COSThe apparent molecular weight (MW) of COS (sample concentration 1.0 mg/ml) was obtained by gel permeation chromatography(GPC) (Agilent 1100, Agilent; Santa Clara, CA, USA). Eluent (0.2 mol/l CH3COOH + 0.15 mol/l CH3COONa) and COS sample solutions were passed through Millipore filters (pore size 0.45 m), with flow rate 1.0 ml/min at 23◦C.