title: Effects of temperature and pH on the helicity of a peptide adsorbed to colloidal silica.
abstract: The conformation of a cationic alpha-helical peptide (DDDDAAAARRRRR) adsorbed to anionic colloidal silica has been investigated by circular dichroism (CD) spectroscopy as a function of temperature and pH in order to examine how the structure of an adsorbed molecule responds to two simultaneous perturbations. Increased temperature destabilizes the helicity of the peptide in solution, while pH changes alter the substrate surface charge and the corresponding strength of the interaction with the peptide. Near neutral pH, the helicity of the adsorbed peptide, which is determined from the intensity of the CD signal at 222 nm, decreases with increasing temperature, similarly to the temperature-dependent behavior observed for the peptide in aqueous solution. By contrast, at basic pH and a strongly negative surface charge, the helicity of the adsorbed peptide increases with temperature. In order to elucidate the origin of the reversal of the temperature dependence of helicity, a statistical model for the conformation of the adsorbed peptide has been formulated based on the Lifson-Roig model for the helix-coil transition of the peptide in solution. The model provides insight into the trends in fractional helicity and reveals that the temperature dependence of the helicity of the adsorbed peptide results from a competition between the intramolecular interactions that promote helicity and the intermolecular interactions with the surface. The statistical model also enables estimation of the free energy contributions from specific aspects of the adsorption process. Through identification of a connection between the conformation of adsorbed peptide and the interactions of the peptide with the surface, this work suggests a route for the control of adsorbate conformation through peptide and surface engineering.
authors.affiliation: Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge 02139, USA.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Read
authors.forename: Anne M
authors.initials: AM
authors.lastname: Mayes
authors.forename: Sandra L
authors.initials: SL
authors.lastname: Burkett
mesh_terms: D000327:Adsorption; D000596:Amino Acids / Q000737:chemistry; D002851:Chromatography, High Pressure Liquid; D003102:Colloids / Q000737:chemistry*; D049691:History, Medieval; D006863:Hydrogen-Ion Concentration; D010455:Peptides / Q000737:chemistry* / Q000302:isolation & purification; D017433:Protein Structure, Secondary; D012822:Silicon Dioxide / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D000596:Amino Acids; D003102:Colloids; D010455:Peptides; D012822:Silicon Dioxide