OBSERVATION OF GANGLIOSIDE LACTONE FORMATION WITH CD SPECTROMETRY

Monolactone (NeuAcl-9NeuAc linkage) formation of a ganglioside, G(D1b) , was observed at 20 degrees C on solutions ranging in concentration f rom 0.008 to 1.7 mM and in pH from 2 to 8.1, by following the change o f ellipicity at 235 nm, where lactonized gangliosides showed a strong Cotton effect. The lactonization was found to be a first-order reactio n in both G(D1b) and H+ in the initial stage but was affected later by a backward reaction, hydrolysis of the lactone linkage. The rate cons tant was 0.08 h(-1) at pH 3. At this pH the reaction went close to equ ilibrium in 24 h at similar to 70% lactonization. At pH higher than 5. 9 no change was observed in the ellipticity within 30 h, suggesting th e possibility that at most 0.6% of the ganglioside lactonized. Lactoni zation of another ganglioside, G(D3), in glacial acetic acid was obser ved with a 0.2-mm cell, agreeing well with a reported result.