Secondary Structure

Secondary structure is the repeating local geometry of the backbone, held together by hydrogen bonds between backbone amide donors and carbonyl acceptors rather than by side chains. The alpha helix is the dominant motif in peptide pharmacology: a right-handed coil of about 3.6 residues per turn in which each carbonyl bonds to the amide four residues along, projecting side chains outward in a pattern that can make one face hydrophobic and the other charged. Beta strands run extended and pair with neighbours into parallel or antiparallel sheets. Turns reverse the chain direction in three or four residues. The random coil is not a structure but the absence of one, a rapidly interconverting ensemble.

The measurement of choice is circular dichroism in the far ultraviolet, where a helix gives a characteristic pair of negative bands near 208 and 222 nanometres and a sheet gives a single broader minimum near 218. This is how helicity is quantified when a peptide is stapled or when trifluoroethanol is used to promote folding. The empirical fact that matters is that most short linear peptides are largely unstructured in water and fold only on meeting their receptor, which is the binding penalty that constraint chemistries remove.

Knowing the structural preference tells you which engineering moves are available. A sequence that wants to be helical can be stapled, cyclised or given helix-favouring residues; a sequence prone to beta-strand pairing is a candidate for aggregation and needs the opposite treatment.

Where it misleads is the assumption that a diagram represents reality in solution. Textbook and modelled structures show the bound or idealised conformation, not the disordered ensemble a free peptide occupies at body temperature, and a helix in a marketing figure is often a computed prediction rather than a measurement.