Small-angle X-ray scattering measurements
SAXS measurements were carried out at BL-6A at the Photon Factory of the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan52,53. The sample solutions were placed in the stainless-steel sample cell with the Edison Bulb path length of 1 mm and 20 μm-thick quartz glass windows. The cell was maintained at around 293 K. PILATUS3 1 M (DECTRIS) was used as a detector. The X-ray wavelength was adjusted to 1.5 Å, and the sample-detector distance was 481 and 1966, mm, calibrated using a silver behenate as a standard sample. These settings provided a detectable Q-range of order 0.2–15 and 0.05–2.5 nm−1, respectively. Sixty frames were collected with the exposure time of 10 s. Since any radiation damages were not seen in all data, all the data in each sample were averaged to improve signal-to-noise ratio. The total integration time was 600 s. The two-dimensional scattering data were circularly averaged to convert into the 1D scattering intensity data, normalizing the scattering intensity to the absolute intensity scale (cm−1) by using water as a reference. The background subtraction was also performed to get the final scattering intensities. The magnitude of the scattering vector is given by Q=(4π/λ)sin θ/2, where λ is the X-ray wavelength and θ is the scattering angle. The software SAngler was used for these data processing54.

Photo-irradiation experiments of SPs
For the trans-to-cis isomerization of trans-2 in SPspiral, a UV LED lamp (λ=365 nm) with the intensity of 17 m W cm−2 (at a distance of 5 cm and using its maximum intensity) was used as a light source. The lamp was placed at a distance of 1 cm from a quartz cuvette (path length: 1 mm) containing SP solution and the UV light was irradiated. Under this condition, an apparent photostationary state (PSS) is typically achieved within 20 min of UV exposure. The photoinduced unfolding of SPhelical into (SPlinear)UV was performed under the careful control of increasing content of cis-2 by the LED lamp with increasing irradiation time and regulating the light intensity upon changing the distance between the cuvette and the new-lights source. When solutions of SPs consisting of trans-2 were exposed to the UV light, a decrease in absorbance around 370 nm was observed in UV–Vis spectra, indicating the extent of photoisomerization of azobenzene unit. The content of cis-2 in UV-irradiated SPs in MCH was determined by comparing the decrease in absorbance intensity of the absorption maximum of the trans-2 by referring the absorption change of trans-2 upon UV irradiation in CDCl3, wherein the content of the cis-isomer was determined by 1H NMR spectroscopy. For the slow cis-to-trans back-isomerization of (SPlinear)UV, a Vis LED lamp (λ=470 nm) at its minimum intensity was used as a light source. The lamp was placed at a distance of 20 cm from a cuvette (path length: 1 mm) containing SP solution and the Vis light was irradiated. Under this condition, about 20 min irradiation is required to reach a PSS. For the fast cis-to-trans isomerization using stronger Vis light, two Vis LED lamps were placed at a distance of 0.5 cm from two opposite sides of the cuvette (path length: 1 mm) containing SP solution. Under this condition, about 30-s irradiation is enough to reach the PSS where the same content of cis-2 is achieved in comparison with the above condition.