Non-Covalent Supramolecular Complexes

Beyond the characterisation of primary structures, the direct analysis tertiary structure states and even non-covalent supramolecular complexes by mass spectrometry have not been considered feasible in previous work. In a few cases tertiary structure-dependences have been found, e. g. specific fragmentations in FAB mass spectra of a-helical polypeptides and some MALDI and PD mass spectra of proteins suggesting some native-like structure of macromolecular ions [106, 107]. This situation has changed drastically recently with the analytical development of ESI-MS. A substantial number of ESI-MS studies have demonstrated the identification of supramolecular complexes of biopolymers, as well as specific non-covalent complexes with low-molecular weight constituents [15—18, 31]. In contrast to other ionisation methods in which, predominantly, singly charged ions are produced (EAB, 

MALDI), the continuous series of multiply charged macro-ions in ESI-MS reflects charge states and distributions characteristic of the solution structure ( charge structure cf. Section 10.2.1) [31]. Moreover, ESI-MS can be readily carried out with aqueous solutions at physiological solution conditions, enabhng comparative studies with other structure determination methods such as NMR [134]. This feasibility for the direct mass spectrometric characterisation of non-covalent complexes has opened new analytical perspectives for supramolecular chemistry and biochemistry [16, 31, 135]. 

although the quantitative characterisation of equihbria of complex formation has not yet been established, the large number of aheady successful apph-cations indicates broad potential for the analysis of non-covalent complexes by ESI-MS. 

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