composition, molecular weight, and length of the segments offers synthetic and engineering challenges for understanding and controlling physical properties of SRT-based materials. To overcome these problems, we recently adopted and improved a known molecular biology technique, rolling-circle amplification (RCA), to create a tandem repeat DNA assembly strategy enabling production of SRT-mimic genes with different lengths from a repetitive building block in a single cloning step.18 We used this RCA method to prepare a library of TR sequences with a controlled distribution of lengths. (Figure 1a). TR proteins can be processed using both thermal and solution methods.26 Figure 1b shows the solution processing of TR powder, which consists of



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dissolving the protein powder in an organic solvent (e.g., Hexafluoroisopropanol, HFIP) and forming it into a particular shape (e.g., a knight piece in the game of chess) by casting. We designed four repetitive TR polypeptides (i.e., TR-n4, TR-n7, TR-n11, and TR-n25, where n denotes the repeat number) based on the crystal-forming polypeptide sequence of PAAASVSTVHHP and the amorphous polypeptide sequence of YGYGGLYGGLYGGLGY (Figure 1c). SDS-Page gel of biosynthetic TR proteins with 4, 7, 11, 25 repeats are shown in Figure 1d.

Proton conductivity of the TR protein films was determined by impedance spectroscopy as shown in Figure 2a. The proton conductivity of TR proteins shows linear scaling with respect to inverse tandem repetition, 1/n (Figure 2b) measured at three temperatures (i.e., 20 °C, 50 °C, and 70 °C). The highest proton conductivity (3.48 mS cm-1) for TR films is measured for TR-n25 at 70 °C when the sample is fully hydrated with water. Fully hydrated TR films swell and form a hydrogen-bonded network that allows for Grotthus-type proton transfer.27 Figure 2c shows the activation energies (Ea) for proton conduction, which were calculated from the temperaturedependent Arrhenius plot. In Figure 2d, we studied the effect of water content on proton conductivity in TR films. Humidity-dependent measurements showed a percolation threshold at a water-volume fraction of 0.1, which is observed in proton-conducting biological materials.

27

This transition is a classic indication of a percolation conductivity model with a power law; σ/σ0 = k (φ-φc)α, where σ is conductivity, σ0 is conductivity at saturation, φ is the water volume fraction, φc is the water volume fraction percolation threshold, α is the power law coefficient and k is a constant.28 The power coefficient α, which denotes the fractal dimensionality of the system, increases linearly with 1/n (Figure 2d inset). In order to confirm the bulk proton conductivity in SRT proteins, impedance spectroscopy measurements were also repeated in deuterium oxide (D2O) to investigate the nature of the charge carrier (Figure S2). We measured elastic incoherent neutron scattering29 over a range of scattering wave vector, Q to quantify the proton concentrations in the protein films. Fixed window scans (Figure 3a) measure the elastic incoherent neutron scattering intensity Iel(Q), which is a measure of the non-mobile hydrogen atoms in the sample. Hydrogen motions faster than the instrumental resolution time window ( ≈ 1 ns) contribute to the inelastic scattering fraction and are excluded from the elastic


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scattering intensity. At very low temperatures (T ≈ 4 K) all hydrogen atoms move slower than the instrumental resolution but as temperature increases, hydrogen motions progressively become faster than the instrumental resolution and fall outside the Iel(Q) measurement window. As a consequence, the elastic scattering intensity decreases with temperature. While the elastic scattered intensity of dry TR proteins steadily decreases with temperature, lower intensity is measured for hydrated TR proteins at T > 280 K, indicating higher mobility upon hydration. We repeated the measurements using deuterated water (D2O) to differentiate water and protein dynamics since deuterium is invisible to neutron scattering (i.e., negligible scattering crosssection), while it can still plasticize the TR proteins like H2O does. MSD (mean square displacement) of mobile hydrogen atoms are calculated from the elastic scattering intensity as a function of the scattering wave vector Q and temperature T through the Debye-Waller factor (Figure 3b). 30 This Gaussian approximation is the standard method for estimating the MSD, but it assumes that all mobile hydrogen atoms undergo the same isotropic motion. Hence, the approximated MSD includes all types of motions (i.e., vibrations, rotations, diffusion) as a single value over a range of temperatures and Q. At very low temperatures (40 K), all samples (dry and hydrated) show a plateau around zero, revealing a very low mobility and no temperature dependence in the T < 70 K temperature range. The first onset of motion is observed at T ~ 70 K for all samples, which is common for proteins and is typically attributed to the onset of methyl group rotations.29 Dry TR proteins show an onset at T ~ 70 K, which is common for proteins and is typically attributed to the onset of methyl group rotations (constant slope from 70 K to 350 K with d / dT ≈ 3.0 · 10-3 Å2/K). Hydrated (either D2O or H2O) TR proteins show a slope of d / dT ≈ 4.4 · 10-3 Å2/K in the 60 – 230 K region and a slope of d / dT ≈ 12.4 · 10-3 Å2/K in the 260 – 350 K region, with a dynamic transition at 245 K. The higher mobility is attributed to the plasticizing effect of water (i.e., disruption of the hydrogen bonded network). Both hydrated TR-n4 and TR-n11 show the same dynamics below 270 K. However, a significant MSD increase in deuterated TR proteins is measured at 270-280 K (i.e., above the melting point of ice).

