Protein-Protein Interaction Probed by Label-free Second Harmonic Light Scattering: Hemoglobin Adsorption on Spectrin...

In this article, we have studied the binding of different naturally occurring hemoglobin (Hb) variants on erythrocyte skeletal protein, spectrin surface using the label free nondestructive second harmonic light scattering (SHLS) technique in aqueous buffer. Hemoglobin variants like sickle hemoglobin (HbS) and hemoglobin E (HbE) were chosen as they associate with sickle cell disease and HbEβ-thalassemia, respectively, and their interaction with spectrin is compared with normal adult hemoglobin (HbA). The concentration dependent change in the second harmonic light intensity from nanomolar spectrin solution has been measured after addition of small aliquots of hemoglobins. From the second harmonic titration data, the binding constant is calculated using a modified Langmuir adsorption model of hemoglobin binding to the spectrin surface. Interestingly, it is found that the binding constant for HbE (13.8 × 10⁸ M^-1) is 1 order of magnitude higher than that of HbS (1.6 × 10⁸ M^-1) or HbA (2.1 × 10⁸ M^-1) which indicates higher affinity of HbE for spectrin compared to HbA and HbS. The number of the Hb molecules bound to the spectrin surface was estimated to be of the order of hundred's which is determined for the first time.