biostudies-literatureUnknown15(58)Merga GThe function of the biological photoswitch phytochrome is initiated by photoisomerization of the methine-bridged tetrapyrrole chromophore, followed by thermal relaxation steps. As a result of this reaction cascade, the protein interconverts between two parental state. These states, denoted as Pr (red absorbing) and Pfr (far-red absorbing), represent the physiologically inactive and active form of the protein, respectively. In this work we studied the primary photoprocesses of two bacterial phytochromes Agp1 and Agp2, in which either Pr or Pfr is the stable dark state, respectively. We employed cryogenic IR difference and resonance Raman spectroscopy between 4 K and 130 K to trap and characterize the species formed on the reaction pathways from Pfr to Lumi-F in Agp2 and Pr to Lumi-R in Agp1RSC advances49889-49899https://www.ebi.ac.uk/biostudies/studies/S-EPMC12703697biostudies-literaturePhotoisomerization of phytochrome's chromophore: a vibrational spectroscopic view on the primary ground state processes.PMC12703697Ebrahim MGroße MPiwowarski PScheerer PSauthof LHildebrandt PMerga GVelazquez Escobar FMichael NKraskov ABartl FfalsePhotoisomerization of phytochrome's chromophore: a vibrational spectroscopic view on the primary ground state processes.The function of the biological photoswitch phytochrome is initiated by photoisomerization of the methine-bridged tetrapyrrole chromophore, followed by thermal relaxation steps. As a result of this reaction cascade, the protein interconverts between two parental state. These states, denoted as Pr (red absorbing) and Pfr (far-red absorbing), represent the physiologically inactive and active form of the protein, respectively. In this work we studied the primary photoprocesses of two bacterial phytochromes Agp1 and Agp2, in which either Pr or Pfr is the stable dark state, respectively. We employed cryogenic IR difference and resonance Raman spectroscopy between 4 K and 130 K to trap and characterize the species formed on the reaction pathways from Pfr to Lumi-F in Agp2 and Pr to Lumi-R in Agp12025-01-01T00:00:00Z2025 Dec2026-06-07T05:20:52.522Z2026-06-07T03:07:03.081ZS-EPMC127036974140368110.1039/d5ra08731g