Anomalous Temperature Dependence of the Triplet-Triplet Energy Transfer in Cereibacter sphaeroides I(L177)H Mutant Reaction Centers

---

Tatiana Yu. Fufina¹, Lyudmila G. Vasilieva¹, Irina B. Klenina¹, Ivan I. Proskuryakov^1,a*

¹Institute of Basic Biological Problems, Russian Academy of Sciences, 142290 Pushchino, Moscow Region, Russia

^* To whom correspondence should be addressed.

Received: March 20, 2024; Revised: August 8, 2024; Accepted: August 22, 2024

---

In photosynthetic reaction centers, quenching of the primary donor triplet state by energy transfer to the carotenoid molecule provides efficient suppression of generation of singlet-excited oxygen, potent chemical oxidant. This process in the Cereibacter sphaeroides reaction centers is thermoactivated, and discontinues at temperatures below 40 K. In these reaction centers, substitution of amino acid residue isoleucine at the 177 position of the L-subunit with histidine results in the sharp decrease of activation energy, so that the carotenoid triplets are populated even at 10 K. Activation energy of the T-T energy transfer was estimated as 7.5 cm^–1, which is more than 10-fold lower than activation energy in the wild type reaction centers. At certain temperatures, the energy transfer in the mutant is decelerated, which is related to the increase of effective distance of the triplet-triplet transfer. To the best of our knowledge, the described mutation presents the first reaction center modification leading to the significant decrease in activation energy of the T-T energy transfer to carotenoid molecule. The I(L177)H mutant reaction centers present a considerable interest for further studies of the triplet state quenching mechanisms, and of other photophysical and photochemical processes in the reaction centers of bacterial photosynthesis.

---

KEY WORDS: reaction center, primary electron donor, carotenoid, triplet-triplet energy transfer, temperature dependence, site-directed mutagenesis, Cereibacter sphaeroides

DOI: 10.1134/S0006297924090049

Publisher’s Note. Pleiades Publishing remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

---