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2 Molecular Structure of GFP-Like Proteins
2.1 The b -Can Fold
The X-ray structures of GFP, first solved by Remington's and Phillips's groups in
1996 [ 13 , 14 ], revealed that its polypeptide chain of 238 amino acids folds into a
rigid, 11-stranded
-can with a central helix running along its axis (Fig. 1a ). It is
capped at either end by short helical sections and loops. The fluorescent chromo-
phore interrupts the central helix and resides close to the geometric center of the
protein. Surrounding residues and structural water molecules anchor the chromo-
phore and hold it tightly in a planar conformation, which is essential for a high
fluorescence quantum yield. Indeed, the isolated chromophore is nonfluorescent in
solution because it rotates around its exocyclic bonds and undergoes fast radiation-
less decay by internal conversion upon reaching an avoided crossing of the S 0 and
S 1 surfaces [ 15 , 16 ]. Over the years, X-ray structures of FPs from many different
species have been solved. They all share the
-can structure, suggesting that this
Fig. 1 Green fluorescent protein, GFP. (a) Cartoon models of the molecular structure. (b)
Maturation scheme. (c) Absorption ( blue ) and emission ( green ) spectra of GFP
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