Fig. 2.

(A) DNP-enhanced MAS NMR has been applied to U-¹⁵N-ChR2 containing 14,15-¹³C all-trans–retinal. (B) A 62-fold signal enhancement is achieved for ¹³C cross-polarization (CP; CP vs. CP + DNP). The ¹³C natural abundance background can be efficiently suppressed by a double quantum filter (DQF; DQF + DNP), resulting in a spectrum with only the resonances of C14 and C15. As a control, one additional CP spectrum was acquired at 850 MHz close to room temperature (CP at RT). The gray bar indicates where the ¹³C14 signal would be expected for a chromophore in the 13-cis,15-syn conformation. (C) ¹³C14-¹³C15 double-quantum (DQ) build-up curve. (D) HCCH dephasing curves for the C14-C15 spin system in ChR2 during two rotor periods reporting on the HCCH dihedral angle.