Photolysis ofHbCO, MbCO, and CO-protoheme has been investigated by measuring transient differential spectra and kinetics of induced absorption after excitation with. a 250-fsec laser pulse at 307 nm. Probing was performed by a part of a continuum pulse between 395 and 445 nm. Photodissociation of the three liganded species occurred within the pulse duration. By contrast, the formation ofdeoxy species appeared with a mean (± SD) response time of 350 ± 50 fsec. This time constant was identical for the three species and independent of the presence or absence of the protein structure. Our results suggest the formation of a transient high-spin in plane iron (II) species which relaxes in 350 fsec to a high-spin stable state with concerted kinetics of CO departure and iron displacement. The spin transition is suspected to occur via liganded excited states which relax in part to nonreactive states with a 3.2-psec time constant.Picosecond spectroscopy has revealed that the dissociation of ligand from the heme in Mb and Hb occurs within 4 psec and that the nonliganded species (Mb or Hb) were formed with a time constant ofill psec (1, 2). This has been confirmed recently by Cornelius et al. (3) who observed the appearance of Mb species after photodissociation of MbO2 with a 12-psec time constant; their results on MbCO showed a "simultaneous development of both bleaching and absorption intensities." From results of studies with 0.5-psec optical pulses at 615 nm in a pump probe experiment, Shank et al. (4) deduced that the photolysis of CO from HbCO should occur in less than 0.5 psec.More recently, Greene et al (5) and Chernoff et al. (6), using 8-psec excitation pulses at 353 and 530 nm, obtained transient absorption spectra in the Soret and visible regions at 10 and 680 psec which indicated that deoxy species formed in a time shorter than their-pulse duration. Terner et al. (7) In the present work, we have limited ourselves to the investigation of the kinetics of formation of the nonliganded species after dissociation of CO from MbCO and HbCO. In order to appreciate the possible influence of polypeptidic environment on the heme reactivity in these hemoproteins, we have also studied the kinetics ofCO dissociation from protoheme and ofdeoxyprotoheme formation. These experiments were carried out with a newly developed spectroscopic technique operating with laser pulses of250 fsec duration to excite the hemes at 307 nm. MATERIAL AND METHODS Preparation ofthe Hemoproteins and Protoheme Solutions.Purified human adult hemoglobin was prepared from fresh human blood by DEAE-Sephadex chromatography (10). The experimental solution was 0.1 mM on the basis of heme; it was diluted in 0.1 M K phosphate buffer at pH 7 or 8. The solution was first deoxygenated under moist argon before equilibration with CO at 1 atm (1 atm = 1.013 x 105 pascals). MetMb (horse heart type III, Sigma) was converted to MbFe2' after addition of a 5 molar excess of freshly prepared Na dithionite (Merck) under strict anaerobic conditions and then equil...

Complete triplet—triplet absorption spectra (2000–10000 Å) were measured in alcoholic solution at 113°K with naphthalene, anthracene, tetracene, and a few methylated derivatives. Several new bands were observed. Calculations of the higher ππ* triplet states energies with the LCAO—MO—SCF Pariser—Parr—Pople method were improved by the use of large configuration interactions. These include many doubly excited configurations with respect to both the ground singlet state S0 and the lowest triplet state T1, thanks to the systematic use of all monoexcitations with respect to S0 plus all monoexcitations with respect to T1. The comparison of all experimental and theoretical results for the three polyacenes allows one to modify a few previous assignments and to give a general tentative assignment for all observed transitions.

We have measured spectral and kinetic differences in protoheme, sperm whale or horse heart myoglobin and human hemoglobin following photodissociation induced by optical pulses of 80 fs duration. Full ligation was performed with oxygen or carbon monoxide. Femtosecond kinetics and transient difference spectra revealed the appearance of a deoxy species with tau approximately equal to 250‐300 fs. The transient deoxy species in myoglobin and hemoglobin evidenced a 3‐4 nm red shift of their delta A spectra compared with the equilibrium delta A spectrum. This shift was not observed after photodissociation of the carbon monoxide liganded protoheme. We proposed that the 250 fs time constant corresponding to the appearance of the deoxy‐like species is related to the displacement of the ferrous iron out of the heme plane. Consequently, the small red shift of the delta A spectra observed in photodissociated hemoproteins may be tentatively attributed to changes in the vibrational modes of either the proximal histidine‐Fe2+ bond and/or of the N4 porph‐Fe‐N epsilon His (F8) bent.