The use of a ditopic Gd(III) paramagnetic probe for investigating alpha-bungarotoxin surface accessibility

Protein surface accessibility is a critical parameter which drives all intermolecular interaction processes. In this respect a big deal of information has been derived by analyzing paramagnetic perturbation profiles obtained from NMR protein spectra, particularly in the case that the effects due to different soluble paramagnets can be compared. Here Gd₂L7, a neutral ditopic paramagnetic NMR probe, has been characterized in terms of structure and relaxivity and its paramagnetic perturbations on α-bungarotoxin CαH signals in ¹H-¹³C HSQC (heteronuclear single quantum coherence) spectra have been analyzed. Then, these signal attenuations have been compared with the ones previously obtained in the presence of GdDTPA-BMA (gadolinium(III) diethylenetriamine-N,N,N′,N'″,N″- pentaacetate-bis(methylamide)). 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 toward the protein surface side where residues involved in the receptor binding are located. The different residence times of the water molecule directly coordinated by the Gd(III) ion measured for the two paramagnets account for the reduced broadening of water signal in the presence of the ditopic probe at equivalent gadolinium concentration. These features make Gd₂L7 a very suitable probe for investigating protein surface accessibility of complex protein systems.