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

Light-harvesting\ncomplex II (LHCII), the most abundant membrane\nprotein in photosystem II, plays dual roles, i.e., efficient light\nharvesting and energy transfer to the reaction center under low light\nconditions and dissipating excess energy as heat to prevent photodamage\nunder high irradiation conditions. The latter process is known as\nnonphotochemical quenching (NPQ). It has been established that both\nthe pH gradient and temperature rise can trigger NPQ, while the transient\nheat release via nonradiative decay of the excess energy, as well\nas the accompanying transient temperature rising of LHCII at room\ntemperature, have not been observed yet. Here we conducted femtosecond\nand nanosecond time-resolved visible pump and mid-infrared probe measurements\non the LHCII trimer, respectively. We detected an excited-state heat\ndissipation-induced transient temperature rise in the LHCII trimer.\nThe results show that the LHCII gets thermal equilibrium with D<sub>2</sub>O medium with a temperature rise of 7 °C under 480 nm\nexcitation (mainly absorbed by Chl<i>b</i> and carotenoid)\nat a power of 0.4 mJ and a pulse duration of 10 ns, fairly consistent\nwith the theoretical estimation of a temperature increase of 9.3 °C.\nFurthermore, we observed the conformational changes of LHCII in response\nto the raised temperature, i.e., from 3<sub>10</sub>-helix/random\ncoil to α-helix. Combining the femtosecond time-resolved visible\npump and mid-infrared probe spectra, the light-induced temperature\njump of LHCII is determined to take place around 60 ns.