Juan Rodriguez

Ultrafast vibrational dynamics of a photoexcited metalloporphyrin

The ultrafast photodynamics of four‐coordinate nickel(II) porphyrins in noncoordinating solvents has been studied using femtosecond time resolved optical spectroscopy. Unambiguous evidence has been found for the formation of a metastable metal (d,d) excited state in less than 350 fs following excitation of the macrocycle. However, the transient absorption spectrum of this ligand‐field electronic excited state continues to evolve and reaches the steady‐state form only after about 20 ps. This spectral behavior and the attendant complex kinetics can be modeled phenomenologically in terms of a broad distribution of the (d,d) excited states evolving to a narrower distribution. The dynamics are associated with vibrational relaxation. Intramolecular and intermolecular contributions to this process are considered.

Time‐resolved and static optical properties of vibrationally excited porphyrins

The effects of nuclear motion on the ground state and excited state optical spectra of porphyrins are examined in a number of experiments designed to generate excess vibrational energy within the macrocycle. These include time‐resolved spectroscopic measurements following ultrafast radiationless transitions, and static measurements in the gas and in the condensed phase at various temperatures. The excess vibrational energy generated by highly exothermic radiationless transitions is found to induce significant red shifts in both the ground state absorption and excited state emission features. As the excess vibrational energy is dissipated on the time scale of about 10 ps, the optical features blue shift to their steady‐state spectral positions. The red shifts found in the time‐resolved spectra are also observed in the ground state absorption spectra of porphyrins in the gas phase at high temperature. We consider various mechanisms for the spectral shifts, including vibrationally induced reduction of the el...

Ultrafast photodissociation of a metalloporphyrin in the condensed phase

The photodissociation of axial ligands from six‐coordinate nickel(II) porphyrins in solution has been studied by transient absorption spectroscopy with subpicosecond resolution. Our observations indicate that the deligation process takes place during the first several picoseconds following optical excitation of the macrocycle as the initially observed 3(π,π*) state relaxes to a dissociative ligand field excited state. No evidence is found for geminate recombination of the two ejected ligands with the four‐coordinate photoproduct. A small fraction of the complexes appear to loose only one ligand. The absorption bands of the initially observed excited state and of the deligated species are also found to undergo a spectral evolution within the first 10 ps similar to that uncovered recently in the photodynamics of four‐coordinate nickel(II) porphyrins in noncoordinating solvents [J. Rodriguez and D. Holten, J. Chem. Phys. 91, 3525 (1989)]. Several possibilities are considered for these time‐dependent spectral shifts, including vibrational dynamics.The photodissociation of axial ligands from six‐coordinate nickel(II) porphyrins in solution has been studied by transient absorption spectroscopy with subpicosecond resolution. Our observations indicate that the deligation process takes place during the first several picoseconds following optical excitation of the macrocycle as the initially observed 3(π,π*) state relaxes to a dissociative ligand field excited state. No evidence is found for geminate recombination of the two ejected ligands with the four‐coordinate photoproduct. A small fraction of the complexes appear to loose only one ligand. The absorption bands of the initially observed excited state and of the deligated species are also found to undergo a spectral evolution within the first 10 ps similar to that uncovered recently in the photodynamics of four‐coordinate nickel(II) porphyrins in noncoordinating solvents [J. Rodriguez and D. Holten, J. Chem. Phys. 91, 3525 (1989)]. Several possibilities are considered for these time‐dependent spectral...

Electronic states and (π, π*) absorption and emission characteristics of strongly coupled porphyrin dimers: sandwich complexes of HfIV and ZrIV

Abstract Time-resolved and steady state optical measurements are reported for the sandwhich complexes consisting of two tetraphenylporphyrin macrocycles held within 3 A by either a Zr IV or Hf IV ion. Compared to monoporphyrins and weakly coupled bisporphyrins, the transition-metal sandwich complexes show new features in both the ground and excited state absorption spectra as well as a broad fluorescence red-shifted to ≈ 1 μm. A model is presented that describes the electronic states responsible for these optical features. The states arise from strong interactions between the porphyrin macrocycles and contain significant charge resonance character.

Elucidation of the role of metal-to-ring charge-transfer excited states in the deactivation of photoexcited ruthenium porphyrin carbonyl complexes

Abstract Deactivation of the lowest excited triplet state, 3 (π, π*), of the Ru(II) porphyrins RuP(CO)(L) is more strongly dependent on temperature than decay of 3 (π, π*) in Pt(II)P and H 2 P (metal-free) complexes containing the same macrocycle P. This and other observations support the proposal that 3 (π, π*) in the RuP(CO)(L) complexes decays in part via a metal-to-ring (d, π*) charge-transfer excited state at higher energy.

