Water-Sensitive Low-Frequency Vibrations of Reaction Intermediates during S-State Cycling in Photosynthetic Water Oxidation

Abstract

In photosynthetic water oxidation, two water molecules are converted to an oxygen molecule through five reaction intermediates, designated S(n) (n = 0-4), at the catalytic Mn cluster of photosystem II. To understand the mechanism of water oxidation, changes in the chemical nature of the substrate water as well as the Mn cluster need to be defined during S-state cycling. Here, we report for the first time a complete set of Fourier transform infrared difference spectra during S-state cycling in the low-frequency (670-350 cm(-1)) region, in which interactions between the Mn cluster and its ligands can be detected directly, in PS II core particles from Thermosynechococcus elongatus. Furthermore, vibrations from oxygen and/or hydrogen derived from the substrate water and changes in them during S-state cycling were identified using multiplex isotope-labeled water, including H2(18)O, D2(16)O, and D2(18)O. Each water isotope affected the low-frequency S-state cycling spectra, characteristically. The bands sensitive only to (16)O/(18)O exchange were assigned to the modes from structures involving Mn and oxygen having no interactions with hydrogen, while the bands sensitive only to H/D exchange were assigned to modes from amino acid side chains and/or polypeptide backbones that associate with water hydrogen. The bands sensitive to both (16)O/(18)O and H/D exchanges were attributed to the structure involving Mn and oxygen structurally coupled with hydrogen in a direct or an indirect manner through hydrogen bonds. These bands include the changes of intermediate species derived from substrate water during the process of photosynthetic water oxidation.