Detailed analysis of the quasielastic broadening is provided in Figure 4, where the full width half maximum Γ(Q) is plotted as function of Q2. TR H2O-protein dynamics (Figure 4a) show a sublinear scaling with Q2 which is characteristic of jump-diffusion models such as the Singwi-



Sjölander (SS) model for combined translational and oscillatory diffusion of H2O molecules.31 However, the evolution of FWHM with Q of TR-n4 and TR-n11 in H2O hydration conditions are very similar, which suggests that the dynamics are similar for both proteins. The elastic incoherent structure factor (EISF) was analyzed as function of Q (Figure 4b), which is defined as the ratio of elastic intensity to the total intensity and provides information on the geometry of the motions and the mobile fraction of hydrogen atoms involved in the dynamic process. EISF was modeled for confined diffusion of hydrogen atoms within a spherical volume as follows: = 3 ⁄ , where j1 is the first order Bessel function and r is the radius of the confined spherical volume. Hence, the EISF of TR proteins is given as: = 1 − + , where pfree is the fraction of mobile hydrogen involved in proton conductivity, and is weighting the EISFsph term. The term 1 – pfree represents the elastic intensity from immobile fraction. We can estimate pfree for hydrated states (0.26 and 0.4) (Table 2). Clearly the TR proteins have increasing free diffusing protons (higher pfree) as the molecular weight increases, which agree with the macroscopic impedance proton conductivity measurements. The free proton content of TR proteins increases as the molecular weight increase, which also agrees with the macroscopic impedance conductivity measurements (Figure 4b-inset). These results reveal that highly repetitive TR films show enhanced proton conductivity due to increased mobile proton concentration.

Furthermore, the mechanical response under large deformation of hydrated TR proteins reveals an elastomeric behavior. Figure 5a shows ultimate strength up to 40 MPa, which also scales inversely as a function of the molecular weight. Interestingly, proton conductivity of TR-n11 increases with strain. Figure 5b shows the dynamic and large changes in proton conductivity of TR-n11 when stretched. We hypothesize that strain-induced alignment of β-sheets and tie chains in the TR proteins improves the conductive pathways in the protein matrix. A similar trend of domain reorientation dependent conductivity has been reported earlier for polymer electrolyte membranes32 as well as nematic elastomers.33 For these systems, the proton conductivity is driven by the orientation of the ordered domains.32 We also measured the conductivity along the parallel and perpendicular directions with respect to strain direction to analyze the transport anisotropy (Figure 5b inset). While the conductivity increases along the stretching direction, it


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remains constant along the perpendicular direction, which agrees with the hypothesis of realignment anisotropy of the β-sheet domains and connecting tie chains. We reported self-healing of SRT proteins earlier,21 which are achieved in mild conditions, by pressing in rubbery state in aquatic media. Similarly, TR films also shows self-healing properties. Figure 5c-inset shows the optical images of self-healed TR-n11 film. We measured proton conductivity of self-healed TR films and compared them to pristine TR films (Figure 5c). For all protein films, 100% recovery in proton conductivity is observed. A slight increase in the proton conductivity is due to mechanically induced domain orientation as discussed earlier.

Mimicking tandem repetitions observed in SRT proteins could also have technological impact. As a comparison to other engineered materials, we show examples of protein conductivity in Figure 5d. TR proteins have the highest bulk proton conductivity values among proteins reported to date. Examples of biological proton conductors are listed in Table S1 (i.e., silk, maleic chitosan, keratin, collagen, reflectin, AoL jelly, melanin, lysozyme, bovine serum albumin). We also performed control experiments by removing histidine sequences from the βsheet domain of TR-n7 polypeptides. Protonation of histidine is known to contribute to the zetapotential of SRT proteins in various pH conditions.34 Without histidine (e.g. poly-A sequences), control Tr-n7 film show significantly lower proton conductivity (Figure 5d) compared to TR films.

