Mechanistic pathways for the photoisomerization reaction of the anchored, tethered chromophore of the photoactive yellow protein and its mutants.

We show by way of physical organic reasoning that the currently known photochemical results of the chromophore of photoactive yellow protein (PYP) are consistent with that expected of a least volume-demanding process for an anchored, tethered chromophore. The primary photoreaction, interestingly, does not appear to involve a hula-twist process. However, the latter might be involved during subsequent transition of dark intermediates. Absorption data of intermediates obtained from a microsecond time-resolved spectroscopic study of three PYP mutants (E46Q, T50V and R52Q) are consistent with the above analyses.