We introduce a method based upon the coupling of mass spectrometry and VUV-synchrotron radiation for biological investigations. This new approach provides information both on the primary structure of full proteins and on the localization of their noncovalent binding sites. This new methodology has been developed through the study of a human intrinsically disordered protein, namely IB5, and its noncovalent interactions with the tannin procyanidin B2 3’OG.