Chromophore Structural\nChange during the Photocycle\nof a Light-Driven Cl<sup>–</sup> Pump from Mastigocladopsis\nrepens: A Cryogenic Raman Study

Microbial rhodopsins are the most widely distributed\nphotoreceptors\nthat bind a retinal Schiff base chromophore. Among them, a light-driven\nCl<sup>–</sup> pump discovered from Mastigocladopsis\nrepens (<i>Mr</i>HR) is distinctive in that\nit has the structural features of both H<sup>+</sup> and Cl<sup>–</sup> pumps. While the photocycle has been characterized by light-induced\nchanges of the absorption spectrum, the structural changes of the\nretinal chromophore remain largely unknown. In this study, we examined\nthe chromophore structural changes of <i>Mr</i>HR by using\ncryogenic Raman spectroscopy. We observed five photointermediatesK,\nL, N1, N2, and <i>Mr</i>HR’that show distinct\nvibrational spectra, indicating atypical chromophore structures, e.g.,\nsmall distortion in the K intermediate and Schiff base configurational\nchange in the <i>Mr</i>HR’ intermediate. Based on\nthe Raman spectra of two N intermediates (N1 and N2), we propose that\nN1 is the Cl<sup>–</sup>-bound state and N2 is the Cl<sup>–</sup>-unbound state, which are responsible for the Cl<sup>–</sup> release and uptake, respectively, to achieve Cl<sup>–</sup> pumping.