We present a novel approach to study properties of normal (HbA) and nonenzymatically glycated (HbA(Ic), HbA(Ia+b)) human hemoglobin using absorption spectroscopy and differential scanning calorimetry. The effect of the presence of the antioxidant fisetin on glycation of HbA is studied. Here, absorption spectroscopy has been fruitfully exploited to observe the formation of the glycated hemoglobin. With the differential scanning calorimetry, we studied the thermal unfolding of the protein hemoglobin at various conditions. The thermogram of the pure HbA showed two transition regions, with the occurrence of a partially unfolded intermediate state (the formation of which is mainly reversible) prior to complete denaturation (irreversible process). The denaturation temperature of HbA was found to be strongly dependent on the heating rate. Furthermore, there is a significant cooperativity between the two transition regions in pure HbA. The overall denaturation for the glycated hemoglobin takes place at a lower temperature, suggesting a decrease in the stability of the protein when it is glycated. In presence of fisetin, glycation is inhibited to a certain extent and the thermograms match well with that of normal HbA. Implications of the results are discussed. (c) 2005 Elsevier Inc. All rights reserved.