The surface accessibility of {\alpha}-bungarotoxin monitored by a novel paramagnetic probe

The surface accessibility of α-bungarotoxin has been investigated by using Gd2L7, a newly designed paramagnetic NMR probe. Signal attenuations induced by Gd2L7 on α-bungarotoxin CαH peaks of H-C HSQC spectra have been analyzed and compared with the ones previously obtained in the presence of GdDTPA-BMA. In spite of the different molecular size and shape, for the two probes a common pathway of approach to the α-bungarotoxin surface can be observed with an equally enhanced access of both GdDTPA-BMA and Gd2L7 towards the protein surface side where the binding site is located. Molecular dynamics simulations suggest that protein backbone flexibility and surface hydration contribute to the observed preferential approach of both gadolinium complexes specifically to the part of the α-bungarotoxin surface which is involved in the interaction with its physiological target, the nicotinic acetylcholine receptor.