This present invention provides methods for structural modification of a molecule containing a hydroxyl group and an oxyanion amide or oxyanion ester group on adjacent or nearby atomic positions. The oxyanion, such as sulfate and phosphate, can be transferred to the hydroxyl group when the molecule is treated with a carbodiimide or various other oxyanion activating agents, resulting in selective oxyanion transfer to the hydroxyl group. Certain polysaccharides, and especially glycosaminoglycans, may be sulfated at a specific hydroxyl group when such hydroxyl group is present adjacent to or nearby a sulfate, phosphate, or other oxyanionic group in ester- or amide-linked forms.