  The surface accessibility of {\alpha}-bungarotoxin has been investigated by
using Gd2L7, a newly designed paramagnetic NMR probe. Signal attenuations
induced by Gd2L7 on {\alpha}-bungarotoxin C{\alpha}H peaks of 1H-13C 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 {\alpha}-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 {\alpha}-bungarotoxin
surface which is involved in the interaction with its physiological target, the
nicotinic acetylcholine receptor.
