Fig. 2.

¹⁵Nζ-¹³C15 correlation experiments provide assignments of the retinal-C15 resonance in each state: dark-adapted (A); light-adapted (B), 4 L's with residual bR₅₆₈ (C), and M_o (D). Note the downfield ¹⁵N shifts observed in the L spectrum. The temperature and wavelength of the light used for preparation of each intermediate is as follows: (A) the dark-adapted state (bR₅₅₅ and bR₅₆₈) obtained by room temperature equilibration in the dark; (B) the light-adapted state (bR₅₆₈) accumulated by 532 nm irradiation of (A) at 275 K; (C) a mixture of 4 L states, with bR₅₆₈ accumulated by 640 nm irradiation of (B) at 150 K; and (D) the early M intermediate accumulated by 532 nm irradiation of (B) at 210 K. After accumulation, all of the intermediates were cooled to 90 K for data acquisition in the dark.