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.
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Abbreviations
- O 2(1Δg):
-
singlet molecular oxygen
- AA :
-
amino-acid
- TPE :
-
tripeptide
- gly-tyr-ala :
-
glycyl-L-tyrosine-alanine
- tyr-gly :
-
L-tyrosyl-glycine
- FFA :
-
furfuryl alcohol
- tyr :
-
L-tyrosine
- gly-tyr :
-
glycyl-L-tyrosine
- DPE :
-
dipeptide
- NaN 3 :
-
sodium azide
- RB :
-
rose bengal
- ZnTPP :
-
Zinc tetraphenyl porphyrine
- MeCN :
-
acetonitrile
- DMA :
-
9,10-dimethyl anthracene
- etOH :
-
ethanol
- TRPD :
-
time-resolved phosphorescence detection
- NH 2 loss:
-
loss of primary amine reactivity
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Miskosky, S., Bertolotti, S.G., García, N.A. et al. On the O2(1Δ g )-mediated photooxidative behaviour of tripeptide glycyl-tyrosyl-alanine in alkaline medium A kinetic study. Amino Acids 4, 101–110 (1993). https://doi.org/10.1007/BF00805805
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DOI: https://doi.org/10.1007/BF00805805