Kinetics and mechanism of the photochemical reaction of 2,2 '-dipyridyl with tryptophan in water: Time-resolved CIDNP and laser flash photolysis study

The mechanism of the reactions between photoexcited 2,2′-dipyridyl and N-acetyl tryptophan has been studied by laser flash photolysis and time-resolved CIDNP (Chemically Induced Dynamic Nuclear Polarization). The transient absorption spectra obtained at different delays after the laser pulse are attributed to the triplet state of dipyridyl and to dipyridyl and tryptophan radicals. Depending on the pH of the solution, all three intermediates can be present in either protonated or deprotonated forms. It is shown that irrespective of pH the primary photochemical step is electron transfer from the tryptophan to triplet dipyridyl followed by protonation/deprotonation of the radicals so formed. The rate constant of the reaction of triplet dipyridyl with tryptophan is close to the diffusion-controlled limit and decreases slightly with increasing pH. The kinetics and the stationary value of the CIDNP are determined by the rates of radical termination, nuclear paramagnetic relaxation, and degenerate electron exchange. The last reaction is important for the protonated tryptophan radical and determines the CIDNP kinetics of tryptophan in acidic conditions. The nuclear relaxation times estimated from the CIDNP kinetics are 44 ± 9 μs for all protons in the dipyridyl radical, 91 ± 20 μs for the β-CH2, 44 ± 9 μs for H2,6, and 63 ± 12 μs for H4 aromatic protons in the tryptophan radical

Mechanisms of reactions of flavin mononucleotide triplet with aromatic amino acids.

Chemical reactions between the photoexcited triplet state of flavin mononucleotide and the aromatic amino acids, N-acetyl tryptophan (TrpH), N-acetyl tyrosine (TyrOH), and N-acetyl histidine (HisH) in aqueous solution have been studied in the pH range 2-12. Across the whole pH range, the principal mechanism of reaction of both TrpH and TyrOH is shown to be electron transfer. For HisH, the mechanism and rate of the reaction depend on the protonation state of the reactants. In acidic conditions (pH < 4), reaction does not occur. At 4 < pH < 11, the reaction proceeds via hydrogen atom abstraction with a rate constant varying from 3.0 x 10(6) to 2.5 x 10(8) M(-1) s(-1). In extremely basic solution (pH > 12) the mechanism switches to electron transfer

Time-resolved CIDNP and laser flash photolysis study of the photoreactions of N-acetyl histidine with 2,2 '-dipyridyl in aqueous solution

The reaction mechanism and details of the formation of CIDNP (chemically induced dynamic nuclear polarization) in the photoreactions of 2,2′-dipyridyl (DP) and N-acetyl histidine (HisH) in aqueous solution have been studied using laser flash photolysis and time-resolved CIDNP techniques. The triplet state TDP reacts with protonated HisH 2+ via hydrogen atom transfer with a rate constant k H = 1.2 × 10 8 M -1 s -1, and with deprotonated His - via electron transfer with k e = 7.5 × 10 9 M -1 s -1. No reaction occurs when the histidine imidazole ring is in its neutral state HisH, or when the dipyridyl triplet is protonated, TDPH +. The nuclear spin-lattice relaxation times in the radicals formed in these reactions have been determined from the CIDNP kinetics: T 1 = 44 ± 9 μs for all DP protons, T 1 = 196 ± 25 μs for the β-CH 2 protons of HisH, and T 1 = 16 ± 5 μs for the H-2 and H-4 protons of HisH. Under strongly basic conditions the CIDNP is greatly affected by degenerate electron exchange between the neutral His • radical and His - anion, with rate constant k ex = 1.5 × 10 8 M -1 S -1. © 2000 American Chemical Society

Time resolved CIDNP study of electron transfer reactions in proteins and model compounds

Intramolecular electron transfer (IET) from tyrosine to tryptophan cation radicals is investigated using time resolved chemically induced dynamic nuclear polarization (CIDNP) spectroscopy in combination with laser flash photolysis. In both the tryptophan-tyrosine dipeptide and the denatured state of hen lysozyme in aqueous solution, the transformation TrpH+. → TyrO. by IET leads to an increase in the tyrosine radical concentration, growth in the tyrosine CIDNP signal, fast decay of the tryptophan CIDNP, and inversion of the phase of the CIDNP of the photosensitizing dye, 2,2′-dipyridyl. IET effects are not observed for mixtures of the amino acid or for the native state of lysozyme. The steady state CIDNP effects seen for denatured lysozyme thus depend not only on the accessibility of the amino acid residues on the surface of the protein but also on the reactivity of the radical intermediates

Effects of surfactants on the photosensitized production of tyrosine radicals studied by photo-CIDNP.

The influence of the surfactants sodium dodecyl sulphate, cetyltrimethyl-ammonium bromide and triton X-100 on the photochemically induced dynamic nuclear polarization (CIDNP) of N-acetyl tyrosine has been investigated. Three photosensitizers were used to generate polarization: thionin, eosin Y and flavin mononucleotide. 600 MHz 1H photo-CIDNP experiments, supported by laser flash photolysis transient absorption measurements, indicate that the neutral triton surfactant has no influence on the nuclear polarization, but that the other two, charged, amphiphiles affect the photochemistry in a variety of ways, depending on the surfactant concentration and the identity of the sensitizer

A previously unknown way of heme detoxification in the digestive tract of cats

Free heme is a highly toxic molecule for a living organism and its detoxification is a very important process, especially for carnivorous animals. Here we report the discovery of a previously unknown process for neutralizing free heme in the digestive tract of domestic cats. The cornerstone of this process is the encapsulation of heme into carbonated hydroxyapatite nanoparticles, followed by their excretion with faeces. This way of heme neutralization resembles the formation of insoluble heme-containing particles in the digestive tracts of other hematophagous species (for example, the formation of insoluble hemozoin crystals in malaria-causing Plasmodium parasites). Our findings suggest that the encapsulation of heme molecules into a hydroxyapatite matrix occurs during the transition from the acidic gastric juice to the small intestine with neutral conditions. The formation of these particles and their efficiency to include heme depends on the bone content in a cat’s diet. In vitro experiments with heme-hydroxyapatite nanoparticles confirm the proposed scenario

Visualising the membrane viscosity of porcine eye lens cells using molecular rotors

The plasma membranes of cells within the eye lens play an important role in metabolite transport within the avascular tissue of the lens, maintaining its transparency over the entire lifespan of an individual. Here we use viscosity-sensitive ‘molecular rotors’ to map the microscopic viscosity within these unusual cell membranes, establishing that they are characterised by an unprecedentedly high degree of lipid organisation

Visualising the membrane viscosity of porcine eye lens cells using molecular rotors

The plasma membranes of cells within the eye lens play an important role in metabolite transport within the avascular tissue of the lens, maintaining its transparency over the entire lifespan of an individual. Here we use viscosity-sensitive ‘molecular rotors’ to map the microscopic viscosity within these unusual cell membranes, establishing that they are characterised by an unprecedentedly high degree of lipid organisation