CONCLUSION In summary, we describe the bulk proton conductivity of self-healing proteins derived from squid ring-teeth sequences, demonstrating that tandem repetition significantly and controllably enhances proton conductivities. SRT proteins have unique advantages including self-healing, and tunable extensibility. We reported here for the first time that these highly stretchable and selfhealing proteins achieve extreme proton conductivity, with peak conductivities of 3.5 mS cm-1, the highest value reported to date for a biological material. In general, proton conductivity scales linearly with the mobility and the concentration of protons in the protein films. For TR samples, the mobility of mobile protons does not change as indicated by the neutron data, and hence an increase in the mobile proton concentration is likely responsible for the increase in conductivities



observed in impedance measurements. Collectively, our results suggest a new basis via tandem repetitions for the discovery and engineering of biomaterials with extreme and dynamic functional properties. Our results offer an intriguing window into the evolutionary dynamics of genetic duplication in naturally occurring tandem-repeat protein materials, which will be the focus of our future studies.


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MATERIALS AND METHODS Protein films: TR proteins are constructed and expressed based on the protocols that are described earlier.18 All proteins (TR-n4, TR-n7, TR-n11, and TR-n25: 15, 25, 42, and 86 kDa respectively) were dissolved in 1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) to a concentration of 50 mg/mL overnight. TR protein solution was cast on polydimethylsiloxane (PDMS) molds and solvent was evaporated overnight at room temperature in a chemical fume hood. The samples were then washed with DI water and dried in ambient conditions. Protein films for thermal measurements were cast on 1 x 1 cm Al-coated (80 nm) silica glass substrates, dried overnight, and washed with DI water.

Proton conductivity: Proton conductivity of the TR protein films (1cm x 3cm x 80 µm) was determined by impedance spectroscopy using a two-probe measurement cell (platinum-coated nickel electrodes) connected to an impedance/gain-phase analyzer (Solatron 1260, analysis with ZView and ZPlot software). Measurements were conducted at an AC amplitude of 100 mV and a frequency interval ranging from 100 Hz to 10 MHz. Impedance data were collected at pH = 7 as a function of temperature for fully hydrated samples (immersed in water) and in a 95 % RH environment after equilibration for 60 minutes for each data point. The real (Z’) and imaginary (Z’’) components of the impedance were measured simultaneously over the defined frequency range (Nyquist plot). The bulk resistance of the films Rb (Ω) can be calculated from the Nyquist plot by extrapolating the impedance line to the x-axis (real impedance where the imaginary impedance is zero, Z’ at Z’’ = 0). The proton conductivity of the film can be calculated using the following equation: =

L ! "

where σ is the proton conductivity (S cm-1), L is the length between the electrodes (0.7 cm), A is the cross sectional area of the membrane sample (cm2). The conductivity values and activation energy of TR films are reported in Table 1.



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Table 1. Conductivity Parameters for TR proteins (*Conductivity values are given in (mS/cm)) Parameter

N=4

N=7

N=11

N=25

N=∞

Mw (kDa)

15

25

42

86

∞

σH, 20 °C, φ=0.3*

0.47 ± 0.03

0.72 ± 0.13

0.93 ± 0.11

1.75 ± 0.21

3.14

σH, 50 °C, φ=0.3*

0.93 ± 0.04

1.19 ± 0.08

1.85 ± 0.10

2.61 ± 0.30

4.11

σH, 70 °C, φ=0.3*

1.38 ± 0.12

1.54 ± 0.4

2.70 ± 0.08

3.48 ± 0.30

4.58

EA, φ=0.3 (eV)

0.08 ± 0.02

0.06 ± 0.03

0.08 ± 0.02

0.05 ± 0.03

0.07

φc (percolation)

0.11

0.11

0.11

0.11

α (percolation)

1.3

1.2

1.1

1

k (percolation)

7.5

7.5

7.5

7.5

Quasi-elastic Neutron Scattering (QENS): Neutron Scattering experiments were performed on the high-flux back-scattering spectrometer (HFBS) and the disk chopper spectrometer (DCS) at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR). Protein films of 200 mg (8 x 3 cm) were cast, washed with DI water, and dried overnight in a vacuum oven at 80 °C. Dry samples were then sealed in the sample cell. Hydrated samples were immersed in H2O for 2 hours, they were blotted to remove excess water (analogous to TGA measurements), and they were then sealed in the sample cell. In elastic fixed window scans (performed on HFBS), the samples were cooled to 4K and elastic intensity was measured while heating to 350K at a heating rate of 1 K/min.

