Fig. 4.

The 2D DARR NMR of rhodopsin and meta II. The expanded regions of the full 2D NMR spectra of rhodopsin (black) and meta II (red) shown contain cross peaks between the retinal and protein ¹³C-labels. The protein and retinal labels are as follows: 4′-¹³C tyrosine and 12, 20-¹³C retinal (A); 4′-¹³C tyrosine, 3-¹³C Cys and 15, 19-¹³C retinal (B); and 3-¹³C Ser, 4′-¹³C tyrosine and 14,15-¹³C retinal (C and D). The intense resonances along the dotted lines correspond to MAS side bands. The unlabeled cross peaks shown in B centered at 25 and 157 ppm correspond to cysteine–tyrosine correlations. The unlabeled cross peaks shown in C centered at 60 and 155 ppm correspond to serine–tyrosine correlations. The unlabeled cross peaks shown in D correspond to correlations between 4′-¹³C tyrosine and natural abundance ¹³C carbons on the tyrosine rings. The rhodopsin and meta II spectra were obtained at -80°C and a spinning speed of 10 kHz (A), 9.5 kHz (B), or 11 kHz (C and D). See the supporting information, which is published on the PNAS web site, for 1D MAS and full 2D DARR spectra.