Figure 4.

Tentative three-dimensional model of FluA complexed with 4-glutarylamidofluorescein (for coloring cf. Fig. 1). The backbone conformation corresponds to the crystal structure of BBP, whereas the five basic residues were introduced with their preferred side-chain rotamers (25) by using InsightII modeling software. The fluorescein structure was taken from the coordinates of its complex with the antibody 4-4-20 (24) and equipped with the aliphatic substituent in an extended standard conformation. The ligand (C, green; O, red; N, blue) was manually placed such that its fluorescein moiety can form hydrogen bonds with the two critical His and Arg residues (magenta), similarly as in the crystal structure of the antibody complex. Another three Arg residues (pink) can participate in electrostatic interactions with the negatively charged hapten.