Electrospray mass spectrometry and UV/visible spectrophotometry studies of aluminum(III)–flavonoid complexes

Aluminum(III) complexes formed by the isoflavonebiochanin‐A and the flavones kaempeferol and quercetin inCH 3 OH, CH 3 CN, i ‐C 3 H 7 OH, and the mixed solventsCH 3 CN/CH 3 OH (1/1 v/v),CH 3 CN/H 2 O (1/1 v/v)CH 3 CN/CH 2 Cl 2 (1/1v/v) andCH 3 CN/ i ‐C 3 H 7 OH(1/1 v/v), were investigated by means ofelectrospray mass spectrometry (ES‐MS) andUV/visible absorbance spectrophotometry. Electrospray mass spectraof solutions that were indicated by their UV/visible spectra tohave formed an Al‐flavonoid complex displayedsingly‐charged ions corresponding in m / z tothe 1:2, dimeric complex, viz.,[AlL 2 ] + ,L=(flavonoid‐H + ) ‐ .Lesser abundant singly‐charged ions corresponding in m / z to[AlR(L)] + , whereR=H ‐ , HO ‐ orCH 3 O ‐ were also sometimes observed.Formation of the 1:2 complex was favored in CH 3 OH relativeto all other solvent systems investigated. The relative amounts ofcomplexed and free flavonoid in solution, as estimated by theappearance of the UV/visible spectra, correlated well with theabundance ratio of the 1:2, Al‐flavonoid dimer to theprotonated flavonoid (indicative of free flavonoid) observedin the ES mass spectrum of the same solution. © 1998 John Wiley& Sons, Ltd.