Magnetic Field Dependence of the Solid-State Photo-CIDNP Effect Observed in Phototropin LOV1-C57S

Abstract

Here we present the magnetic field dependence of the solid-state Photo-CIDNP effect observed in phototropin LOV1-C57S using 13 C magic-angle spinning (MAS) NMR spectroscopy. Both dark and light spectra were measured at 4.7 T (i.e., 200 MHz 1 H frequency) using a spinning frequency of 8 kHz. An Avance 200 MHz spectrometer equipped with 4-mm MAS probe (Bruker, Karlsruhe, Germany) was used for the 13 C MAS NMR experiments. The sample was packed into a 4-mm sapphire rotor and inserted into the MAS probe. For a homogeneous sample distribution against the rotor wall, the sample was frozen at a very low spinning frequency of 500 Hz. Freezing was monitored on the proton tuning channel as a shift of ca. 0.1 MHz. The variable temperature unit on the spectrometer was set to 235 K. The spinning frequency was in- creased to 8 kHz after the sample is completely frozen. This frequency and set temperature were used for all 13 C MAS NMR measurements. A simple Hahn-echo pulse sequence with two-pulse phase modulation proton-decoupling was used. Continu- ous illumination was supplied by a 1 kW xenon lamp. The cycle delay was 2 s, and the measurement time was about 12 h. In the spectrum obtained in the dark, standard broad protein responses appear. Under illumination, several strong additional light-induced signals appeared. In contrast to the entirely emissive (negative) peaks in the photo-CIDNP MAS NMR spectra observed at 2.3 T (i.e., 100 MHz 1 H frequency), the first observation of this effect in a nonphotosynthetic system, the light induced 13 C NMR peaks at 4.7 T show mixed absorptive/emissive enhancement pattern. This pattern is reminiscent of the spectra observed by liquid state photo-CIDNP of a LOV2 sample. The observed solid-state photo-CIDNP effect is strongly magnetic field dependent, and this field-dependence is well distinguished for the various nuclei. This large difference in magnetic field dependence reflects the large variety of hyperfine factors found in this comparable small-sized and asymmetric radical pair. Keywords photo-CIDNP; MAS; NMR; phototropin; LOV