Fig. 6. DNP-enhanced ¹⁵N-¹³C TEDOR spectra of ¹³C_10–18-KR2 recorded under different trapping conditions.

(A) In the dark state, cross peaks between the pSB nitrogen (173.3 ppm) and C14/C15 occur. (B) Upon illumination at 110 K, a mixture of new cross peaks and residual dark state populations arises. The main new population stems from the K-state. (C) A subsequent thermal relaxation step at 190 K depopulates the K-state and increases the L-state. The K- and L-state ¹³C chemical shifts agree with those described above (Fig. 3B). They correlate with pSB signals at 176.1 ppm (K-state) and 181.3 ppm (L-state). (D) Trapping the O-state by illumination at room temperature followed by flash freezing does not alter the C14/C15 chemical shifts significantly, but an additional cross peak between C15 and a new pSB nitrogen resonance at 182 ppm is observed. The signal at 181/175 ppm could be from a remaining L-state population but is not considered further because of its low signal-to-noise ratio. Spectra were recorded with a TEDOR mixing time of 6 ms.