Folding dynamics and mechanism of β-hairpin formation

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Abstract
Protein chains coil into α-helices and β-sheet structures. Knowing the timescales and mechanism of formation of these basic structural elements is essential for understanding how proteins fold1. For the past 40 years, α-helix formation has been extensively investigated in synthetic and natural peptides2,3,4,5, including by nanosecond kinetic studies6,7. In contrast, the mechanism of formation of β structures has not been studied experimentally. The minimal β-structure element is the β-hairpin, which is also the basic component of antiparallel β-sheets. Here we use a nanosecond laser temperature-jump apparatus to study the kinetics of folding a β-hairpin consisting of 16 amino-acid residues. Folding of the hairpin occurs in 6 µs at room temperature, which is about 30 times slower than the rate of α-helix formation6,7. We have developed a simple statistical mechanical model that provides a structural explanation for this result. Our analysis also shows that folding of a β-hairpin captures much of the basic physics of protein folding, including stabilization by hydrogen bonding and hydrophobic interactions, two-state behaviour, and a funnel-like, partially rugged energy landscape.