Proton escape hole on the surface of B-state GFP, after removing a few surface water molecules for better visualization. While the Cro is in its excited state, the proton can hop along the pathway denoted 1, 2, 3, corresponding to the three relevant oxygen atoms of Tyr-66 (black), Thr-203, and His-148. Also seen are Glu-222 (red, under Phe-223 and Gln-204), Ser-205, “the” water molecule (w, white), and the 202–204 triplet (see text). The space filling view is color coded by amino-acid side-chain characteristics: Greens, aliphatic; gray, aromatic; red, carboxylic (Glu, Asp); dark blues, charged amino (Arg, Lys, His); cyan, uncharged amino; yellow, sulfur; and orange, Ser and Thr. Water oxygens are gray. Coordinates from PDB file 1Q4A.