Kinetics of the homogeneous exchange of ?-lactalbumine adsorbed on titanium oxide surface

Abstract

The homogeneous exchange process whereby alpha-lactalbumine molecules adsorbed on hydrophilic titanium oxide particles are replaced by alpha-lactalbumine molecules in solution has been investigated by means of a 125I radio-labeling technique, alpha-lactalbumine is a compact and highly negatively charged protein, making this study complementary to previous work devoted to the general understanding of the exchange mechanisms of adsorbed proteins on solid surfaces. The isotherm of alpha-lactalbumine exhibits bimodal adsorption shape, and the exchange process whereby adsorbed proteins are replaced by new incoming ones from the bulk solution has been studied at both the upper and the lower plateau of the isotherm. In the upper plateau the exchange process was found to be of first order with respect to the bulk molecules, and the release rate constant was equal to 0.914 L. mol-1.s-1. This behavior is identical to what has been observed with other proteinic systems. In the lower plateau domain, in contrast, the protein release process is independent of the concentration of proteins in the bulk, but the release rates are higher than the pure desorption rates. This constitutes, to our knowledge, a behavior that never before has been observed and that remains to be explained.

FTIR−ATR and Radiolabeling Study of the Adsorption of Ribonuclease A onto Hydrophilic Surfaces: Correlation between the Exchange Rate and the Interfacial Denaturation

“…This can be explained by the fact that proteins which denature on a surface become more tightly bound on it. Moreover, as found in previous studies [3][4][5] the percentage of irreversibly bound molecules decreases with C bulk (Figure 4). This dependence can be the result of a competition between the exchange process in which adsorbed proteins are replaced by new proteins from the bulk and the relaxation of bound proteins from reversibly to irreversibly adsorbed states.…”

“…They are respectively equal to (15.2 ( 1.0) × 10 3 L mol -1 h -1 , 6.6 × 10 -5 s -1 . The value of k e lies between the values obtained for the homogeneous exchange processes of the R-lactalbumin 5 and of lysozyme molecules 4 on the same TiO 2 surface (see Table 1). These proteins are of similar size and shape but differ in their native structural stability, hydrophobocity, and charge.…”

“…This result is in agreement with previous data for different systems. [2][3][4][5][6] In addition, adsorbed molecules can also be released in the absence of proteins in solution as indicated by the value of K 1 different from zero for C bulk ) 0. According to Figure 3, K 1 varies with the bulk concentration C bulk of the protein solution in contact with the layer as where, k e , and k d can be interpreted as the exchange and desorption rate constants.…”

“…Among them, the exchange process, by which proteins present at the sorbent surface are replaced by molecules from the bulk, is of prime importance. Our previous studies were largely devoted to this subject: we investigated the mechanism of homogeneous [2][3][4][5][6] and heterogeneous protein exchange onto hydrophobic 2 and hydrophilic surfaces 6 under uncontrolled [2][3][4] or well-defined hydrodynamic conditions. 6 We demonstrated that structural stability of the adsorbed proteins greatly affects the exchange process.…”

FTIR-ATR and Radiolabeling Study of Structural Modifications during Protein Adsorption on Hydrophilic Surfaces. 2. The Case of Apo-α-lactalbumine

“…Apo-lac was 125 I radiolabeled by using exactly the same labeling procedure as that for Rnase (see paper 1). It was also demonstrated previously that 125 I radiolabeling had an incidence neither on the uptaken protein amount nor on the release kinetics of apo-lac. The concentrations of the labeled protein solutions were determined by absorbance at 280 nm, assuming an extinction coefficient of 2.04 cm 2 ·mg -1 .…”