Department of Chemistry, University of California , Berkeley, California 94720, United States.
J Phys Chem B. 2014 Jan 23;118(3):659-67. doi: 10.1021/jp408584v. Epub 2014 Jan 10.
Femtosecond stimulated Raman spectroscopy (FSRS) is used to examine the structural dynamics of the para-hydroxycinnamic acid (HCA) chromophore during the first 300 ps of the photoactive yellow protein (PYP) photocycle, as the system transitions from its vertically excited state to the early ground state cis intermediate, I0. A downshift in both the C7═C8 and C1═O stretches upon photoexcitation reveals that the chromophore has shifted to an increasingly quinonic form in the excited state, indicating a charge shift from the phenolate moiety toward the C9═O carbonyl, which continues to increase for 170 fs. In addition, there is a downshift in the C9═O carbonyl out-of-plane vibration on an 800 fs time scale as PYP transitions from its excited state to I0, indicating that weakening of the hydrogen bond with Cys69 and out-of-plane rotation of the C9═O carbonyl are key steps leading to photoproduct formation. HOOP intensity increases on a 3 ps time scale during the formation of I0, signifying distortion about the C7═C8 bond. Once on the I0 surface, the C7═C8 and C1═O stretches blue shift, indicating recovery of charge to the phenolate, while persistent intensity in the HOOP and carbonyl out-of-plane modes reveal HCA to be a cissoid structure with significant distortion about the C7═C8 bond and of C9═O out of the molecular plane.
飞秒受激拉曼光谱（FSRS）用于研究对羟基肉桂酸（HCA）发色团的结构动力学在光激活黄色蛋白（PYP）光循环的前 300 皮秒，因为系统从垂直激发态过渡到早期的基态顺式中间态 I0。光激发后 C7═C8 和 C1═O 伸缩的蓝移表明发色团在激发态中向越来越多的醌式形式转移，这表明电荷从酚部分向 C9═O 羰基转移，这种转移在 170 fs 内继续增加。此外，在 PYP 从激发态过渡到 I0 的 800 fs 时间尺度上，C9═O 羰基的面外振动也发生蓝移，这表明与 Cys69 的氢键减弱和 C9═O 羰基的面外旋转是导致光产物形成的关键步骤。在 I0 的形成过程中，HOOP 强度在 3 ps 时间尺度上增加，这表明 C7═C8 键发生了扭曲。一旦在 I0 表面，C7═C8 和 C1═O 伸缩蓝移，表明电荷恢复到酚盐，而 HOOP 和羰基面外模式的持续强度表明 HCA 是一种顺式结构，C7═C8 键发生了显著扭曲，C9═O 离开分子平面。
