Structural investigations on betacyanin pigments by LC NMR and 2D NMR spectroscopy

Four betacyanin pigments were analysed by LC NMR and subjected to extensive NMR characterisation after isolation. Previously, low pH values were applied for NMR investigations of betalains resulting in rapid degradation of the purified substances thus preventing extensive NMR studies. Consequently, up to now only one single 13C NMR spectrum of a betalain pigment, namely that of neobetanin (=14,15-dehydrobetanin), was available. Because of its sufficient stability under highly acidic conditions otherwise detrimental for betacyanins, this pigment remained an exemption. Since betalains are most stable in the pH range of 5–7, a new solvent system has been developed allowing improved data acquisition through improved pigment stability at near neutral pH. Thus, not only 1H, but for the first time also partial 13C data of betanin, isobetanin, phyllocactin and hylocerenin isolated from red-purple pitaya [ Hylocereus polyrhizus (Weber) Britton & Rose, Cactaceae] could be indirectly obtained by gHSQC– and gHMQC–NMR experiments. Four betacyanin pigments hitherto characterised by mass spectrometric and proton nuclear magnetic resonance (NMR) analyses were subjected to extensive NMR characterisation. A new solvent system considering the pH stability characteristics of betalain pigments was developed. For the first time, 13C NMR data of betanin, isobetanin, phyllocactin and hylocerenin could thus be obtained.