Surface plasmon resonance (SPR) is one of the leading tools in biomedical research. The challenge in its use is the controlled positioning of one of the components of an interaction on a carefully designed surface. Many attempts in interaction analysis fail due to the non-functional or unsuccessful immobilization of a reactant onto the complex matrix of that surface. The most common technique for linking ligands covalently to a hydrophilic solid surface is amine coupling via reactive esters. In this chapter detailed methods and problem discussions will be given to assist in fast decision analysis to optimize immobilization and regeneration. Topics in focus are different coupling techniques for small and large molecules, streptavidin-biotin sandwich immobilization, and optimizing regeneration conditions.
The role of electrostatics in the adsorption process of proteins to preformed negatively-charged (phosphatidylcholine/phosphatidylglycerol) and neutral (phosphatidylcholine) liposomes was studied. The interaction was monitored at low ionic strength for a set of model proteins as a function of pH. The adsorption behavior of trypsin inhibitor (pI = 4.6), myoglobin (pI = 7.4), ribonuclease (pI = 9.6), and lysozyme (pI = 10.7) with preformed liposomes was investigated, along with changes in the electrophoretic mobility of liposomes through the adsorption of charged proteins. Mean protein charge was determined by acid/base titration. Significant adsorption of the proteins to negatively-charged liposomes was only found at pH values where the number of positive charge moieties exceeds the number of negative charge moieties on the protein by at least three charge units. Negligible adsorption to liposomes composed of zwitterionic lipids was observed in the pH range tested (4-9). The absolute value of the electrophoretic mobilities of negatively-charged, empty liposomes decreased after adsorption of positively-charged proteins. With increasing protein to phospholipid ratio, the drop in the electrophoretic mobility leveled off and reached a plateau; protein adsorption profiles showed a similar shape. Analysis of the data demonstrated that neutralization of the liposome charge due to the adsorption of the positively-charged proteins is the controlling factor in their adsorption. The plateau level reached depended on the type of protein and the pH of the incubation medium. This pH dependency could be ascribed to the mean positive charge of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)
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