Observation of ganglioside lactone formation with CD spectrometry

Monolactone (NeuAc1-9NeuAc linkage) formation of a ganglioside, GD1b, was observed at 20 degrees C on solutions ranging in concentration from 0.008 to 1.7 mM and in pH from 2 to 8.1, by following the change of ellipicity at 235 nm, where lactonized gangliosides showed a strong Cotton effect. The lactonization was found to be a first-order reaction in both GD1b and H+ in the initial stage but was affected later by a backward reaction, hydrolysis of the lactone linkage. The rate constant was 0.08 h-1 at pH 3. At this pH the reaction went close to equilibrium in 24 h at approximately 70% lactonization. At pH higher than 5.9 no change was observed in the ellipticity within 30 h, suggesting the possibility that at most 0.6% of the ganglioside lactonized. Lactonization of another ganglioside, GD3, in glacial acetic acid was observed with a 0.2-mm cell, agreeing well with a reported result.