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Flavodoxin Relaxes in Microseconds Upon Excitation of the Flavin Chromophore: Detection of a UV–Visible Silent Intermediate by Laser Photocalorimetry

A small contraction concomitant with the relaxation of the protein in ca. 3 μs is observed upon ns‐laser excitation at 455 nm of the Cys69Ala (C69A) mutant of flavodoxin II from Azotobacter vinelandii. This constitutes another example of detection of a UV–vis silent transient species through a photocalorimetric technique. The contraction is attributed to reorganization of protein dihedral bonds and of water molecules at relatively long distances from the flavin chromophore, after the protein has received the heat shock from the relaxing photoproduced charge transfer state. This study constitutes a preliminary step towards the understanding of the origin of protein movements taking place upon electron transfer reactions, e.g. between a photoinduced electron donor (or acceptor) and an accepting (or donating) cofactor in a protein.