Lifetimes of intermediates in the β-sheet to α-helix transition of β-lactoglobulin by using a diffusional IR mixer

The extremely slow alpha -helix/beta -sheet transition of proteins is a crucial step in amylogenic diseases and represents an internal rearrangement of local contacts in an already folded protein. These internal structural rearrangements within an already folded protein are a critical aspect of biological action and are a product of conformational flow along unknown metastable local minima of the energy landscape of the compact protein. We use a diffusional IR mixer with time-resolved Fourier transform IR spectroscopy capable of 400-mus time resolution to show that the trifluoroethanol driven beta -sheet to a-helix transition of beta -lactoglobulin proceeds via a compact beta -sheet intermediate with a lifetime of 7 ms, small compared with the overall folding time of beta -lactoglobulin.