  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. Glu10, Gln10, Asp10, Asn10, and Lys10 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.
Glu10, and Gln10, exhibit a second transition at lower temperatures connected
with side chain ordering. This occurs in 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.
Asp10 and Asn10, 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 Lys10, 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.
