Abstract
It has been shown that anti-PAH mAb can bind a particular cross-reactant by adopting two distinct "red" and "blue" conformations of its binding sites [N.M. Grubor et al. PNAS 102, 2005, 7453-7458] In the case of red conformation of pyrene (Py)/anti-PAH mAb (with a broad fluorescence (0,0)-band with fwhm similar to 140 cm(-1)), the central role in complex formation was played by pi-pi interactions. The nature of the blue-shifted conformation with very narrow fluorescence (0,0)-band (fwhm similar to 75 cm(-1)) was left unclear due to the lack of suitable data for comparison. In this work, we suggest spectroscopic and modeling results obtained for the blue conformation of Py in several mAb (including 4D5 mAb) are consistent with pi-cation interactions, underscoring the importance of pi-cation interaction in ligand binding and stabilization in agreement with earlier modeling studies [J-L Pellequer, et al. J. Mol. Biol. 302, 2000, 691-699]. We propose considerable narrowing of the fluorescence origin band of ligand in the protein environment could be regarded as a simple indicator of pi-cation interactions. Since 4D5 mAb forms only the blue-shifted conformation, while anti-PAH and 8E11 mAbs form both blue- and red-shifted conformations, we suggest mAb interactions, with Py molecules lacking H-bonding functionality, may induce distinct conformations of mAb binding sites that allow binding by pi-pi and/or pi-cation interactions. (C) 2010 Elsevier B.V. All rights reserved.