Anthocyanins are well known to exist with different colors as a function of pH, as they exist in an equilibrium system.  In acidic media, anthocyanins are red due to the flayvium cation form of the equilibrium dominating.  At neutral or basic pH, the quinoidal base form is prevalent and the color turns blue.  When the pH is in-between, the carbinol and chalcone pseudobase forms are most important and the color fades or turns clear.  The anthocyanin equilibrium with respect to pH has been well studied by UV-Vis and NMR.  However, very few studies have used vibrational spectroscopy to investigate the equilibrium of the anthocyanin forms.  This work provides a Raman study of grape anthocyanins as a function of pH, showing the dominate form of the resonance. MicroRaman spectra were recorded using blue light with wavelength 473nm.  This did not eliminate fluorescence but reduced it enough to observe a sufficient Raman signal.  Infrared was attempted, but unsuccessful due to absorption of water.

pH studies of grape anthocyanins using resonance Raman spectroscopy

244th National Meeting (2012)

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