Investigation of Transient Temperature Rising of Light-Harvesting Complex II by Nonradiative Heat Dissipation at the Protein Level

Light-harvesting complex II (LHCII), the most abundant membrane protein in photosystem II, plays dual roles, i.e., efficient light harvesting and energy transfer to the reaction center under low light conditions and dissipating excess energy as heat to prevent photodamage under high irradiation conditions. The latter process is known as nonphotochemical quenching (NPQ). It has been established that both the pH gradient and temperature rise can trigger NPQ, while the transient heat release via nonradiative decay of the excess energy, as well as the accompanying transient temperature rising of LHCII at room temperature, have not been observed yet. Here we conducted femtosecond and nanosecond time-resolved visible pump and mid-infrared probe measurements on the LHCII trimer, respectively. We detected an excited-state heat dissipation-induced transient temperature rise in the LHCII trimer. The results show that the LHCII gets thermal equilibrium with D₂O medium with a temperature rise of 7 °C under 480 nm excitation (mainly absorbed by Chl<i>b</i> and carotenoid) at a power of 0.4 mJ and a pulse duration of 10 ns, fairly consistent with the theoretical estimation of a temperature increase of 9.3 °C. Furthermore, we observed the conformational changes of LHCII in response to the raised temperature, i.e., from 3₁₀-helix/random coil to α-helix. Combining the femtosecond time-resolved visible pump and mid-infrared probe spectra, the light-induced temperature jump of LHCII is determined to take place around 60 ns.