12d

left-handed (b) right-handed 

8c

A

C

solvent

solvent

Si

Si s porou

-8

-7

-6

-5

-4

-3

-2

-1

0

0.0

29

∆H 6 30

2

)

Neutron reflectivit

y, log10

R

B

0.25 0.20 0.15 0.10 0.05

6

5

4

3

2

1

0

60 40 20 0

i S m fro e distanc

(Å) e surfac

Si

porous

Si

solvent

-0.5

4.0

6.5

Q, r Momentum transfe

z ) -1 (Å

solvent nSLD

ρn

 (10-6 ) -2 ÅnSLD, 

ρn (10-6 Å-2)

solvent

Co (d) 7

Zn 6 Mn 7

Mn Zn

Zn

Mn

 Co8

Zn 3 Mn 9

5

9

13

17

21

25

6 5 4 3 2 1 0

0

1

2

3

4

20

15

10

5

0

0

ρ nSLD solvent 2, n

2

 (10-6 ) -2 Å

nSLD solvent 1, 

ρ

n

1 (10

Information gain, 

Δ

24 20 16 12 8 4

 (h) t Counting time, 

BIOSCIENCES

8 NIST CENTER FOR NEUTRON RESEARCH

Conductivity and dynamics of 

biomimetic synthetic polypeptides

A. Pena-Francesch,1 H. Jung,1 M. Tyagi,2,3 and M. C. Demirel1

S

quid-inspired synthetic proteins self-assemble into a 

hydrogen-bonding physically crosslinked network that 

gives rise to remarkable thermal and protonic conductivity. 

Understanding the chain dynamics of biomimetic structural 

proteins and their role in heat and proton transport will help 

design novel biodegradable protein-based materials with 

programmable properties for bioelectronics and thermal 

switching devices. 

Proteins are heteropolymers that provide a variety of building 

blocks for designing novel biological materials. Proteins are 

diverse but often display substantial similarity in sequence 

and three-dimensional structure. A new family of repetitive 

structural proteins with remarkable properties was recently 

identified in the tentacles of several squid species. Using the 

tools of molecular biology and proteomics, we demonstrated 

that these squid ring teeth (SRT) proteins have segmented 

semi-crystalline morphology with repetitive amorphous and 

crystalline domains. However, a clear relationship between 

the protein architecture (repetition and length of crystalline/

amorphous domains), the self-assembled nanostructure, and 

conducting properties remains elusive due to complexity of 

native amino acid sequences.

To investigate the genetic basis of material properties in 

natural and artificial SRT sequences, we have developed a new 

approach for the design and production of structural proteins. 

These synthetic proteins have identical sequences but increasing 

repeat unit number n (n = 4, 7, 11 and 25) and molecular 

weights (15, 25, 42 and 86) kDa. Synthetic SRT proteins have 

remarkable protonic conductivity in the range of (1 to 5) mS/cm, 

which is the highest reported to date for protein-based materials 

as measured by Electrochemical Impedance Spectroscopy. In 

addition, synthetic SRT proteins exhibit a programmable thermal 

conductivity from 0.3 W/mK to 1.4 W/mK.

To investigate these vibrational dynamics, we turned to elastic 

and quasi-elastic neutron scattering (QENS) using the High-Flux 

Backscattering Spectrometer (HFBS) and Disk-chopper timeof-flight spectrometer (DCS) at the NIST Center for Neutron 

Research. QENS can measure molecular dynamic processes 

such as rotations, relaxations and diffusive motions with 1 Å 

to 30Å and pico- to nanosecond resolution by directly probing 

1 Pennsylvania State University, University Park, PA 16801

2 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899

3 University of Maryland, College Park, MD 20742

b

a 2.5

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

FWHM (meV)

2.0

1.5

1.0

0.5

MSD of H (Å2

)

0.0

0 50 100 150 200 250 300

Temperature (K)

Q2

 (Å–2)

Tg (hydrated D2O)

Ambient

Tg (ambient)

Hydrated (D2O)

TR-n4 (ambient)

TR-n11 (ambient)

TR-n4 (D2O)

TR-n11 (D2O)

