Distinction of Sialyl Anomers on ESI- and FAB-MS/MS: Stereo-Specific Fragmentations

An anomeric pair of the lysoglyceroganglioside 1- O-octadecyl-3- O-( N-acetyl)neuraminyl- sn-glycerol sodium salt was studied to see if sialic anomers were distinguishable by mass spectra. It was evident that, in the electrospray ionization and fast-atom bombardment product-ion spectra: (1) in the positive MS 2 product-ion spectrum, the β- anomer showed an unexpected aglycone-side sodiated sodium alkoxide ion, which was absent for the α-anomer; (2) in both polarities the β-anomer showed dehydration much more easily than the α-anomer; and (3) in the negative MS 2 product-ion spectrum, the β-anomer also readily showed decarboxylation. Our hypothesis is that, although several easily interconvertible conformations may be allowed, the one having the large aglycone in the equatorial orientation affects the collision-induced dissociation fragmentations. Anomers are distinguishable by ESI product-ions. An example is a lysoglyceroganglioside, 1- O-octadecyl-3- O-( N-acetyl)neuraminyl- sn-glycerol sodium salt. Conformational flexibility of the pyranose ring may play an important role.