Isolation and structural characterization of N-acetyl- and N-glycolylneuraminic-acid-containing GalNAc-G(D1a) isomers, IV(4)GalNAcIV(3)Neu5AcII(3)Neu5GcGgOse(4)Cer and IV(4)GalNAcIV(3)Neu5GcII(3)Neu5AcGgOse(4)Cer, from bovine brain