title: Side-chain and backbone ordering in homopolymers.
abstract: In order to study the relation between backbone and side-chain ordering in proteins, we have performed multicanonical simulations of deka-peptide chains with various side groups. Glu(10), Gln(10), Asp(10), Asn(10), and Lys(10) were selected to cover a wide variety of possible interactions between the side chains of the monomers. All homopolymers undergo helix-coil transitions. We found that peptides with long side chains that are capable of hydrogen bonding, i.e., Glu(10), and Gln(10), exhibit a second transition at lower temperatures connected with side-chain ordering. This occurs in the gas phase as well as in solvent, although the character of the side-chain structure is different in each case. However, in polymers with short side chains capable of hydrogen bonding, i.e., Asp(10) and Asn(10), side-chain ordering takes place over a wide temperature range and exhibits no phase transition-like character. Moreover, non-backbone hydrogen bonds show enhanced formation and fluctuations already at the helix-coil transition temperature, indicating competition between side-chain and backbone hydrogen bond formation. Again, these results are qualitatively independent of the environment. Side-chain ordering in Lys(10), whose side groups are long and polar, also takes place over a wide temperature range and exhibits no phase transition-like character in both environments. Reasons for the observed chain length threshold and consequences from these results for protein folding are discussed.
authors.affiliation: Department of Physics, Michigan Technological University, Houghton, Michigan 49931, USA.
authors.forename: Yanjie
authors.initials: Y
authors.lastname: Wei
authors.forename: Walter
authors.initials: W
authors.lastname: Nadler
authors.forename: Ulrich H E
authors.initials: UH
authors.lastname: Hansmann
mesh_terms: D006860:Hydrogen Bonding; D008958:Models, Molecular; D011108:Polymers / Q000737:chemistry*; D013696:Temperature
chemical_list: D011108:Polymers