Quantitative Description Of Adsorption Research Articles

26] Methods for studying protein adsorption

Proteins are interfacially active molecules; a statement that is demonstrated easily by the spontaneous accumulation of proteins at interfaces.1–4 Why do proteins show the propensity to adsorb to interfaces and why do they adsorb so tenaciously? For some proteins, the tendency to adsorb is due to the nature of side chains present on the surface of the protein. Protein is an amphoteric polyelectrolyte.5 Its amino acids have different characteristics: some are apolar and like to be buried inside the protein globule, whereas others are polar and charged and are often found on the outside protein surface. A strong, long-ranged electrostatic attraction between a charged adsorbent and oppositely charged amino acid side chains will lead to a significant free energy change favoring the adsorption process. In other cases, the interfacial activity of the protein may be driven by its marginal structural stability.6 The compactness of the native structure of the protein is due to the optimal amount of apolar amino acid residues. The stability of such a structure depends on the combination of hydrophobic interactions between the hydrophobic side chains, hydrogen bonds between the neighboring side chains and along the polypeptide chains, and the Coulomb interactions between charged residues and van der Waals interactions. An adsorbent surface can “compete” for the same interactions and minimize the total free energy of the system by unfolding the protein structure: the adsorption process may result in a surface-induced protein denaturation.7,8 Elements of the secondary structure of the protein (α helix and β sheet) together with the supersecondary motifs form a compact globular domain. Some proteins are built from more than one domain. In a multidomain protein, it is possible that one domain will dominate the adsorption property of the whole macromolecule at a particular type of interface. For example, acid-pretreated antibodies bind with their constant fragments to a hydrophobic surface.9 In order to completely characterize and predict protein adsorption, one would like to have a quantitative description of adsorption. This description is typically obtained by measuring the adsorption isotherm, adsorption and desorption kinetics, conformation of adsorbed proteins, number and character of protein segments in contact with the surface, and other physical parameters related to the adsorbed protein layer, such as layer thickness and refractive index. This article describes a selected set of techniques and protocols that will provide answers about the mechanism of protein adsorption onto and desorption from surfaces. The reader is referred to the specialized monographs1–4 and a review 10 on protein adsorption for a more comprehensive coverage of various aspects of protein–surface interactions.

Effect of Solution Ionic Strength and Iron Coatings on Mineral Grains on the Sorption of Bacterial Cells to Quartz Sand

Understanding the interaction between bacterial cells and solid surfaces is essential to our attempts to quantify and predict the transport of microbes in groundwater aquifers, whether from the point of view of contamination or from that of bioremediation. The sorption of bacterial cells suspended in groundwater to porous medium grains was examined in batch studies. Bacterial sorption to clean quartz sand yielded equilibrium, linear, adsorption isotherms that varied with the bacterial strain used and the ionic strength of the aqueous solution. Values of K(d) (the slope of the linear sorption isotherm) ranged from 0.55 to 6.11 ml g, with the greatest sorption observed for the highest groundwater ionic strength. These findings are consistent with the interpretation that an increasingly compressed electrical double layer results in stronger adsorption between the like-charged mineral surface and the bacterial cells. When iron-oxyhydroxide-coated sand was used, however, all of the added bacteria were adsorbed up to a threshold of 6.93 x 10 cells g of coated sand, beyond which no further adsorption occurred. The irreversible, threshold adsorption is the result of a strong electrostatic attraction between the sesquioxide coating and the bacterial cells. Experimental results of adsorption in mixtures of quartz and Fe(III)-coated sand were successfully predicted by a simple additive model for sorption by the two substrate phases. Even small amounts of Fe(III)-coated sand in a mixture influenced the extent of adsorption of bacterial cells. A quantitative description of adsorption in the mixtures can be realized by using a linear isotherm for reversible adsorption to the quartz grains with a y intercept that represents the number of cells irreversibly adsorbed to the Fe(III)-coated sand.

Calculation of adsorption-related parameters from a.c. polarographic data:: Basis and Computer Programs

Phase-selective alternating current polarography can be advantageously used for the observation and quantitative description of adsorption at a solution—electrode interface; for example, in the absence of a faradaic process, the quadrature current component is simply related to the interfacial differential capacity. Such a.c. measurements are especially advantageous for the occasional investigation of adsorption. The basis of using such measurements is considered; the data analysis is examined; specifics for computer calculation of differential capacity, surface charge density, and relative surface excess, and the requirements for data smoothing are described. The computer programs developed are sufficiently general for handling special situations.