The invention relates to aldonic acid esters of starch fractions or starch fraction derivatives which are selectively oxidised on the reducing chain end to form aldonic acids, and to solids and solutions containing said aldonic acid esters. The invention also relates to methods for producing said aldonic acid esters, to methods for producing pharmaceutical active ingredients coupled to polysaccharides or polysaccharide derivatives on free amino functions, and to pharmaceutical active ingredients thus obtained.