Quasielastic experiments on dry proteins were performed on HFBS, with energy resolution of 1 µeV, dynamic range of ± 15 µeV and Q-range of 0.25 Å-1 – 1.8 Å-1, at 295K. The resolution function was measured at 4K (signal is expected to be completely elastic). Quasielastic experiments on H2O-SRT were performed on DCS, with energy resolution of 64 µeV (wavelength λ = 6 Å), dynamic range of ± 0.5 meV and Q-range of 0.1 Å-1 – 2 Å-1, at 295K (Figure S1a). Vanadium was used as the resolution function. DAVE software, developed by NCNR,35 was used to analyze the data. A Lorentzian function was fitted to describe the quasielastic broadening, and the full width at half maximum (FWHM) and the area of the fitted peak was analyzed as function of Q (Figure S1b).


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Table 2. Protein Samples studied by QENS and related parameters Sample n Mw (kDa) p (H2O) r (H2O) (Å)

TR-n4 4 15 0.26 3

TR-n11 11 42 0.40 3



ACKNOWLEDGMENTS MCD, BDA, APF, and HJ were supported partially by the Army Research Office under grant No. W911NF-16-1-0019, and Materials Research Institute of the Pennsylvania State University.

AUTHOR CONTRIBUTIONS MCD conceived the idea and supervised the research. BDA introduced the tandem-repeat strategy in consultation with MCD. APF performed the proton and neutron measurements. HJ worked on the cloning, recombinant expression and purification of proteins together with BDA. MAH contributed in understanding of proton conductivity. MT supervised APF for neutron measurements and contributed to the interpretation of results. All authors contributed to writing and revising the manuscript, and agreed on the final content of the manuscript.

ADDITIONAL INFORMATION The authors declare no competing financial interests. Correspondence and requests for materials should be addressed to MCD.


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Page 19 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

(circularized unit)

(repeat unit n=1)

i

(random RCA products)

ii

Cloning Vector digestion and circularization

RCA iii

(repeat unit n=X)

TR Polypeptide

iv

v

digestion

n=1

TR Expression Vector

n=2

size selection

expression

n=3

(repeat unit n=X)

b

(TR products with size distribution)

c

d

MW (kDa)

Tandem Repeat (TR) Proteins

n=25

100

Squid-inspired Tandem Repeat (TR) Proteins

STGTLSYGYGGLYGGLYGGLGYGPAAASVSTVHHP

70 55

amorphous (disordered)

n=11

crystalline (β-sheet)

25

Tandem repeat unit “n”

(

n=7

35

n=4

)n

15 10

Figure 1. a) Tandem Repeat construct: The PD-RCA workflow: i) The repeat unit is excised from a cloning vector and circularized by intramolecular ligation. ii) The circularized repeat unit is expanded in a rolling-circle amplification (RCA). iii) The RCA products are digested, leading to a pool of tandem-repeat fragments of various sizes. iv) The digested RCA products are sizeselected and cloned into an expression vector. v) Polypeptides with the desired molecular weight are expressed. b) TR polypeptides can be shaped into any three-dimensional geometry by solution processes or melting. For example, a knight-shaped sample prepared by dissolving the TR protein powder in hexafluoro-2-propanol (HFIP) and then casting in a PDMS mold. c) The repeat unit contains one amorphous region (red), one crystalline region (green). d) The molecular weight of four TR proteins are confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE).



