INVESTIGATION OF PROTEIN-PROTEIN NONCOVALENT INTERACTIONS IN SOYBEAN AGGLUTININ BY ELECTROSPRAY-IONIZATION TIME-OF-FLIGHT MASS-SPECTROMETRY

Noncovalent interaction in soybean agglutinin (SBA) were studied on an electrospray ionization (ESI) time-of-flight mass spectrometer constr ucted recently at the University of Manitoba. The high m/z range and h igh sensitivity of the instrument together with mild ESI interface con ditions turned out to be ideal for detecting this noncovalently bonded tetrameric protein (MW approximately 116 000 Da) in low charge states (z = 23 to 27). By altering the acetonitrile content of the SBA solut ions it was shown that the observed SBA tetramers are due to structura lly specific noncovalent associations in solution. Octamers and dodeca mers (MW approximately 350 000 Da) were also detected. Information on the quaternary structure of the tetramers was obtained by analyzing th e fragment-ion spectrum resulting from the collision-induced dissociat ion of the tetramer ions.