Osmotically Induced Helix-Coil Transition in Poly(glutamic acid)

¹ Oak Ridge National Laboratory
² State University of New York at Stony Brook

^* To whom correspondence should be addressed. E-mail: stanleycb{at}ornl.gov.

Submitted on September 23, 2007
Revised on November 1, 2007
Accepted on 18 December 2007

	Abstract

Protein folding and conformational changes are influenced by protein-water interactions and, as such, the energetics of protein function are necessarily linked to water activity. Here, we have chosen the helix-coil transition in poly(glutamic acid) as a model system to investigate the importance of hydration to protein structure by using the osmotic stress method combined with circular dichroism spectroscopy. Osmotic stress is applied using poly(ethylene glycol) MW 400 as the osmolyte. The energetics of the helix-coil transition under applied osmotic stress allows us to calculate the change in the number of preferentially included water molecules per residue accompanying the thermally induced conformational change. We find that osmotic stress raises the helix-coil transition temperature by favoring the more compact -helical state over the more hydrated coil state. The contribution of other forces to -helix stability also are explored by varying pH and studying a random copolymer, poly(glutamic acid-r-alanine). In this paper, we clearly show the influence of osmotic pressure on the peptide folding equilibrium. Our results suggest that in order to study protein folding in vitro the osmotic pressure, in addition to pH and salt concentration, should be controlled to better approximate the crowded environment inside cells.

Key Words: osmotic stress, poly(amino acid)s, poly(ethylene glycol), protein hydration, solvent accessible surface area