Elexsys 2 e500 epr spectrometer

EPR thylakoid samples were prepared as discussed above at a final concentration of 1.6 mg ml^–1 Chl S. leopoliensis membranes. The flavodoxin samples contained 100 μM deuterated flavodoxin53 (link) in the presence of 0.3 mM DCPIP and 10 mM sodium ascorbate as donors to P700⁺. The samples were placed in quartz EPR tubes, degassed in a nitrogen box, capped and illuminated for 10 s at room temperature with visible light, prior to freezing in liquid N₂. For the low temperature electron transfer experiments, S. leopoliensis native membrane and membrane/Pt-nanoparticle samples were placed in quartz EPR tubes and dark-adapted for 20 min at room temperature prior to freezing in liquid nitrogen. Both samples contained 1.6 mg ml^–1 Chl, 0.3 mM DCPIP and 10 mM sodium ascorbate. cw X-band (9.5 GHz) EPR measurements were carried out with a Bruker ELEXSYS II E500 EPR spectrometer (Bruker Biospin Corp, Rheinstetten, Germany) equipped with a TE102 rectangular EPR resonator (Bruker ER 4102ST) and a helium gas-flow cryostat (ICE Oxford, UK). Temperature control was provided by an ITC (Oxford Instruments, UK). The samples were transferred from liquid nitrogen to the pre-cooled resonator at 10 K. To measure the low temperature light-induced protein activity, X-band EPR spectra were recorded at 10 K shortly after continuous illumination with a white light LED (Thorlabs).

EPR measurements were performed
at the Advanced EPR Facility at Argonne
National Laboratory. CW X-band (9–10 GHz) EPR experiments were
carried out with a Bruker ELEXSYS II E500 EPR spectrometer (Bruker
Biospin, Rheinstetten, Germany), equipped with a TE₁₀₂ rectangular
EPR resonator (Bruker ER 4102ST). Note that the CW EPR technique uses
field modulation with lock-in detection which leads to first derivative-type
spectra. Measurements were performed at room temperature (T = 295 K). Data processing was done using Xepr software
(Bruker BioSpin, Rheinstetten). For “as-cast/dry/35%RH”
measurements, samples cast onto quartz substrates of approximately
2 mm × 15 mm were placed in quartz EPR tubes. For increasing
RH samples (Figure S2), solutions were
deposited in the bottom of EPR tubes and dried at 60 °C under
vacuum and then equilibrated in an RH chamber before sealing. Spin
quantification of a single crystal of CuSO₄·H₂O with a known spin concentration was used as a reference
sample. Spin quantifications were done by comparing double integrals
of the experimental and reference EPR. Spin concentration was calculated
based on the dry mass of the polymer deposited in the EPR tube.