Pyrroloquinoline quinone (PQQ) is reduced to pyrroloquinoline quinol (PQQH₂) by vitamin C, and PQQH₂ produced is recycled to PQQ by air oxidation in buffer solution at pH 7.4

Figures & data

Fig. 1. Molecular structures of PQQNa₂, PQQH₂, vitamin C (ascorbic acid, AsH₂), and ascorbate monoanion (AsH⁻).

Fig. 2. The UV–vis absorption spectra of PQQNa₂ (–––), PQQH₂ (·····), and Vit C (AsH⁻) (- - -) with the same concentrations of 4.45 × 10⁻⁵ M in 0.05 M phosphate-buffered solution (pH 7.4) at 25.0°.

Note: The spectra of PQQNa₂ and PQQH₂ were reported in a previous study (see Fig. 2 in Ref. [26]).

Table 1. Values of UV–vis absorption maxima (${\mathit{\lambda }}_{max}^i$) and molar extinction coefficients (${\mathit{\epsilon }}_{max}^i$) for PQQNa₂, PQQH₂, and Vit C in 0.05 M phosphate-buffered solution (pH 7.4).

	${\mathit{\lambda }}_{max}^1$/nm (ε^1/M^−1 cm^−1)	${\mathit{\lambda }}_{max}^2$/nm (ε^2/M^−1 cm^−1)	${\mathit{\lambda }}_{max}^3$/nm (ε^3/M^−1 cm^−1)	${\mathit{\lambda }}_{max}^4$/nm (ε^4/M^−1 cm^−1)	${\mathit{\lambda }}_{max}^5$/nm (ε^5/M^−1 cm^−1)	${\mathit{\lambda }}_{max}^6$/nm (ε^6/M^−1 cm^−1)
PQQNa2	307.5 sh^a^b (10,800)	304.0 sh^b (11,100)	249^c (26,600)	267 sh^c (20,500)	331^c (12,700)	477^c (692)
PQQH2	307.5 sh^b (37,700)	304.0^c (40,000)	340 sh^c (11,500)	405 sh^c (2410)	499^c (1170)
Vit C	307.5 sh (82.6)	304.0 sh (193)	266.0 (15,200)

^a sh: shoulder.

^b The λ_max and ε_max values of PQQNa₂ and PQQH₂ at 307.5 and 304.0 nm were estimated from the UV–vis absorption spectra shown in Fig. 2.

^c The λ_max and ε_max values of PQQNa₂ and PQQH₂ in 0.05 M phosphate-buffered solution (pH 7.0) were reported in a previous study (Ref. [26]).

Fig. 3. Changes in the absorption spectrum of PQQNa₂ and PQQH₂ during the reaction of PQQNa₂ with Vit C in 0.05 M phosphate-buffered solution (pH 7.4) at 25.0 °C.

Notes: [PQQNa₂]_t = 0 = 4.45 × 10⁻⁵ M. [Vit C]_t = 0 = 5.36 × 10⁻⁴ M. The spectra were recorded at (A) 6-min interval and (B) 3-h interval. The downward arrow indicates a decrease (↓) (PQQNa₂), and an upward arrow indicates an increase (↑) (PQQH₂) in the absorbance with time.

Fig. 4. Formation of PQQH₂ due to the reaction of PQQNa₂ with Vit C, and decay of PQQH₂ in the presence of air.

Notes: (A) Time dependence of the absorbance of PQQH₂ at 307.5 nm, after the reduction of PQQNa₂ with Vit C in 0.05 M phosphate buffer (pH 7.4) at 25.0 °C under nitrogen atmosphere (○). [PQQNa₂]_t = 0 = 4.45 × 10⁻⁵ M and [Vit C]_t = 0 = 5.36 × 10⁻⁴ M. (B) PQQH₂ solution produced due to the reaction of PQQNa₂ with Vit C was exposed to air, after keeping the solution under nitrogen atmosphere for 4.5 day. A decrease in the absorbance of PQQH₂ at 307.5 nm was observed in the presence of air (○).

Fig. 5. Formation of PQQH₂ due to the reaction of PQQNa₂ with Vit C, and decay of PQQH₂ in the presence of air.

Notes: (A) Time dependence of the absorbance of PQQH₂ at 307.5 nm, after the reduction of PQQNa₂ with Vit C in 0.05 M phosphate buffer (pH 7.4) at 25.0 °C under nitrogen atmosphere. [PQQNa₂]_t = 0 = 4.45 × 10⁻⁵ M. [Vit C]_t = 0 = 5.36 × 10⁻⁴ M (○) and 5.36 × 10⁻³ M (●). (B) PQQH₂ solution produced by the reaction of PQQNa₂ with Vit C was exposed to air, after keeping the solution under nitrogen atmosphere for 4.5 day. A decrease in the absorbance of PQQH₂ at 307.5 nm was observed in the presence of air.

Fig. 6. PQQH₂ solution produced due to the reaction of PQQNa₂ ([PQQNa₂]_t = 0 = 4.45 × 10⁻⁵ M) with Vit C ([Vit C]_t = 0 = 5.36 × 10⁻⁴ M) was exposed to air, after keeping the solution under nitrogen atmosphere for 4.5 day.

Notes: The spectra were recorded at 36-min time intervals. A downward arrow indicates a decrease (↓) (PQQH₂), and an upward arrow indicates an increase (↑) (PQQNa₂) in the absorbance with time.

Fig. 7. Recycling of PQQH₂ and PQQ under the coexistence of Vit C.

