Piotr Filipiak

Head-to-Tail Interactions in Tyrosine/Benzophenone Dyads in the Ground and the Excited State: NMR and Laser Flash Photolysis Studies

The formation of head-to-tail contacts in de novo synthesized benzophenone/tyrosine dyads, bp logical sum Tyr, was probed in the ground and excited triplet state by NMR techniques and laser flash photolysis, respectively. The high affinity of triplet-excited ketones towards phenols was used to trace the geometric demands for high reactivity in the excited state. A retardation effect on the rates with increasing hydrogen-bond-acceptor ability of the solvent is correlated with ground-state masking of the phenol. In a given solvent the efficiencies of the intramolecular hydrogen-atom-transfer reaction depend strongly on the properties of the linker: rate constants for the intramolecular quenching of the triplet state cover the range of 10(5) to 10(8) s(-1). The observed order of reactivity correlates to a) the probability of close contacts (from molecular-dynamics simulations) and b) the extent of the electronic overlap between the pi systems of the donor and acceptor moieties (from NMR). A broad survey of the NMR spectra in nine different solvents showed that head-to-tail interactions between the aromatic moieties of the bp logical sum Tyr dyads already exist in the ground state. Favourable aromatic-aromatic interactions in the ground state appear to correspond to high excited-state reactivity.

Sensitized photooxidation of s-methylglutathione in aqueous solution: intramolecular (S∴O) and (S∴N) bonded species.

Nanosecond laser flash photolysis was used to generate sulfur radical cations of the thioether, S-methylglutathione (S-Me-Glu), via the one-electron oxidation of this thioether by triplet 4-carboxybenzophenone. The purpose of this investigation was to follow the neighboring group effects resulting from the interactions between the sulfur radical cationic sites and nearby lone-pair electrons on heteroatoms within the radical cation, especially the electron lone-pairs on heteroatoms in the peptide bonds. The tripeptide, S-Me-Glu, offers several possible competing neighboring group effects that are characterized in this work. Quantum yields of the various radicals and three-electron bonded (both intramolecular and intermolecular) species were determined. The pH dependence of photoinduced decarboxylation yields was used as evidence for the identification of a nine-membered ring, sulfur-nitrogen, three-electron bonded species. The mechanisms of the secondary reactions of the radicals and radical cations were characterized by resolving their overlapping transient-absorption spectra and following their kinetic behavior. In particular, sulfur-oxygen and sulfur-nitrogen three-electron bonded species were identified where the oxygen and nitrogen atoms were in the peptide bonds.

Photosensitized oxidation of methionine-containing dipeptides. From the transients to the final products.

The Met residue oxidation has been studied for decades. Although many efforts have been made on the identification of free radicals, some doubts remain about their final fates, i.e., the nature of stable oxidation products. The photosensitized oxidation processes of two peptides, methionyl lysine (Met-Lys) and lysyl methionine (Lys-Met), were investigated using 3-carboxybenzophenone (3CB) as a sensitizer. Therefore, not only the transients were characterized but also the final products (by high-performance liquid chromatography and mass spectrometry) together with the quantum yields. As for the transients, the sulfur radical cations stabilized by a two-center three electron bonds with a nitrogen (S.·.N)(+) were identified in the case of Met-Lys. On the other hand, in Lys-Met, the intermolecular (S.·.S)(+) radical cations were found. The peptide-3CB adduct was the only stable product detected and was accompanied neither by sulfoxide formation nor by decarboxylation. It shows that both (S.·.N)(+) and (S.·.S)(+) radicals are converted into the relatively long-lived α-(alkylthio)alkyl radicals, which add to the 3CB-derived radicals. This addition reaction prevented all other oxidation processes such as formation of sulfoxide. The lysine residue was totally protected, which may also be of importance in biological processes.

Efficient Photochemical Oxidation of Anisole in Protic Solvents: Electron Transfer driven by Specific Solvent-Solute Interactions

The dynamics of the bimolecular quenching of triplet excited benzophenone by anisole was studied by nanosecond flash photolysis. We carried out a detailed study of the solvent dependence of the reaction rates and efficiencies in a number of protic and non-protic solvents. These studies were augmented by theoretical modelling and experimental investigation of solute/solvent interactions in the triplet excited and the ground state, respectively. The triplet quenching that follows Stern-Volmer kinetics in all cases is profoundly dependent on the nature of the solvent, with the highest reactivity being consistently found in protic solvents. The results in non-protic solvents are compatible with unproductive quenching via a charge-transfer state, whereas the generally fast quenching in protic solvents is accompanied by efficient formation of free-radical products. Analysis of the solvent dependence in terms of Marcus theory reveals the impact of specific solvation of benzophenone by protic solvents on the ET driving force and kinetics. Specific solvation is found to support efficient free radical ion formation in media of moderate and low polarity as well.

