Structure and Conformation of Polypeptides

Polypeptides and poly(a-amino acid)s are polymers of a-amino acids, with the chemical structure I  

They can be synthesized easily, up to a molecular weight of 106, by polymerization of N-carboxy anhydride derivatives of amino acids. When the configuration of the chiral carbon atoms is l for all the amino acid monomer units along the macromole-cular chains, they can be considered simple synthetic analogues of proteins. 

Polypeptides and poly(a-amino acid)s have a quite unique position amongst synthetic polymers. The reason for this is that most common synthetic polymers have very little long range order in solution and their properties are the products of statistical random coil conformations. Polypeptides, in contrast, can adopt well defined, ordered structures typical of those existing in proteins, such as a-helix and P-struc-tures. Moreover, the ordered structures can undergo conformational changes to the random coil state as cooperative transitions, analogous to the denaturation of proteins. 

The most widely known ordered structure in polypeptides is the a-helix. When the repeating units are amino acid residues of l configuration, the a-helix is right-handed i.e., the chain backbone follows the pattern of the thread of a right-hand screw. Each turn of the helix is made up of 3.6 residues, so the helix is nonintegral. An exact repeat of the backbone structure occurs every 18 monomeric units. This repeat corresponds to five turns of the helix, with a linear translation along the axis of 27 A and a pitch of 5.4 A. The helix diameter, neglecting side chains, is about 6 A. 

All side chains of L-amino acid residues point away from the axis thus the structure can accommodate almost any kind of flexible side chain, and it is not necessary that the macromolecules have chemically identical repeating units. Since the chain backbone is the same for all the amino acids, units with different side groups can be accommodated within the same structure. 

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