Antioxidant capacity of (+)-catechin visible-light photoirradiated in the presence of vitamin B₂

Figures & data

Scheme 1. Possible pathways involved in the photosensitized oxidation of CTQ, CTC, and RSC. S represents the photosensitizer and dCTQ denotes generically to the substrates CTQ, CTC, and RSC.

Scheme 2. Chemical structures of CTQ, CTC, RSC, RB, and Rf.

Figure 1. Spectral changes of 0.4 mM CTQ + 0.02 mM Rf vs. 0.02 mM Rf upon visible-light photoirradiation, in buffer pH 7 (main figure) and in buffer pH 9 (insert). Numbers on the spectra represent irradiation time, in seconds.

Figure 2. Spectral changes of 0.7 mM RSC + 0.02 mM Rf vs. 0.02 mM Rf, upon visible-light photoirradiation. Insert: spectral changes for 0.4 mM CTC + 0.02 mM Rf vs. 0.02 mM Rf, upon visible-light photoirradiation. Solvent: buffer pH 7. Numbers on the spectra represent irradiation time, in seconds.

Figure 3. Profiles of oxygen consumption, upon Rf-sensitized photoirradiation (a) CTC, (b) CTQ, and (c) RSC, in buffer pH 7 (main figure) and pH 9 (insert). [dCTQ] = 0.4 mM.

Table 1 Relative rates of oxygen consumption by CTQ, CTC, and RSC, sensitized by RB (A_556 nm = 0.6) and 0.02 mM Rf. Solvents: buffer pH 7 and buffer pH 9.

	v-O2 rel RB pH = 7	v-O2 rel Rf pH = 7	v-O2 rel RB pH = 9	v-O2 rel Rf pH = 9
CTQ	1.00	0.49	1.00	0.75
CTC	0.26	0.09	0.33	0.16
RSC	0.68	1.00	0.45	1.00

Figure 4. Stern–Volmer plot for the quenching of O₂ -phosphorescence emission by dCTQ: (a) CTQ, (b) CTC, and (c) RSC, upon RB-sensitized photoirradiation, in D₂O pD 7. Inset: profiles of oxygen consumption, in buffer pH 7, upon Rf-sensitized photoirradiation (a) 0.02 mM Rf + 0.4 mM CTQ; (b) 0.02 mM Rf + 0.4 mM CTQ + 1 mg/100 ml SOD; (c) 0.02 mM Rf + 0.4 mM CTQ + 10 mM NaN₃; (d) 0.02 mM Rf + 0.4 mM CTQ + 1 mg/100 ml CAT; (e) 0.02 mM Rf + 0.4 mM CTQ + 1.0 mM D-Mannitol (1.0 mM).

Table 2 Rate constants for the reactive (k_r) and overall (k_t) quenching of singlet oxygen by CTQ, CTC, and RSC; k_r/k_t ratios; rate constants for the quenching of electronically excited singlet state of Rf (¹Kg); rate constants for the quenching of electronically excited triplet state of Rf (¹Kg) × 2. Solvents: buffer pH 7 and buffer pH 9. k_t values were determined in buffer pD 7.

	kt (×10^−7M^−1 s^−1) RB pD = 7	kr (×10^−7 M^−1 s^−1) RB pH = 7	kr/kt RB pH = 7	kr (×10^−7 M^−1 s^−1) RB pH = 9	^1kq (×10^−7 M^−1 s^−1) Rf pH = 7	(×10^−7 M^−1 s^−1) Rf pH = 7
CTQ	8.2 ± 0.8	1.3 ± 0.1	0.16	16 ± 2	236 ± 24	312 ± 31
CTC	7.3 ± 0.7	0.7 ± 0.1	0.10	8.1 ± 0.8	254 ± 25	830 ± 83
RSC	2.1 ± 0.2	0.8 ± 0.1	0.03	8.0 ± 0.8	256 ± 25	358 ± 36