X-ray crystallography and nuclear magnetic resonance measurements provide us with atomically resolved structures of an ever-growing number of biomolecules. These static structural snapshots are important to our understanding of biomolecular function, but real biomolecules are dynamic entities that often exploit conformational changes and transient molecular interactions to perform their tasks. Nuclear magnetic resonance methods can follow such structural changes, but only on millisecond timescales under non-equilibrium conditions. Time-resolved X-ray crystallography has recently been used to monitor the photodissociation of CO from myoglobin on a subnanosecond timescale1, yet remains challenging to apply more widely. In contrast, two-dimensional infrared spectroscopy, which maps vibrational coupling between molecular groups and hence their relative positions and orientations2,3,4,5,6,7,8,9,10,11, is now routinely used to study equilibrium processes on picosecond timescales. Here we show that the extension of this method into the non-equilibrium regime12,13 allows us to observe in real time in a short peptide the weakening of an intramolecular hydrogen bond and concomitant opening of a β-turn. We find that the rate of this process is two orders of magnitude faster than the ‘folding speed limit’ established for contact formation between protein side chains14.
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This work was financially supported by a Postdoctoral Research Fellowship from the Deutsche Forschungsgemeinschaft to C.K.; by grants from the SNF to P.H.; and grants from the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie to W.S. Author Contributions C.K. designed the molecule together with W.S., C.K. synthesized it and ran the experiment together with J.H. C.K., J.H. and P.H. wrote the manuscript. M.K. and P.H. were responsible for the molecular dynamics simulations. 2D-IR and transient 2D-IR spectroscopy is a major theme of the group of P.H.
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Topics in Current Chemistry (2017)