Confined diffusion

TR-n4 (ambient)

TR-n11 (ambient)

Localized motion

TR-n4 (D2O)

TR-n11 (D2O)

∆ MSD

350 400 450 500

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

FIGURE 1: a) Measured mean square displacement, or vibrational amplitude, of 

hydrogen atoms in the TR films as a function of sample temperature. As seen, the 

dense, hydrogen-bonded network opens up and allows for increased movement 

following hydration. b) Full-width at half-maximum of the quasi-elastic peaks of 

the ambient and hydrated proteins as a function of the square of the scattering 

wavevector, Q 2. Error bars represent one standard deviation.
A

A

A

right-han

A

C

6

iiNIST CENTER FOR NEUTRON RESEARCH FOREWORD aAf

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Foreword

0.10

6

5

4

3

2

1

0

S

-0.5

4.0

6.5

Momentum tra

solvent nSLD

 (10-6 Å-2) nSLD, 

ρn (10-6 Å-2)

12

Counting t

BIOSCIENCES

2019 ACCOMPLISHMENTS AND OPPORTUNITIES 9

the self-diffusion of hydrogen atoms. The hydrated chain 

dynamics were measured using deuterated water (D2O), as 

deuterium has a negligible neutron scattering cross-section 

and therefore it does not contribute to the signal. The mean 

square displacement (MSD) of hydrogen atoms in the protein 

(shown in Fig. 1a) was calculated from the scattering intensity 

in fixed window elastic scans, as a function of temperature, 

using the Debye–Waller factor (which is a standard Gaussian 

approximation method). Tandem Repeat (TR) polypeptides 

under ambient conditions show very localized motions 

at T > 70K, which is common for proteins and typically 

originates from methyl group rotations, and exhibit a glass 

transition around 450K. Therefore, neutron spectroscopy 

does not show significant segmental or backbone motion 

at room temperature (300K). Thus, the disordered chains 

are constrained in a dense hydrogen-bonding network with 

minimal vibrational freedom. Conversely, in the hydrated 

state, the MSD of the hydrogen atoms increases and we 

observe much larger vibrational amplitudes, as well as 

a decrease in the glass transition temperature (250K). 

In this case, the water molecules break the hydrogen 

bonding between the disordered chains, allowing for more 

delocalization of the hydrogen motion. Protein chain dynamics 

were further investigated by quasi-elastic measurements. 

Figure 1 shows the full-width at half-maximum, Γ (Q), of 

the quasi-elastic peaks of ambient and D2O-hydrated TRn4 and TR-n11 proteins, plotted as function of Q2

. Under 

ambient conditions (<1 % relative humidity during our 

QENS measurements), TR proteins show a Q-independent 

behavior (15 μeV) characteristic of localized motions (that is, 

methyl group rotations). On the other hand, D2O-hydrated 

TR proteins show a Q-independent plateau at low Q values 

(0.15 meV), while a linear scaling with Q2 is observed at 

higher Q. This two-regime Q dependence is characteristic 

of diffusion in a confined space and can be described by 

the Volino and Dianoux (VD) model for bounded diffusion 

in a potential of spherical symmetry, which corresponds to 

confined diffusive motions of the amorphous segments within 

the β-sheet nanocrystals.

In order to understand the transport processes at the molecular 

level, we have designed variants of squid-inspired proteins 

with variable repeat unit number (n). Our results of elastic 

fixed window scan data collected at HFBS shows the hydration 

dynamics of two variants of biomimetic SRT proteins changes 

upon hydration for both polypeptides. QENS analysis at HFBS and 

DCS are necessary to understand the protein chain dynamics. 

Based on these results, we concluded that the hydration plays 

a key role in localized motions, and faster self-diffusion of 

amorphous chains in SRT polypeptides, which enhances the 

thermal conductivity. The thermal conductivity values scales 

with increasing repeat unit number n (enabling precise control 

of the physical properties by sequence design), and are highly 

dependent on protein hydration (enabling fast reversible 

switching of thermal conductivity), with a switch ratio of 4.5 at 

room temperature, which is among the highest reported to date.