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Electrostatic Interactions Modulate the Conformation of Collagen I

Uwe Freudenberg ^*, Sven H. Behrens , Petra B. Welzel ^*, Martin Müller ^*, Milauscha Grimmer ^*, Katrin Salchert ^*, Tilman Taeger , Kati Schmidt , Wolfgang Pompe  and Carsten Werner ^* 

^* Leibniz Institute of Polymer Research Dresden, Max Bergmann Center of Biomaterials Dresden, Dresden, Germany; BASF Aktiengesellschaft, Ludwigshafen, Germany; Technische Universität Dresden, Department of Materials Science, Max Bergmann Center of Biomaterials Dresden, Dresden, Germany; and Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Canada

Correspondence: Address reprint requests to C. Werner, Tel.: 49-351-465-8531; E-mail: werner{at}ipfdd.de.

The pH- and electrolyte-dependent charging of collagen I fibrils was analyzed by streaming potential/streaming current experiments using the Microslit Electrokinetic Setup. Differential scanning calorimetry and circular dichroism spectroscopy were applied in similar electrolyte solutions to characterize the influence of electrostatic interactions on the conformational stability of the protein. The acid base behavior of collagen I was found to be strongly influenced by the ionic strength in KCl as well as in CaCl₂ solutions. An increase of the ionic strength with KCl from 10^–4 M to 10^–2 M shifts the isoelectric point (IEP) of the protein from pH 7.5 to 5.3. However, a similar increase of the ionic strength in CaCl₂ solutions shifts the IEP from 7.5 to above pH 9. Enhanced thermal stability with increasing ionic strength was observed by differential scanning calorimetry in both electrolyte systems. In line with this, circular dichroism spectroscopy results show an increase of the helicity with increasing ionic strength. Better screening of charged residues and the formation of salt bridges are assumed to cause the stabilization of collagen I with increasing ionic strength in both electrolyte systems. Preferential adsorption of hydroxide ions onto intrinsically uncharged sites in KCl solutions and calcium binding to negatively charged carboxylic acid moieties in CaCl₂ solutions are concluded to shift the IEP and influence the conformational stability of the protein.