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Zimmermann, R. ; Kratzmüller, Th. ; Erickson, D. ; Li, D. ; Braun, H.-G. ; Werner, C.
Ionic Strength-Dependent pK-Shift in the Helix-Coil Transition of Grafted Poly(L-glutamic acid) Layers Analyzed by Electrokinetic and Ellipsometric Measurements

Surface-bound layers of poly(L-glutamic acid) prepared by a recently described "grafting-from" method1were analyzed with respect to electrical charging and structural alterations upon variation of pH andconcentration of the background electrolyte in aqueous solutions. The microslit electrokinetic setup (MES)2,3was utilized for the combined determination of zeta potential and surface conductivity on the basis ofstreaming potential and streaming current measurements at polypeptide layers in contact with aqueouselectrolyte solutions of varied composition. In situ ellipsometry was applied at similar samples immersedin identical aqueous solutions to investigate the influence of the solution pH on the structure of the polypeptidelayers. Zeta potential and Dukhin number versus pH plots revealed the dissociation behavior of the surface-bound polypeptides indicating a significant shift of the pK of their acidic side chains correlating with theconcentration of the background electrolyte potassium chloride and the related variation of the Debyescreening length.
Surface conductivity data pointed at a more expanded structure of the polypeptide layerin the fully dissociated state as an increased ion conductance in this part of the interface was determined.The occurrence of a strong increase of the thickness and a corresponding decrease of the refractive indexfor the coil state of the layer strongly supports the findings of the electrokinetic measurements. This fullyreversible "switching" of the layer structure was attributed to helix-coil transitions within the graftedpolypeptides induced by the dissociation of carboxylic acid functions of the polypeptide side chains. Theshift of the "switching pH" of the surface-bound poly(L-glutamic acid) layers at varied concentrations ofthe background electrolyte was interpreted as a result of the pK shift of the carboxylic acid groups of thepolypeptide side chains. The observed patterns prove that the electrostatic interactions causing this shiftoccur within but not between the grafted chains.

Langmuir 20, 2369-2374


March 2004
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