Helix-Coil Transition at a Glycine Following a Nascent α-Helix: A Synergetic Guidance Mechanism for Helix Growth.

A detailed understanding of forces guiding the rapid folding of a polypeptide from an apparently random coil state to an ordered alpha-helical structure following the rate-limiting preorganization of the initial three residue backbones into helical conformation is imperative to comprehending and regulating protein folding and for the rational design of biological mimetics. However, several details of this process are still unknown. First, although the helix-coil transition was proposed to originate at the residue level (J. Chem. Phys. 1959, 31, 526-535; J. Chem. Phys. 1961, 34, 1963-1974), all helix-folding studies have only established it between time-averaged bulk states of a long-lived helix and several transiently populated random coils, along the whole helix model sequence. Second, the predominant thermodynamic forces driving either this two-state transition or the faster helix growth following helix nucleation are still unclear. Third, the conformational space of the random coil state is not well-defined unlike its corresponding alpha-helix. Here we investigate the restrictions placed on the conformational space of a Gly residue backbone, as a result of it immediately succeeding a nascent ahelical turn. Analyses of the temperature-dependent ID-, 2D-NMR, FT-IR, and CD spectra and GROMACS MD simulation trajectory of a Gly residue backbone following a model alpha-helical turn, which is artificially rigidified by a covalent hydrogen bond surrogate, reveal that: (i) the alpha-helical turn guides the phi torsion of the Gly exclusively into either a predominantly populated entropically favored alpha-helical (alpha-phi) state or a scarcely populated random coil (RC-phi) state; (ii) the alpha-phi state of Gly in turn favors the stability of the preceding alpha-helical turn, while the RC-phi state disrupts it, revealing an entropy-driven synergetic guidance for helix growth in the residue following helix nucleation. The applicability of a current synergetic guidance mechanism to explain rapid helix growth in folded and unfolded states of proteins and helical peptides is discussed.