Löwenich Dennis, Kleinermanns Karl, Karunakaran Venugopal, Kovalenko Sergey Alexander
Institute for Physical Chemistry, Heinrich-Heine-University, Düsseldorf, Germany.
Photochem Photobiol. 2008 Jan-Feb;84(1):193-201. doi: 10.1111/j.1751-1097.2007.00219.x.
Photoreduction of cytochrome c (Cyt c) has been reinvestigated using femtosecond-to-nanosecond transient absorption and stationary spectroscopy. Femtosecond spectra of oxidized Cyt c, recorded in the probe range 270-1000 nm, demonstrate similar evolution upon 266 or 403 nm excitation: an ultrafast 0.3 ps internal conversion followed by a 4 ps vibrational cooling. Late transient spectra after 20 ps, from the cold ground-state chromophore, reveal a small but measurable signal from reduced Cyt c. The yield phi for Fe3+-->Fe2+ photoreduction is measured to be phi(403) = 0.016 and phi(266) = 0.08 for 403 and 266 nm excitation. These yields lead to a guess of the barrier E(f)(A) = 55 kJ mol(-1) for thermal ground-state electron transfer (ET). Nanosecond spectra initially show the typical absorption from reduced Cyt c and then exhibit temperature-dependent sub-microsecond decays (0.5 micros at 297 K), corresponding to a barrier E(A)(b) = 33 kJ mol(-1) for the back ET reaction and a reaction energy DeltaE = 22 kJ mol(-1). The nanosecond transients do not decay to zero on a second time scale, demonstrating the stability of some of the reduced Cyt c. The yields calculated from this stable reduced form agree with quasistationary reduction yields. Modest heating of Cyt c leads to its efficient thermal reduction as demonstrated by differential stationary absorption spectroscopy. In summary, our results point to ultrafast internal conversion of oxidized Cyt c upon UV or visible excitation, followed by Fe-porphyrin reduction due to thermal ground-state ET as the prevailing mechanism.
利用飞秒至纳秒瞬态吸收光谱和稳态光谱对细胞色素c（Cyt c）的光还原过程进行了重新研究。在270 - 1000 nm探测范围内记录的氧化态Cyt c的飞秒光谱表明，在266 nm或403 nm激发下具有相似的演化过程：超快的0.3 ps内转换，随后是4 ps的振动冷却。20 ps后来自冷基态发色团的晚期瞬态光谱显示出还原态Cyt c的一个小但可测量的信号。对于403 nm和266 nm激发，Fe³⁺→Fe²⁺光还原的产率φ分别测得为φ(403) = 0.016和φ(266) = 0.08。这些产率使得我们猜测热基态电子转移（ET）的势垒E(f)(A) = 55 kJ mol⁻¹。纳秒光谱最初显示出还原态Cyt c的典型吸收，然后呈现出温度依赖的亚微秒衰减（297 K时为0.5 μs），这对应于反向ET反应的势垒E(A)(b) = 33 kJ mol⁻¹和反应能ΔE = 22 kJ mol⁻¹。纳秒瞬态在第二个时间尺度上不会衰减到零，这表明一些还原态Cyt c具有稳定性。从这种稳定的还原形式计算出的产率与准稳态还原产率一致。差分稳态吸收光谱表明，适度加热Cyt c会导致其有效的热还原。总之，我们的结果表明，氧化态Cyt c在紫外或可见光激发后发生超快内转换，随后由于热基态ET导致铁卟啉还原是主要机制。
