Asp85 is the only internal aspartic acid that gets protonated in the M intermediate and the purple‐to‐blue transition of bacteriorhodopsin A solid‐state13C CP‐MAS NMR investigation

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Abstract
High‐resolution solid‐state13C NMR spectra of the ground state and M intermediate of the bacteriorhodopsin mutant D96N with the isotope label at [4‐13C]Asp and [11‐13C]Trp were recorded. The NMR spectra show that Asp85 is protonated in the M intermediate. The environment of Asp85 is quite hydrophobic. On the other hand, Asp212 remains deprotonated and a slight shift to lower field indicates a more hydrophilic environment. Asp85 also protonates in the purple‐to‐blue transition or bacteriorhodopsin in the deionized membrane, where it experiences a similar environment to M. The shift of Trp resonances in M reflect a conformational change of the protein in forming the M intermediate.