Protein molecules immobilized at a material surface play a key role in many biosensing, tissue engineering, enzymatic catalysis, bioseparation, and bioelectronics applications. The tendency of proteins to attach to interfacial regions is well documented (Haynes and Norde 1994; Malmsten 1998a, b; Van Tassel 2003). Ionic, van der Waals, solvation, and donor-acceptor interactions all play important roles in rendering the interfacially adsorbed state to be thermodynamically favored over the solution state (Haynes and Norde 1994). Proteins are colloidal objects that possess a distribution of surface charge and, in an electrolytic solution, a distribution of weakly associated counterions. Their interaction with a solid substrate is thus sensitive to the substrate's charge distribution. By controlling the electric potential of an adsorbing surface, one alters this charge distribution and, therefore, the surface-protein interaction. In principle, the adsorption process maybe controlled in this way, perhaps leading to adsorbed layers of enhanced density or preferred orientation or spatial distribution. However, the interaction between a protein and an adsorbing surface is complex, and predicting adsorbed layer properties by considering the contributions from

Paul R. Van Tassel: Department of Chemical Engineering, Yale University, New Haven, CT 06520-8286, USA, E-mail: [email protected]

Principles and Practice Proteins at Solid-Liquid Interfaces Philippe Dejardin (Ed.) © Springer-Verlag Berlin Heidelberg 2006

the interaction modes given above remains a significant challenge. Thus, while influencing an adsorbed protein layer through the substrate's electric potential is both possible and desirable (Asanov et al. 1997,1998; Bernabeu and Caprani 1990; Bos et al. 1994; Brusatori et al. 2003; Brusatori and Van Tassel 2003; Feng and Andrade 1994; Fievet et al. 1998; Fraaije et al. 1990; Khan and Wernet 1997), the process is as yet poorly understood.

In this contribution, we review our recent work investigating the influence of substrate electric potential on protein adsorption kinetics (Brusa-tori et al. 2003; Brusatori and Van Tassel 2003). We begin with a brief presentation of certain basic theoretical considerations. We then introduce the method by which we measure protein adsorption kinetics under electric potential control: optical waveguide lightmode spectroscopy (OWLS). Next, we introduce some key results and discuss them in the context of findings by other groups. By summarizing several important results and open questions, we hope to guide future efforts to produce controlled layers of adsorbed protein through the control of substrate electric potential.

Proteins are composed of amino acids, some of which contain acidic/basic sites. Thus, at all but the isoelectric pH, a protein molecule possesses a net charge and therefore migrates in response to an electric field. Since the charge distribution is generally not spherically symmetrical, the electric field also imposes a torque on the molecule, causing it to rotate. Of course, electric-field-induced migration and rotation must compete against the molecule's thermal diffusive motion, so these influences are only observed in excess of some threshold field strength. Due to screening by solution ions, electric field effects become appreciable only within a few Debye lengths of a charged surface and are most important at or near direct contact, where properties such as the rate of attachment, mean protein orientation, and the rate and extent of subsequent postadsorption changes in orientation and conformation may be profoundly affected.

The transport of protein in a flowing solution is described by where c is the protein concentration, t is the time, v is the fluid flow velocity vector, D is the diffusivity, q is the effective protein charge, E is the electric field vector, and Ç is the friction coefficient (a measure of the viscous drag

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