Asp<sup>85</sup> is the only internal aspartic acid that gets protonated in the M intermediate and the purple&#x2010;to&#x2010;blue transition of bacteriorhodopsin A solid&#x2010;state<sup>13</sup>C CP&#x2010;MAS NMR investigationG. Metz, Martin Engelhard|FEBS Letters|1992Cited by 138