The solvent in CNBr cleavage reactions determines the fragmentation efficiency of ketosteroid isomerase fusion proteins used in the production of recombinant peptides

Abnormal fragmentation during cyanogen bromide polypeptide cleavage rarely occurs, although parallel side reactions are known to typically accompany normal cleavage. We have observed that cyanogen bromide cleavage of highly hydrophobic fusion proteins utilized for production of recombinant peptides results in almost complete abolishment of the expected reaction products when the reaction is carried out in 70% trifluoroacetic acid. On the basis of mass spectrometric analysis of the reaction products, we have identified a number of fragments whose origin can be attributed to incomplete fragmentation of the fusion protein, and to unspecific degradation affecting the carrier protein. Substituting the solvent in the reaction media with 70% formic acid or with a matrix composed of 6M guanidinium hydrochloride in 0.1M HCl, however, was found to alleviate polypeptide cleavage. We have attributed the poor yields of the CNBr cleavage carried out in 70% TFA to the increased hydrophobicity of our particular fusion proteins, and to the poor solubilizing ability of this reaction medium. We propose the utilization of chaotropic agents in the presence of diluted acids as the preferred cyanogen bromide cleavage medium of fusion proteins in order to maximize cleavage efficiency of hydrophobic sequences and to prevent deleterious degradation and structural modifications of the target peptides.

8:The Dual Role of Heme as Cofactor and Substrate in the Biosynthesis of Carbon Monoxide

Carbon monoxide (CO) is a ubiquitous molecule in the atmosphere. The metabolism of mammalian, plastidic, and bacterial cells also produces CO as a byproduct of the catalytic cycle of heme degradation carried out by the enzyme heme oxygenase (HO). The biological role of CO spans the range from toxic to cytoprotective, depending on concentration. CO generated by the catalytic activity of HO is now known to function in several important physiological processes, including vasodilation, apoptosis, inflammation, and possibly neurotransmission. Consequently, understanding the details of the reaction that leads to the formation of this important gaseous molecule from heme has become an important aspect in the study of the chemistry and biochemistry of HO, which utilizes heme in the dual capacity of substrate and cofactor. In this chapter, a summary, and when appropriate, discussion of the current understanding of the structural, dynamical, and reactive properties that allow HO to breakdown heme into iron, biliverdin, and CO is presented.

Quinone Substituted Porphyrin Dimers: New Photosynthetic Model Systems

The 1988 Nobel Prize in Chemistry was awarded to Johann Deisenhofer, Hartmut Michel, and Robert Huber for their elucidation of the X-ray crystal structure of the reaction center (RC) from the photosynthetic bacterium Rhodopseudomonas viridis(Deisenhofer et al. 1984). More recently, structural information for the RC of Rhodobacter sphaeroideshas also become available (Allen et al., 1986;Chang et al., 1986). Six tetrapyrrolic subunits are found at the active sites of these two structurally similar RCs: A dimeric bacteriochlorophyll “special pair” (P), two “accessory” bacteriochlorophylls (Bchls), and two bacteriopheophytins (Bphs), all held in a well-defined but skewed geometry along a C2 axis of symmetry. The Bchls are separated from P by center-to-center distances of ca. 11 A and interplane angles of ca. 70°. The Bphs in turn are separated by similar distances and angles from the Bchls. Four of these six prosthetic groups are currently considered to define the relevant electron transport chain (Kirmaier and Holten, 1987). This consists in sequence of the photosensitizer (P), an “accessory” Bchl, an intermediate Bph, and a quinone acceptor (Q). In R. sphaeroides, Q is an ubiquinone; in R. viridis, it is a menaquinone. In both cases, Q lies roughly 13-14 A away from the corresponding Bph center. Charge separation between P* and Bph entities is known to occur on a time scale of 2-4 ps with nearly 100% quantum efficiency (Woodbury et al., 1985;Martin et al., 1986;Wasielewski and Tiede, 1986;Kirmaier and Holten, 1988). Furthermore this process exhibits activationless behavior, increasing in rate by a factor of two at liquid helium temperature (Woodbury et al., 1985;Fleming et al., 1988). Subsequent electron transfer from Bph- to Q to give P+-Bchl-Bph-Q-occurs in 200 ps, also with 100% quantum yield (Kirmaier and Holten, 1987).