Protein hydrophobic collapse and early folding steps observed in a microfluidic mixer

We demonstrate that the sub-millisecond protein folding process referred to as "collapse'' actually consists of at least two separate processes. We observe the UV fluorescence spectrum from naturally occurring tryptophans in three well-studied proteins, cytochrome c, apomyoglobin, and lysozyme, as a function of time in a micro. uidic mixer with a dead time of similar to 20 mu s. Single value decomposition of the time-dependent spectra reveal two separate processes: 1), a spectral shift which occurs within the mixing time; and 2), a. uorescence decay occurring between; 100 and 300 ms. We attribute the first process to hydrophobic collapse and the second process to the formation of the first native tertiary contacts.