Fig. 1. Experimental approach.

(A) Schematic photocycle of KR2. (B) Light-induced cryo-trapping for dynamic nuclear polarization (DNP)–enhanced magic angle spinning (MAS) NMR. (C) Retinal and KR2 isotope-labeling schemes. KR2 proteoliposomes were incubated with AMUPol in D₂O/H₂O/glycerol (see Materials and Methods). (D) DNP-enhanced ¹³C spectra of ¹³C_10–18-KR2. (i) ¹³C CP spectra reveal a DNP signal enhancement of 40. (ii) ¹³C DQF spectra in the dark state and (iii) after illumination at 110 K. (iv) Difference spectrum (light dark). (v) Pure light-induced spectrum obtained by subtracting the remaining dark state contribution (see fig. S1). Retinal peak assignment is based on data in Fig. 2. mw, microwave.