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  • 1
    Electronic Resource
    Electronic Resource
    Influence of the pH on the photodynamic effect in lysozyme A comparative kinetic study with the sensitized photooxidation of isolated amino acids (1995)
    Springer
    Amino acids 9 (1995), S. 123-134 
    Details
    ISSN: 1438-2199
    Keywords: Amino acids ; Eosin ; Lysozyme ; Photodynamic effect ; Photooxidation ; Singlet oxygen
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The kinetics of the eosin-sensitized photooxidation ([O2(1Δg)]-mediated) of the protein lysozyme (Lyso) was investigated under two different pH conditions (pH 7 and pH 11). Rates of oxygen consumption and the fade in the protein fluorescence spectrum upon sensitized irradiation were monitored. Parallel studies on both denatured Lyso (absence of the four-S-S- bridges in the protein) and different mixtures of the photooxidizable amino acids of Lyso were also carried out. The mixtures maintained the same molar ratio as in the native protein, and were selected just in order to throw into relief the preferential amino acids that were being photooxidized at both pH values. Under work conditions Lyso was only photooxidizable at pH 7, whereas the opposite accounted for the denatured protein: only measurable oxygen consumption was detected at pH 11. Nevertheless, Lyso at pH 11, evidenced an important physical quenching of O2(1Δg) due to the Tyr and Trp residues. The results for the native protein were interpreted on the basis of a previously described dark complex Eosin-Lyso, which selectively favours the photooxidation of the bounded protein. The Trp residues were the main reactive entities in the native protein. The photodinamic effect in denatured Lyso was characterized by the prevalence of Tyr residues as photooxidizable targets. In the discussion of the results, a comparisson with the photooxidation kinetics of the mixtures of free amino acids was made.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00805833
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  • 2
    Electronic Resource
    Electronic Resource
    On the O2(1Δ g )-mediated photooxidative behaviour of tripeptide glycyl-tyrosyl-alanine in alkaline medium A kinetic study (1993)
    Springer
    Amino acids 4 (1993), S. 101-110 
    Details
    ISSN: 1438-2199
    Keywords: Amino acids ; Singlet-oxygen ; Peptides ; Photooxidation ; Kinetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The type II singlet molecular oxygen [O2(1Δ g )]-mediated photo-oxidation of the tripeptide gly-tyr-ala was studied. It has two non-oxidizable amino-acids (gly and ala) bonded to the oxidizable one, tyr. Overall (k t) and reactive (k r) rate constants for the interaction were determined by time-resolved methods (IR emission of O2(1Δ g )) and stationary photolysis, in water at pH 11.5 as well as in alkaline non-aqueous etOH-MeCN (80:20, v/v, 10 mM in KOH) solutions. An important solvent polarity effect onk t was detected; the rate constant increasing one order of magnitude in going from the organic mixture to water (k t H2O = 2 × 109 M−1 s−1). Nevertheless,k r does not parallel this trend; gly-tyr-ala being less photooxidizable in a more polar environment. The effective quantum yield (∅ r ) forTPE photooxidation is much higher in etOH-MeCN (∅ r = 0.056) than in water (∅ r = 0.023). Results are discussed on the basis of the formation of an exciplex with polar character between the TPE and O2(1Δ g ). Two remarkable points should be taken into account: a) the rate costants for the interaction of O2(1Δ g ) with gly-tyr-ala are practically the same as for free tyr. b) New -NH2 groups are generated upon sensitized irradiation. Both findings indicate that the peptide bonds in the TPE break as a result of the photooxidation. A thorough analysis with data for tyrosine and related dipeptides is undertaken.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00805805
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  • 3
    Electronic Resource
    Electronic Resource
    Influence of the nuclear and extranuclear substitution on the singlet molecular oxygen [O 2(1Δg)]-mediated photooxidation of tyrosine derivatives: A kinetic study (1995)
    Springer
    Amino acids 8 (1995), S. 367-377 
    Details
    ISSN: 1438-2199
    Keywords: Amino acids ; Tyrosine ; Tyrosine derivatives ; Singlet oxygen ; Photooxidation ; Kinetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The effect of the substitution pattern on the kinetics of the Type II (O2(1Δg)-mediated) dye-sensitized photooxidation of a series of nine tyrosine derivatives was investigated. Overall (kt) and reactive (kr) rate constants for the interaction of the excited oxygen species with the amino acid derivatives were determined. A parallel study on solvent and pH effects was carried out. The presence of different substituents in nuclear positions or in the amino acid side chain greatly affect the photooxidation rates. An upper limit for photooxidation quantum yield, calculated from the kinetic data, varies from 0.03 to 0.25, being the higher for halogenated tyrosines and the lower for esterified tyrosines and for the nitro-derivative. The variation of solvent polarity and pH of the reaction medium confirm that the presence of the ionized phenolate group in tyrosine, clearly dominates the quenching process. As already postulated for generic phenolic derivatives, it proceeds through a polar intermediate complex which posses some component of charge-transfer character. Esterification of the carboxilic acid of tyrosine selectively decreases the contribution of the reactive step to the overall process of O2(1Δg) quenching. An amide group in the same position does not produce noticiable changes in this sense. The presence of a highly deactivating nitro group in nuclear positions greatly diminishes the magnitude of both overall and reactive interactions. For all three, o-, m- and p-tyrosine the values of photooxidation quantum yields show an excellent parallelism with the rates of consumption of the — NH2 group of the amino acid chain, upon sensitized irradiation. It could react, in the cases of 0- and m-tyrosine in a secondary, non photochemical, step.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00806554
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