Spectral evidence for sub-picosecond iron displacement after ligand detachment from hemoproteins by femtosecond light pulses.

Martin, Jean−Louis; Poyart, Claire; Lecarpentier, Yves; Astier, R.; Antonetti, A.

doi:10.1002/j.1460-2075.1983.tb01663.x

Cited by 30 publications

(17 citation statements)

References 22 publications

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“…However, it is widely recognized that the CO and NO serve as excellent probes of protein dynamics on time scales from picoseconds to seconds. Studies of heme photophysics (Cornelius et al;, Martin et al, 1983bPetrich et al, 1987) are key to understanding the nature of the barrier to ligand rebinding (Franzen et al, 1995b;Walda et al, 1994) and the mechanism of photolysis of diatomic -acid ligands (Hoffman and Gibson, 1978).…”

Section: Introductionmentioning

confidence: 99%

Heme Photolysis Occurs by Ultrafast Excited State Metal-to-Ring Charge Transfer

Franzen¹,

Kiger²,

Poyart³

et al. 2001

Biophysical Journal

120

176

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Ultrafast time-resolved resonance Raman spectra of carbonmonoxy hemoglobin (Hb), nitroxy Hb, and deoxy Hb are compared to determine excited state decay mechanisms for both ligated and unligated hemes. Transient absorption and Raman data provide evidence for a sequential photophysical relaxation pathway common to both ligated and unligated forms of Hb* (photolyzed heme), in which the excited state 1Q decays sequentially: 1Q-->Hb*I-->Hb*II-->Hb ground state. Consistent with the observed kinetics, the lifetimes of these states are <50 fs, approximately 300 fs, and approximately 3 ps for 1Q, Hb*I, and Hb*II, respectively. The transient absorption data support the hypothesis that the Hb*I state results from an ultrafast iron-to-porphyrin ring charge transfer process. The Hb*II state arises from porphyrin ring-to-iron back charge transfer to produce a porphyrin ground state configuration a nonequilibrium iron d-orbital population. Equatorial d-pi* back-bonding of the heme iron to the porphyrin during the lifetime of the Hb*II state accounts for the time-resolved resonance Raman shifts on the approximately 3 ps time scale. The proposed photophysical pathway suggests that iron-to-ring charge transfer is the key event in the mechanism of photolysis of diatomic ligands following a porphyrin ring pi-pi* transition.

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Section: Introductionmentioning

confidence: 99%

Heme Photolysis Occurs by Ultrafast Excited State Metal-to-Ring Charge Transfer

Franzen¹,

Kiger²,

Poyart³

et al. 2001

Biophysical Journal

120

176

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“…However, the use of an excitation at 610 nm, which could directly excite one of the possible initial acceptors (bacteriochlorophyll), made it difficult to reach definite conclusions. In the present work, the earliest events in the charge separation process occurring in the reaction center of Rhodopseudomonas sphaeroides R-26 have been investigated by using a femtosecond spectroscopic technique, which associates a 100-fsec time resolution with tunability of the excitation in the near-infrared region (12,13).…”

mentioning

confidence: 99%

Femtosecond spectroscopy of electron transfer in the reaction center of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26: Direct electron transfer from the dimeric bacteriochlorophyll primary donor to the bacteriopheophytin acceptor with a time constant of 2.8 ± 0.2 psec

Martin

Breton²,

Hoff

et al. 1986

Proc. Natl. Acad. Sci. U.S.A.

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370

184

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The primary light-induced charge separation in reaction centers from Rhodopseudomonas sphaeroides R-26 has been investigated after excitation with laser pulses of 150 fsec duration within the longwave absorption band of the primary donor at 850 nm. An excited state of the primary donor, characterized by a broad absorption spectrum extending over the whole spectral range investigated (545-1240 nm), appeared within 100 fsec and gave rise to stimulated emission in the 870-to 1000-nm region with a 2.8-psec lifetime. The photooxidation of the primary donor, as measured at 1240 nm, and the photoreduction of the bacteriopheophytin acceptor, monitored at 545 nm and 675 nm, have been found to proceed simultaneously with a time constant of 2.8 ± 0.2 psec. Kinetics of absorbance changes at other probe wavelengths gave no indication that an accessory bacteriochlorophyll is involved as a transient electron acceptor.The mechanism by which light is converted into chemical free energy involves a sequence of very fast events between the absorption of a photon by the antenna pigments and the completion of the primary charge separation in the chlorophyll-protein complex, called the reaction center. Picosecond spectroscopy on reaction centers isolated from photosynthetic bacteria has revealed the presence of shortlived intermediates in this electron transfer process (1-11). However, the first steps take place in a time shorter than the duration of the pulses generally used and therefore have not been kinetically resolved. A previous study using subpicosecond pulses suggests that an electron has been transferred in -4 psec (5). However, the use of an excitation at 610 nm, which could directly excite one of the possible initial acceptors (bacteriochlorophyll), made it difficult to reach definite conclusions. In the present work, the earliest events in the charge separation process occurring in the reaction center of Rhodopseudomonas sphaeroides R-26 have been investigated by using a femtosecond spectroscopic technique, which associates a 100-fsec time resolution with tunability of the excitation in the near-infrared region (12, 13).Reaction centers from photosynthetic bacteria can be isolated in a functionally intact state and contain three polypeptides, four bacteriochlorophylls, two bacteriopheophytins, two quinones, and one non-heme iron atom (14). In the case of the reaction center from R. sphaeroides R-26, the main absorption bands of the pigments are located at 865, 800, 760, 590, and 540 nm. The 865-nm band, which bleaches upon (photo)oxidation of the reaction center, is ascribed to the primary donor (P). ENDOR data (15,16) indicate that in oxidized P (P+) the hole left by the removal of the electron is delocalized over a dimer of bacteriochlorophylls. Concomitant with the (photo)oxidation of P, the 800-nm band undergoes a change, which appears to be largely a blue shift. This shift is currently assigned to an electrochromic effect of P+ on the Qy transition of the two other bacteriochlorophyll molecules (called B) w...

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“…(1983) do not assign the Hb* states to either a particular electronic or structural conformation of the porphyrin. They note, however, that similar excited states with identical lifetimes have been observed after photoexcitation of the Hb deoxy solutions (Martin et al, 1983b). This suggests that the Hb* absorption bands are due to deligated Hb species.…”

Section: Protein Dynamicsmentioning

confidence: 61%