Reaching the protein folding speed limit with large, sub-microsecond pressure jumps

Biomolecules are highly pressure-sensitive, but their dynamics upon return to ambient pressure are often too fast to observe with existing approaches. We describe a sample-efficient method capable of large and very fast pressure drops (<1 nanomole, >2,500 atmospheres and <0.7 microseconds). We validated the method by fluorescence-detected refolding of a genetically engineered lambda repressor mutant from its pressure-denatured state. We resolved barrierless structure formation upon return to ambient pressure; we observed a 2.1 +/- 0.7 microsecond refolding time, which is very close to the 'speed limit' for proteins and much faster than the corresponding temperature-jump refolding of the same protein. The ability to experimentally perform a large and very fast pressure drop opens up a new region of the biomolecular energy landscape for atomic-level simulation.