Femtosecond UV Studies of the Electronic Relaxation Processes in Cytochrome c

Abstract

We report on an experimental study with UV and visible ultrafast time-gated emission and transient absorption of the early photodynamics of horse heart Cytochrome c in both ferric and ferrous redox states. A clear separation in time and energy of tryptophan and haem emission is observed. Excitation of the haem via resonant energy transfer from the tryptophan residue is observed in the subsequent haem electronic relaxation. Different Trp-haem energy transfer time constants of the ferrous and ferric forms are obtained. An almost instantaneous relaxation to the lowest singlet excited state (corresponding to the so-called Q band) characterizes the earliest electronic dynamics of the haem, independent of excitation energy, while dark intermediate states govern the ground-state recovery. The information gathered in these two experiments and in the literature allows us to propose a simple scheme for the electronic relaxation leading to ligand dissociation.

Investigations of heme distortion, low-frequency vibrational excitations, and electron transfer in cytochrome c

“…The heme excited state can undergo nonradiative relaxation via an intermediate state with one electron transferred from the porphyrin π* to the iron centered orbitals (50,51). In the intermediate state, the half-filled π orbital will be delocalized on the porphyrin periphery, allowing for an increased electron transfer rate from the environmental electron donor to the heme.…”

Photo-induced dynamics of the heme centers in cytochrome bc1

“…It is to keep in mind that not all ferrous c 1 -and b-hemes take part in these photo-reactions, and that, while the remaining excited c 1 -hemes exibit relaxation mechanisms that are mainly illustrated by broad ESA signals, the coolingdown of the remaining excited b-hemes do not show any significant ESA. Besides pointing to the fact that the c 1 -and bhemes behave and therefore function differently, these spectral differences call for further studies such as time-resolved fluorescence measurements 16 in order to evaluate the nature of these non-dissociative and non-oxidizing pathways We confirm that the ferric form of b-heme types in the bc 1 complex is electronically inert under our excitation regime, i.e. that it does not undergo photo-reduction to any disernable extent.…”

“…34,35 Correspondingly, we conclude that the short-lived broad ESA monitored for the heme c 1 are also indicative of the heme's CT state. Since the SE, ESA and the formation of a 6-c ferrous c-heme are concomitant, we suggest that an initially excited c-heme, having lost its reduced 6-c character via an impulsive CT from the heme macrocycle to the metal center, 16 gives rise to the SE and ESA, and relaxes within 190 fs into its hot ground state through a non-dissociative pathway. The bleach of the 1.6-ps DADS is slightly broader and redshifted compared to the static c 1 -heme -band.…”

“…11,12 For example, the hemes in cyt c and cyt b 5 , which are structurally comparable to the c 1 -and b-hemes of the bc 1 complex, respectively, 13,14 are known to dissociate from their axial ligand within femtoseconds as a response to light absorption. [14][15][16] While the early steps of the heme photo-cycle still raises questions, it is generally accepted that photoexcitation of the heme is followed by an electron transfer (ET) to the iron d-orbitals, 16 which in turn is believed to trigger the ligand dissociation in ferrous complexes. This photodissociation seems to be a universal feature in cyt's 17 but its relation to the overall functionality of the protein is unclear.…”

Ultrafast dynamics of the photo-excited hemes b and cnin the cytochrome b6f complex

“…However the yield of energy transfer also depends on the overlap between the hemes fluorescence and the Car absorbance. The fluorescence of embedded cyt is expected to be minimal in the region bellow 550 nm 36 with minor overlap with the Car absorption spectrum. Hence the probability of energy transfer from the hemes to Car is assumed to be negligible.…”