Adsorption behavior of different polypeptides in the 3 kDa molecular weight range at an Si 0.8Ti 0.2O 2–aqueous solution interface from low ionic strength solutions

Abstract

The adsorption and desorption kinetics of six polypeptides, denoted by 1, 2, 3A, 3B, 4 and bee venom melittin, close in molecular weight but differing in their apparent charge state and hydrophobicity are investigated at a given bulk concentration (10 −6 M) by means of optical waveguide light mode spectroscopy at an Si 0.8Ti 0.2O 2–aqueous solution interface. At pH 8.0 and a salt concentration of 0.01 M, it is found that four of these polypeptides, 1, 2, 4 and melittin display quasi identical adsorption kinetics and differ only significantly with respect to their desorption kinetics. These observations could be rationalized by the calculation of their charge state in the framework of the Linderström-Lang model: in contrast with predictions based on their amino acid sequence, all these molecules display only a very small charge due to p K a modifications in this low ionic strength medium. In marked contrast to the four other polypeptides, the molecules 3A and 3B, identical in their amino acid composition but differing in the localization of four amino acids (through two point mutations), display smaller adsorption rates when put in contact with the sorbent. Moreover, their desorption kinetics could be fitted by means of only one exponential decay function whereas two exponentials where necessary for the first class of polypeptides. No evident reason is found to explain this peculiar adsorption behavior since 3A and 3B display the same average (calculated) charge state and hydrophobicity as the other investigated molecules. The polypeptides 1, 2, 3A, 3B display an adsorption hysteresis: their desorption rate constant depends markedly on the contact time between the polypeptide solution and the sorbent. In that sense, the interfacial behavior of these molecules is quite similar to that of large globular proteins.