Light-Controlled Spin Filtering in Bacteriorhodopsin

The role of the electron spin in chemistry and biology has received much attention recently owing to to the possible electromagnetic field effects on living organisms and the prospect of using molecules in the emerging field of spintronics. Recently the chiral-induced spin selectivity effect was observed by electron transmission through organic molecules. In the present study, we demonstrated the ability to control the spin filtering of electrons by light transmitted through purple membranes containing bacteriorhodopsin (bR) and its D96N mutant. The spin-dependent electrochemical cyclic voltammetry (CV) and chronoamperometric measurements were performed with the membranes deposited on nickel substrates. High spin-dependent electron transmission through the membranes was observed; however, after the samples were illuminated by 532 nm light, the spin filtering in the D96N mutant was dramatically reduced whereas the light did not have any effect on the wild-type bR. Beyond demonstrating spin-dependent electron transmission, this work also provides an interesting insight into the relationship between the structure of proteins and spin filtering by conducting electrons.

surface using the drop-casting method and left to dry in a vacuum of 10 -2 Torr for 48 hours.
Three types of samples were prepared, each three times. The types are as follows: a.
Different amounts of drop-casted bR (WT and D96N) on Ni with 0.78 cm 2 active area b.
bR deposition using an electric field. c. Chronoamperommetry was measured both on the same samples as used for the cyclic voltammetry and on new fresh samples in order to reduce the measurement time and to avoid Ni oxidation. In this paper the chronoamperommetry results represent the freshly made samples.

Electrochemical measurements.
Cyclic voltammetry and chronoamperometry were measured using a potentiostat . The electrochemical measurements were performed under ambient conditions in the dark and during illumination at 532 nm. The laser used in this work is a Temperature Controlled Laser Diode Mount (TCLDM9, 3mW power, ThorLabs, USA).
The diameter of the spot is 5.6 millimeters. When light shines on the electrochemical cell, the light is scattered and covers the whole area of the active surface. The working electrode is nickel; underneath, a magnet is placed whose direction can be flipped (0.35 T). The counter electrode is platinum and the reference electrode used is KCl-saturated calomel. The electrolyte used in all electrochemical measurements consists of a 10 mM tris(hydroxymethyl)aminomethane) (TRIS) buffer with 50 mM NaCl at pH 9, with the addition of a redox couple; a basic pH was chosen in this work in order to increase the M's intermediate lifetime. The redox couple selected for the current study was To eliminate the possibility that the spin polarization effect comes from the Ni surface, cyclic voltammetry was applied to a clean Ni surface under the same conditions and with the same electrolytes as stated above. Figure S2 shows the cyclic voltammetry of a clean Ni surface in TRIS buffer and 5 mM Fe 2+ /Fe 3+ . Figure S2: CV curves of bare Ni in 5 mM K 4 [Fe(CN) 6 ]/K 3 [Fe(CN) 6 ] redox couple in TRIS buffer, pH 9, in a magnetic field (0.35 T) that is orthogonal to the surface of the working electrode. The scan rate is 100 mV/s. Figure S3 shows cyclic voltammetry measurements of WT bR in the dark and during illumination at 532 nm. It can be clearly seen that the effect of light on WT bR cannot be 4 detected under these experimental conditions. The ratio in the current through the membranes containing the mutant and the WT bR is shown in Figure S4. Figure S3: CV curves of WT bR in the dark and during illumination. The electrolyte used is 5 mM K 4 [Fe(CN) 6 ]/K 3 [Fe(CN) 6 ] redox couple in TRIS buffer, pH 9, in a magnetic field (0.35T) that is perpendicular to the surface of the working electrode. The scan rate is 100 mV/s. Figure S4: The current through the purple membrane containing the mutant D96N (black) and through the WT bR (red) measured for different concentrations of the deposition solution at the peak of the oxidation potential. The results are shown for a magnetic field pointing upwards. A similar trend was shown for the opposite magnetic field.

Membrane orientation
For estimating the samples' orientation, we deposited the bR using an applied electric field. Following the deposition step, we dried the samples and measured the CPD and the electrochemical signal. The electric field used in this work was ± 40 V/cm. All the samples were measured 3 times for statistical purposes.  Adv. Mater. 2007, 19, 2433-2438 that the electric field induces the orientation of the bR sample. It was found that whereas in the case of the WT there is some effect of the field, its effect is smaller on the mutant orientation.
However, when no field is applied, the CPD shows results similar to those obtained when a field is applied, so that the substrate is biased positively relative to the counter electrode. The reproducibility of CPD measurements for all samples indicates that the membrane is deposited when the more negative side is pointing towards the substrate.

AFM measurements
AFM measurements were performed on thin and thick drop-casted WT and D96N. Figure   S7 shows that good surface coverage was obtained for all samples.