Impedance (-Im) Z'' (kΩ)

φ = 0.3 70 °C

30

TR-n4 TR-n7 TR-n11 TR-n25

25 20 15 10 5 0

0

10

20

30

40

50

60

70

n=25

n=∞ 4.0

b

35

Conductivity σ H (mS/cm)

a

n=7

n=4 TR-n25 TR-n11 TR-n7 TR-n4

3.0 2.5 2.0

70 °C

1.5

50 °C

1.0 20 °C 0.5

φ = 0.3

0.0 0.00

80

n=11

3.5

0.05

0.10

0.15

0.20

0.25

Reciprocal repeat units 1/n

Impedance (Re) Z' (kΩ) T (°C)

-2.4

50

39

30

21

13

φ = 0.3

-2.6

Ea = 0.05 eV

-2.8 Ea = 0.08 eV

-3.0


-3.2

-3.4

2.9

3.0

d

1.2

Reduced conductivity σH/σ H0

c

60

Ea = 0.06 eV

1.0

α

72

Conductivity log σ H (S/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

0.8

1.8 1.6 1.4 1.2 1.0 0.8 0.0

0.1

0.2

0.3

1/n

0.6 0.4


φc 0.2

Ea = 0.08 eV

3.1

3.2

3.3

3.4

3.5

0.0 0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

H2O volume fraction φ

1000/T (K-1)

Figure 2. Proton Conductivity of TR Proteins: a) Characteristics of proton conduction with a semicircle in the high-frequency range (parallel resistance and capacitance due to the conductivity cell architecture) and a linear region in the low frequency range (ionic diffusion). b) Proton conductivity of TR proteins as function of number of tandem repeats 1/n. c) Activation energies (Ea) for conduction were calculated from the temperature-dependent Arrhenius plot. d) Reduced proton conductivity as function of water volume fraction φ shows agreement with percolation theory and a power law is fitted to the data with φc as percolation threshold and α as power law coefficient. Inset shows a linear dependence of α on 1/n reciprocal repeat unit number.


Page 21 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b

Figure 3. a) Elastic fixed window scans of TR-n4 and TR-n11 proteins as a function of temperature. b) Elasticity intensity is converted to mean square deviation (MSD) using DebyeWaller factor: (DWF) = # , %⁄# , % = 40( = )* −1/3 ^2 〈*^2 〉.



a

b

Figure 4. a) Lorentzian full width half max (FWHM) fitted to the H2O/protein quasielastic spectra measured on the Disk Chopper Spectrometer (DCS). b) Elastic incoherent structure factor (EISF) of the H2O/protein quasielastic spectra. EISF was modeled for confined diffusion of hydrogen atoms within a spherical volume, EISF456 Q = 3j Qr⁄Qr , where j1 is the first order Bessel function and r is the radius of the confined spherical volume. Hence, EISF:; Q = 1 − p=>?? + p=>?? EISF456 Q, where pfree is the fraction of mobile hydrogen involved in proton conductivity. The term 1 – pfree represents the elastic intensity from immobile fraction. Impedance measurements (from Figure 2) are compared with the EISF data in the inset.


Page 22 of 23

b

2.6

σ||

2.2

electrodes 2.0

σ 2.5

1.8 1.6 1.4

TR-n11 φ = 0.3 20 °C

1.2 1.0

c

stretching

2.4

σ H (mS/cm)

a

0

50

σ⊥

1.5 1.0

0

50 100 150 200

Strain (%)

100

d

σ || 2.0

150

200

250

300

Strain (%) Proton Conductivity σH (mS/cm)

4.0

Proton conductivity σ H (mS/cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Proton conductivity σ H (mS/cm)

Page 23 of 23

20 °C

2.0

pristine healed 1.5

1.0

healing area

0.5

0.0

TR-n4

TR-n7

TR-n11

TR-n25

3.5 3.0

TR-n∞

TR-n25 TR-n11

2.5 L. pealei

L. vulgaris I. illec.

2.0

AoL jelly

TR-n7

1.5 reflectin

TR-n4

1.0 0.5 0.0

melanin silk BSA (mat) TR-n7 (polyA, no H) (cocoon) pAsp silk (rec.) YYACAYY gladius collagen lysozyme keratin BSA

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 20 40

Reciprocal Molecular Weight 104/Mw (mol g-1)

Figure 5. a) Mechanical testing of TR-proteins shows ultimate strength up to 40 MPa (inset shows 1/n dependence). b) Proton conductivity increases as a function of applied strain. Proton conductivity of TR-n11 protein is constant in perpendicular strain direction, but it scales linearly in parallel direction with applied strain. c) Proton conductivity comparison of self-healed and pristine TR-films. Inset shows the optical images of self-healing in a TR-n11 film d) Bulk Proton Conductivity of Protein Based Materials are compared as function of inverse molecular weight (Mw). TR proteins have the highest bulk proton conductivity among all known biological materials (see supplementary information for a detailed analysis). However, we note that conductivity in biomaterials is strongly dependent on the water content and temperature.


PROGRAMMABLE PROTON CONDUCTION IN STRETCHABLE AND

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