An approach to the precise chemoenzymatic synthesis of 13C-labeled sialyloligosaccharide on an intact glycoprotein:: A novel one-pot [3-13C]-labeling method for sialic acid analogues by control of the reversible aldolase reaction, enzymatic synthesis of [3-13C]-NeuAc-α-(2→3)-[U-13C]-Gal-β-(1→4)-GlcNAc-β- sequence onto glycoprotein, and its conformational analysis by developed NMR techniques