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. 2005 Nov 11;90(3):1009–1018. doi: 10.1529/biophysj.105.069393

FIGURE 1.

FIGURE 1

A proton pathway in GFP (16) leads from Glu-5 on the bottom of the β-barrel (not shown), via an inward-pointing aspartate (Asp-82), the backbone carbonyl of Gln-69, the hydroxyl of Ser-72, the buried Glu-222, and Ser-205 to the hydroxyl of Tyr-66 on the GFP Cro. A short side-branch of the HB pathway (yellow) connects to the N2 atom of the imidazolidinone ring, whose protonation leads to zwitterion formation. Atomic coordinates from Protein DataBank file 1GFL (5) with the HB O–O distances in Å. (Color code: Carbon atoms, gray; oxygens, red; nitrogens, blue.) Hydrogens are not depicted, since they are unobservable in the x-ray structure.