Formation of a Three-Electron Sulfur–Sulfur Bond as a Probe for Interaction between Side Chains of Methionine Residues

The mechanism of oxidation processes of l-Met-(Pro)n-l-Met peptides that contain two Met residues located on the N- and C-terminal and separated by a defined number (n = 0-4) of proline residues was investigated in aqueous solutions using pulse radiolysis. The use of such peptides allowed for distance control between the sulfur atoms located in the side chains of the Met residues. The formation of a contact between the side chains of the Met residues was probed by the observation of transients with σ*-type 2c-3e S∴S and S∴O bonds as well as of α-(alkylthio)alkyl radicals (αS). This approach enabled the monitoring, in real time, of the efficiency and kinetics of interactions between methionine side chains. Such knowledge is important, inter alia, for long-distance electron transfer processes because methionine side chains can serve as relay stations and also for many aspects of protein folding when the formation of a contact between two amino acid residues in an unfolded polypeptide chain plays a central role in protein-folding mechanisms. The yields of these transients (measured as G-values) were found to be dependent on the number of Pro residues; however, they were not dependent in a simple way on the average distance ⟨rS-S⟩ between the sulfur atoms in Met residues. A decrease in the yield of the (S∴S)(+) species with an increase in the number of Pro residues in the bridge occurred at the expense of an increase in the yields of the intramolecular three-electron-bonded (S∴O)(+) radical cations and αS radicals. A detailed understanding of these trends in the chemical yields was developed by modeling the underlying chemical kinetics with Langevin dynamical simulations of the various oligoproline peptide chains and combining them with a simple statistical mechanical theory on the end-to-end contact rates for polymer chains. This analysis showed that the formation of a contact between terminal Met residues in the peptides with 0-2 Pro residues was controlled by the activated formation of (S∴S)(+) whereas in the peptides with 3 and 4 Pro residues, by the relative diffusion of the sulfur radical cation and unoxidized sulfur atom. In this picture, the dynamics of the other radical products can be seen to be only indirectly dependent on the length of the proline bridges because their formation is in competition with (S∴S)(+) formation.

New Insights into the Reaction Paths of 4-Carboxybenzophenone Triplet with Oligopeptides Containing N- and C-Terminal Methionine Residues

The oxidation processes of l-Met-(Pro)n-l-Met peptides that contain two Met residues located on the N and C termini and separated by a defined number (n = 0-4) of proline residues were investigated in aqueous solutions using laser flash photolysis. The use of such peptides allowed for distance control between the sulfur atoms located in the side chains of the Met residues. Interactions between side chains of the Met residues were probed by the observation of transients with σ*-type 2c-3e (S∴S)+, (S∴N)+, and (S∴O)+ bonds as well as of α-(alkylthio)alkyl radicals (αS). This approach enabled the monitoring, in real time, of the efficiency and kinetics of interactions between amino acid chains. Such knowledge is important, inter alia, for long-distance electron-transfer (ET) processes because amino acid side chains can serve as relay stations. The yields of these transients (measured as quantum yields (Φ)) were found to be dependent on the number of Pro residues, however, not dependent in a simple way on the average distance between sulfur atoms in Met residues. A decrease in the yield of the (S∴S)+ species with the number of Pro residues occurred at the expense of an increase in the yields of intramolecular three electron-bonded (S∴O)+/(S∴N)+ radical cations and αS radicals. These observations were rationalized by the fact that the time required for adequate overlap of the bonding orbitals is a key factor effecting the yield of the (S∴S)+ species. The time, however, can be controlled not only by the average distance but also by the specific geometrical and conformational properties of the peptide molecules.

Reactions of hydrogen atoms with α -(alkylthio) carbonyl compounds. Time-resolved ESR detection and DFT calculations

The pulse radiolysis of acidic aqueous solutions of 2-(methylthio)ethanoic acid and S-ethylthioacetone yields acetic acid radicals, ˙CH2COOH, and acetone radicals, ˙CH2-C(O)CH3, respectively. These radicals were identified by time-resolved electron spin resonance. Based on the analogy to previous work on α-(methylthio)acetamide where acetamide radicals were identified in similar situations, the mechanism of the reactions is assigned as bimolecular homolytic substitution (SH2). Support for this assignment is given by density functional theory (DFT) calculations and radical-scavenging experiments with tert-butanol. DFT computations were made on the thermo-chemistry of the SH2 reaction pathway and on the various alternatives to SH2 such as H-abstraction/β-scission.