ESI investigation of non-covalent complexes between phosphorylated daidzein derivatives and insulin

Since the end of the last century, ESI-MS has begun to be used to reveal the existence of non-covalent complexes, providing important stoichiometric information. Many researchers have reported the use of ESI-MS to determine dissociation constants of such complexes. Gas phase dissociation constants measured in this way have been found to correlate well with those measured by solution based techniques. (1.2) In this article, ESI results show that all the phosphorylated daidzein derivatives can form non-covalent complexes with the protein insulin, while non-covalent complexes were not detected in solutions of unphosphorylated daidzein and insulin. The relative affinity of each non-covalent complex was obtained according to its different decomposition orifice voltage. Compound e has the highest disappearing orifice voltage and therefore the strongest binding affinity. The relative stability of the non-covalent complexes was closely associated with the length of the hydrophobic chain.