Effect of Amino Group Charge on the Photooxidation Kinetics of Aromatic Amino Acids

Abstract

The kinetics of the photooxidation of aromatic amino acids histidine (His), tyrosine (Tyr), and tryptophan (Trp) by 3,3',4,4'-benzophenonetetracarboxylic acid (TCBP) has been investigated in aqueous solutions using time-resolved laser flash photolysis and time-resolved chemically induced dynamic nuclear polarization. The pH dependence of quenching rate constants is measured within a large pH range. The chemical reactivities of free His, Trp, and Tyr and of their acetylated derivatives, N-AcHis, N-AcTyr, and N-AcTrp, toward TCBP triplets are compared to reveal the influence of amino group charge on the oxidation of aromatic amino acids. The bimolecular rate constants of quenching reactions between the triplet-excited TCBP in the fully deprotonated state and tryptophan, histidine, and tyrosine with a positively charged amino group are kq = 2.2 × 10(9) M(-1) s(-1) (4.9 < pH < 9.4), kq = 1.6 × 10(9) M(-1) s(-1) (6.0 < pH < 9.2), and kq = 1.5 × 10(9) M(-1) s(-1) (4.9 < pH < 9.0), respectively. Tryptophan, histidine, and tyrosine with a neutral amino group quench the TCBP triplets with the corresponding rate constants kq = 8.0 × 10(8) M(-1) s(-1) (pH > 9.4), kq = 3.0 × 10(8) M(-1) s(-1) (pH > 9.2), and kq = (4.0-10.0) × 10(8) M(-1) s(-1) (9.0 < pH < 10.1) that are close to those for the N-acetylated derivatives. Thus, it has been established that the presence of charged amino group changes oxidation rates by a significant factor; i.e., His with a positively charged amino group quenches the TCBP triplets 5 times more effectively than N-AcHis and His with a neutral amino group. The efficiency of quenching reaction between the TCBP triplets and Tyr and Trp with a positively charged amino group is about 3 times as high as that of both Tyr and Trp with a neutral amino group, N-AcTyr and N-AcTrp.

Toward MRI and Optical Detection of Zwitterionic Neurotransmitters: Near-Infrared Luminescent and Magnetic Properties of Macrocyclic Lanthanide(III) Complexes Appended with a Crown Ether and a Benzophenone Chromophore

“…49 Photoreduction of BP and its derivatives by the H-abstraction process has been studied in the presence of various hydrogen-containing substrates, including aliphatic hydrocarbons, alcohols and ethers, [50][51][52][53] nucleic acids, 54-55 lipids 47 and amino acids. [56][57][58] These studies have been mainly conducted in non-aqueous media and the determined rate constants fall in the range 10 4 to 10 9 M -1 s -1 , while Porter has reported that protons can quench the phosphorescence of BP in aqueous solution with a rate constant of 6•10 8 M -1 s -1 . 59…”

Oxidation of Purine Nucleotides by Triplet 3,3′,4,4′-Benzophenone Tetracarboxylic Acid in Aqueous Solution: pH-Dependence

“…The pH dependent protonation states of reactive species together with the acidity constants employed in calculations are listed in Tables 2 and 1, correspondingly. The equilibrium concentrations of the protonation forms of TCBP were calculated using two mean ground-state acidity constants p K a1 = 2.9 and p K a2 = 4.9 for H 2 O, 13 and p K a1 *=2.7 and p K a2 *=4.8 for D 2 O.…”

Time‐Resolved Chemically Induced Dynamic Nuclear Polarization of Biologically Important Molecules

“…[74] Third, capabilities of CIDNP can be extended by designing novel pulse sequences aimed at polarization transfer to target nuclei of choice and at fast 2D-NMR detection. Fourth, progress can be achieved in designing efficient photo-sensitizers [53] with the aim to detect biomolecules at very low concentrations. Last but not least, to 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 probe the structure of short-lived radicals one can study not only 1 H-CIDNP, but also CIDNP of hetero-nuclei, such as 13 C or 15 N. Such studies can be done either by direct detection of polarization of hetero-nuclei [55a,56] or by polarization transfer to protons followed by 1 H-NMR detection [30a] .…”

“…As photosensitizers, which are used to initiate a photo‐reaction giving rise to CIDNP, various dyes are used, which are listed in Scheme . These dyes are 2,2‐dipyridyl (DP),– 4‐carboxy‐benzophenone (4‐CBP),,, 3,3′,4,4′‐tetracarboxybenzophenone (TCBP),, and flavins with different substituents (flavin mononucleotide, FMN, is shown in Scheme ) ,,,. For running experiments under special conditions, i. e., at very low concentrations, one can also use other dyes, such as fluorescein .…”

“…[43a,47a,c,52] For running experiments under special conditions, i. e., at very low concentrations, one can also use other dyes, such as fluorescein. [53] After photo-excitation by a short laser pulse, these dyes go to the excited triplet state; subsequently, electron transfer or H-transfer gives rise to the formation of an RP in the triplet state. This RP comprises the radical of a dye and the radical of the biomolecule under study.…”