Do strong intramolecular hydrogen bonds persist in aqueous solution?: Variable temperature gradient 1H, 1H-13C GE-HSQC and GE-HMBC NMR studies of flavonols and flavones in organic and aqueous mixtures

Intramolecular hydrogen bonds in crystals and in apolar media are well documented, however, the degree to which they persist in aqueous solution is controversial. We report here variable temperature gradient H-1, H-1-C-13 Gradient Enhanced Heteronuclear Single Quantum Correlation (GE-HSQC) and Gradient Enhanced Heteronuclear Multiple Bond Coherence (GE-HMBC) NMR studies of the flavonols quercetin and kaempferol and the flavone luteolin, in organic solvents and in mixtures of organic-aqueous solutions. It is demonstrated that the strong intramolecular hydrogen bond of the -CO(4) and -OH(5) moieties persists over a wide range of aqueous mixtures and, thus, provide a rare example of non-charged intramolecular hydrogen bonds, which is not overwhelmed by solvation with protic solvents, in particular in aqueous solution. (C) 2002 Elsevier Science Ltd. All rights reserved.