Effects of High Hydrostatic Pressure on the Spectral and Temporal Characteristics of Fluorescence of Photosystem II Core Complex of Thermosthicus Vulcanus

Abstract

Recent steady-state and time-resolved spectroscopy investigations have revealed that Photosystem II core complexes (PSII CCs) are capable of undergoing marked light-induced structural reorganizations even upon the formation of stable charge separation state PSII_C. These reversible changes observed at physiological and cryogenic temperatures lead to the gradual formation of light adapted charge-separated state PSII_L. It has been proposed that the underlying physical mechanisms involve complex dielectric relaxation processes due to the generation of stationary and transient electric fields, in which structural rigidity and flexibility of the related protein complexes play equally important roles. In order to gain further insights into the nature of structural dynamics of PSII, here, the response of the chlorophyll-a transient fluorescence in PSII CC prepared from Thermosthicus vulcanus was studied at 78 K under high hydrostatic pressures applied either at room temperature or at 78 K. PSII CC exhibits remarkable flexibility against high hydrostatic pressures up to 12 kbar and cryogenic temperatures down to 78 K, as evidenced by the fair shape overlap between the initial fluorescence spectrum at ambient conditions and the final fluorescence spectra recorded under various pressure-temperature treatments. This observed reversibility further implies that the variations in these parameters do not significantly disrupt the pigment binding pockets within PSII CC. However, as is typical of glassy protein samples, the pressure-induced spectral and kinetic effects were contingent on the sample's treatment history. These effects, such as bathochromic shifts and broadenings of the spectra, were not only quantitatively greater, but also qualitatively different, such as the disruption of antenna energy transfer pathways or inhibition of the induction of variable chlorophyll fluorescence when pressure was applied at ambient temperature compared to 78 K. The relatively modest spectral shift rates, not exceeding about − 20 cm⁻¹/kbar, further suggest the absence of strongly coupled chlorophyll units significantly contributing to PSII CC fluorescence.