Ultrafast relaxation of zinc protoporphyrin encapsulated within apomyoglobin in buffer solutions

Abstract

The relaxation dynamics of a zinc protoporphyrin (ZnPP) in THF, KPi buffer, and encapsulated within apomyoglobin (apoMb) was investigated in its excited state using femtosecond fluorescence up-conversion spectroscopy with S-2 excitation (lambda(ex) = 430 nm). The S-2 -> S-1 internal conversion of ZnPP is ultrafast (tau < 100 fs), and the hot S-1 ZnPP species are produced promptly after excitation. The relaxation dynamics of ZnPP in THF solution showed a dominant offset component (tau = 2.0 ns), but it disappeared completely when ZnPP formed aggregates in KPi buffer solution. When ZnPP was reconstituted into the heme pocket of apoMb to form a complex in KPi buffer solution, the fluorescence transients exhibited a biphasic decay feature with the signal approaching an asymptotic offset: at lambda(em) = 600 nm, the rapid component decayed in 710 fs and the slow one in 27 ps; at lambda(em) = 680 nm, the two time constants were 950 fs and 40 ps. We conclude that (1) the fast-decay component pertains to an efficient transfer of energy from the hot S-1 ZnPP species 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 S-1 ZnPP species; and (3) the offset component is due to S-1 -> T-1 intersystem crossing of the surviving cold S-1 ZnPP species. The transfer of energy through bonds might lead the dative bond to break, which explains our observation of the degradation of ZnPP-Mb samples in UV-vis and CD spectra upon protracted excitation.