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 Chlb 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.

中文翻译：

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蛋白质水平非辐射散热对光捕获复合物 II 瞬态升温的研究


光捕获复合物 II （LHCII） 是光系统 II 中最丰富的膜蛋白，具有双重作用，即在弱光条件下有效地收集光和能量传输到反应中心，并在高照射条件下将多余的能量以热量的形式耗散以防止光损伤。后一种过程称为非光化学淬灭 （NPQ）。已经确定 pH 梯度和温度升高都可以触发 NPQ，而通过多余能量的非辐射衰减释放的瞬态热量，以及 LHCII 在室温下伴随的瞬态温度升高，尚未观察到。在这里，我们分别在 LHCII 三聚体上进行了飞秒和纳秒时间分辨可见光泵浦和中红外探针测量。我们在 LHCII 三聚体中检测到激发态散热诱导的瞬态温升。结果表明，LHCII 在 D₂O 介质下获得热平衡，在 480 nm 激发下（主要被 Chlb 和类胡萝卜素吸收），功率为 0.4 mJ，脉冲持续时间为 10 ns，与温度升高 9.3 °C 的理论估计相当一致。 此外，我们观察到 LHCII 响应温度升高的构象变化，即从 3_{个 10} 螺旋/无规卷曲变为 α 螺旋。结合飞秒时间分辨可见光泵浦和中红外探针光谱，确定 LHCII 的光致温度跳跃发生在 60 ns 左右。