Ultrafast Relaxation of Zinc Protoporphyrin Encapsulated within Apomyoglobin in Buffer Solutions
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- 作者：Liyang Luo ; Chin-Hao Chang ; Yue-Ching Chen ; Tung-Kung Wu ; Eric Wei-Guang Diau
- 刊名：Journal of Physical Chemistry B
- 出版年：2007
- 出版时间：July 5, 2007
- 年：2007
- 卷：111
- 期：26
- 页码：7656 - 7664
- 全文大小：392K
- 年卷期：v.111,no.26(July 5, 2007)
- ISSN：1520-5207
文摘
The relaxation dynamics of a zinc protoporphyrin (ZnPP) in THF, KPi buffer, and encapsulated withinapomyoglobin (apoMb) was investigated in its excited state using femtosecond fluorescence up-conversionspectroscopy with S2 excitation (
ex = 430 nm). The S2 
S1 internal conversion of ZnPP is ultrafast (
<100 fs), and the hot S1 ZnPP species are produced promptly after excitation. The relaxation dynamics ofZnPP in THF solution showed a dominant offset component ( = 2.0 ns), but it disappeared completelywhen ZnPP formed aggregates in KPi buffer solution. When ZnPP was reconstituted into the heme pocket ofapoMb to form a complex in KPi buffer solution, the fluorescence transients exhibited a biphasic decayfeature with the signal approaching an asymptotic offset: at em = 600 nm, the rapid component decayed in710 fs and the slow one in 27 ps; at em = 680 nm, the two time constants were 950 fs and 40 ps. Weconclude that (1) the fast-decay component pertains to an efficient transfer of energy from the hot S1 ZnPPspecies to apoMb through a dative bond between zinc and proximal histidine of the protein; (2) the slow-decay component arises from the water-induced vibrational relaxation of the hot S1 ZnPP species; and (3) theoffset component is due to S1 T1 intersystem crossing of the surviving cold S1 ZnPP species. The transferof energy through bonds might lead the dative bond to break, which explains our observation of the degradationof ZnPP-Mb samples in UV-vis and CD spectra upon protracted excitation.
ex = 430 nm). The S2 S1 internal conversion of ZnPP is ultrafast ( <100 fs), and the hot S1 ZnPP species are produced promptly after excitation. The relaxation dynamics ofZnPP in THF solution showed a dominant offset component ( = 2.0 ns), but it disappeared completelywhen ZnPP formed aggregates in KPi buffer solution. When ZnPP was reconstituted into the heme pocket ofapoMb to form a complex in KPi buffer solution, the fluorescence transients exhibited a biphasic decayfeature with the signal approaching an asymptotic offset: at em = 600 nm, the rapid component decayed in710 fs and the slow one in 27 ps; at em = 680 nm, the two time constants were 950 fs and 40 ps. Weconclude that (1) the fast-decay component pertains to an efficient transfer of energy from the hot S1 ZnPPspecies to apoMb through a dative bond between zinc and proximal histidine of the protein; (2) the slow-decay component arises from the water-induced vibrational relaxation of the hot S1 ZnPP species; and (3) theoffset component is due to S1 T1 intersystem crossing of the surviving cold S1 ZnPP species. The transferof energy through bonds might lead the dative bond to break, which explains our observation of the degradationof ZnPP-Mb samples in UV-vis and CD spectra upon protracted excitation.