Adsorption and conformation change of short helical peptides on silica and aluminosilicate surfaces

Abstract

Motivated by the challenges in understanding important features of protein adsorption, the interactions between (-helical peptides and a carefully selected set of model surfaces were studied. The peptide sequences contained three blocks of two to five residues each, with the side chains of the N-terminal and C-terminal blocks having negative and positive charges, respectively, and the central block having uncharged side chains. The conformation of a variety of such peptides was studied in solution by circular dichroism (CD) and H nuclear magnetic resonance (NMR) spectroscopy, in order to characterize the degree of xc-helicity in solution, as a function of temperature, pH, and chemical denaturant (urea) concentration. Intramolecular electrostatic interactions arising from the charged side chains, together with the central block of gc-helix-forming alanine residues, were found to stabilize oc-helicity. These interactions were balanced by the natural tendency toward disordered structures, which resulted in fractional oc-helicities between 25% and 50% when in solution. Adsorption isotherms for the peptide Ac-DDDDAAYAARRRR-Am on amorphous colloidal silica nanoparticles were studied in detail. A greater amount of peptide was adsorbed at basic pH than at neutral pH. The was fit with Langmuir and Frumkin isotherms, and the free energy, enthalpy, and entropy of adsorption were calculated. The enthalpy of adsorption at pH 9 (-17 kJ mol'1) was consistent with calculations of the electrostatic interaction between the screened silica surface charge and the dipolar charge distribution on the peptide arising from the charged side chains.

The entropy of adsorption at pH 9 (15 J mol'l K-) arose in part from conformational changes which were observed in the CD and NMR spectra of adsorbed molecules. Solution 1H NMR spectra of peptide adsorbed to colloidal silica resolved resonances from aspartate and alanine, but not from arginine; this was shown to arise from selective immobilization of positively-charged arginine side chains due to electrostatic interactions with the negatively-charged silica, which resulted in a specific orientation of the peptide on the surface. Conformationally-sensitive resonances from aspartate and alanine residues displayed peak shifts characteristic of reduced ac-helicity. The CD spectra of adsorbed peptides also indicated a decrease in oc-helicity. Taken together, these measurements supported the view that loss of o-helicity of adsorbed molecules propagates from the arginine terminus, which is directly adjacent to the surface, into the alanine and arginine segments, which extend into solution. Because conformational changes have important implications for the functionality of adsorbed molecules, the extent of c-helicity loss was measured quantitatively as a function of temperature and pH for the peptide Ac-DDDDAAAARRRRR-Am. At neutral pH, the c-helicity of adsorbed peptides decreased with increasing temperature, much like the behavior observed for peptides in solution. In contrast, at basic pH, the ca-helicity of adsorbed peptides increased with increasing temperature. These results were interpreted using a statistical model based on modifications to existing theories of the helix-coil transition to include residue-specific electrostatic interactions between the surface and charged peptide side chains.