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 CaCl(2) 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 CaCl(2) 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 CaCl(2) solutions are concluded to shift the IEP and influence the conformational stability of the protein.

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Thin films of cellulose are advantageous for analytical studies in aqueous environments to investigate various factors determining the performance of cellulose-based products. However, the weak fixation of cellulose layers on common carrier materials often limits this approach. To address this problem, we suggest a novel maleic anhydride copolymer precoating technique which allows for the covalent attachment of cellulose thin films through esterification. Maleic anhydride copolymers were deposited and covalently bound onto planar, aminosilane-modified glass or silicon oxide surfaces. Cellulose was subsequently immobilized on top of the copolymer precoatings by spin coating from N-methylmorpholine-N-oxide/dimethyl sulfoxide solutions. The resulting cellulose films were thoroughly characterized with respect to layer thickness, morphology, chemical constitution, and electrical charging. The stability of the layers against shear stress was demonstrated in aqueous solutions and the covalent attachment of the cellulose to the copolymer films was proven by means of dissolution experiments followed by ellipsometry and high-resolution X-ray photoelectron spectroscopy.

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