Intrinsic tryptophan fluorescence quenching by iodine in non-canonical amino acid reveals alteration of the hydrogen bond network in the photoactive orange carotenoid protein

The Orange Carotenoid Protein (OCP) regulates cyanobacterial photosynthetic activity through photoactivation in intense light. A hydrogen bonding network involving the keto-carotenoid oxygen and Y201 and W288 residues prevents the spontaneous activation of dark-adapted OCP. To investigate role of hydrogen bonds in OCP photocycling, we introduced non-canonical amino acids near the keto-carotenoid, particularly iodine at the meta-position of Y201. This modification significantly increased the yield of red OCP photoproducts, albeit with a shorter lifetime. Changes in tryptophan fluorescence during photocycling influenced by the presence of an iodine near W288 and revealed interactions between Y201 and W288 in the absence of carotenoid in the C-domain. We propose that upon the relaxation of red states, a ternary complex with the carotenoid is formed. Analysis of spectral signatures and interaction energies indicates that specific iodo-tyrosine configuration enhances interactions between the carotenoid and W288. •3-iodination of Y201 increases the yield of OCP photoproducts and their relaxation rate.•3-iodination of Y201 shifts H-bonding of OCP carotenoid to interact with W288.•In the absence of carotenoid in the C-domain, Y201 interacts with W288.