Table 2. Changes in the absorbance of PQQH₂ at 307.5 nm due to the reaction of PQQNa₂ with vitamin C in the buffer solution (pH 7.4) at 25.0 °C under nitrogen atmosphere.

	at t = 0 min		at t = 15 h
	$\begin{array}{c}{\text{Abs}}_{\text{initial}}^{\text{total}}\\ ={\text{Abs}}_{\text{initial}}^{{\text{PQQNa}}_2}+{\text{Abs}}_{\text{initial}}^{\text{Vit C}}\\ =\mathit{\epsilon }[{\text{PQQNa}}_2]+{\mathit{\epsilon }}^{\text{Vit C}}[\text{Vit C}]\end{array}$	${\text{Abs}}_{\text{initial}}^{\text{total}}$	$\begin{array}{c}{\text{Abs}}_{\text{final}}^{\text{total}}\\ ={\text{Abs}}_{\text{final}}^{{\text{PQQH}}_2}+{\text{Abs}}_{\text{final}}^{\text{Vit C}}\\ ={\mathit{\epsilon }}^{{\text{PQQH}}_2}[{\text{PQQH}}_2]+{\mathit{\epsilon }}^{\text{Vit C}}[\text{Vit C}]\end{array}$	${\text{Abs}}_{\text{final}}^{\text{total}}$
Reaction 5 (low conc. of Vit C)	= 10,800[4.45 × 10^−5] + 82.6[5.36 × 10^−4]	0.525 (calcd.)	= 37,700[4.45 × 10^−5]+82.6[4.47 × 10^-4]	1.715 (calcd.)
	= 0.481 + 0.044		=1.678 + 0.037
	=0.500^a + 0.044	0.544 (obsd.)	=1.423^b + 0.037	1.460 (obsd.)
			$\begin{array}{c}\text{ratio}\\ ={\text{Abs}}_{\text{final}}^{{\text{PQQH}}_2}(\text{obsd}.)/{\text{Abs}}_{\text{final}}^{{\text{PQQH}}_2}(\text{calcd}.)\\ =1.423/1.678=0.848\end{array}$
	at t = 0 min		at t = 15 h
Reaction 5 (high conc. of Vit C)	=10,800[4.45 × 10^−5] + 82.6[5.36 × 10^-3]	0.924 (calcd.)	=37,700[4.45 × 10^−5] + 82.6[5.27 × 10^−3]	2.120 (calcd.)
	=0.481 + 0.443		=1.678 + 0.435
	=0.935^a + 0.443	1.378 (obsd.)	=1.867^b + 0.435	2.302 (obsd.)
			$\begin{array}{c}\text{ratio}\\ ={\text{Abs}}_{\text{final}}^{{\text{PQQH}}_2}(\text{obsd}.)/{\text{Abs}}_{\text{final}}^{{\text{PQQH}}_2}(\text{calcd}.)\\ =1.867/1.678=1.11\end{array}$

^a ${\text{Abs}}_{\text{initial}}^{{\text{PQQH}}_2}$ (obsd.) value was obtained by subtracting the contribution of ${\text{Abs}}_{\text{initial}}^{\text{Vit C}}$ (calcd.) from ${\text{Abs}}_{\text{initial}}^{\text{total}}$ (obsd.).

^b ${\text{Abs}}_{\text{final}}^{{\text{PQQH}}_2}$ (obsd.) value was obtained by subtracting the contribution of ${\text{Abs}}_{\text{final}}^{\text{Vit C}}$ (calcd.) from ${\text{Abs}}_{\text{final}}^{\text{total}}$ (obsd.).

Table 3. Changes in the absorbance of PQQH₂ at 307.5 nm due to the reaction of PQQH₂ with air (molecular oxygen) in the buffer solution (pH 7.4) at 25.0 °C.

	at t = 0 min	at t = 15 h
	${\text{Abs}}_{\text{initial}}^{\text{total}}$	$\begin{array}{c}{\text{Abs}}_{\text{final}}^{\text{total}}\\ ={\text{Abs}}_{\text{final}}^{{\text{PQQNa}}_2}+{\text{Abs}}_{\text{final}}^{\text{Vit C}}\\ ={\mathit{\epsilon }}^{{\text{PQQNa}}_2}[{\text{PQQNa}}_2]+{\mathit{\epsilon }}^{\text{Vit C}}[\text{Vit C}]\end{array}$	${\text{Abs}}_{\text{final}}^{\text{total}}$
Reaction 5 (low conc. of Vit C)		=10,800[4.45 × 10^−5] + 82.6[4.47 × 10^−4]	0.518 (calcd.)
		= 0.481 + 0.037
	1.420 (obsd.)	=0.507^a + 0.037	0.544 (obsd.)
		$\begin{array}{c}\text{ratio}\\ ={\text{Abs}}_{\text{final}}^{{\text{PQQNa}}_2}(\text{obsd}.)/{\text{Abs}}_{\text{final}}^{{\text{PQQNa}}_2}(\text{calcd}.)\\ =0.507/0.481=1.05\end{array}$
	at t = 0 min	at t = 38 h
Reaction 5 (high conc. of Vit C)		=10,800[4.45 × 10^−5] + 82.6[5.27 × 10^−3]	0.916 (calcd.)
		=0.481 + 0.435
	2.305 (obsd.)	=1.030^a + 0.435	1.465 (obsd.)
		$\begin{array}{c}\text{ratio}\\ ={\text{Abs}}_{\text{final}}^{{\text{PQQNa}}_2}(\text{obsd}.)/{\text{Abs}}_{\text{final}}^{{\text{PQQNa}}_2}(\text{calcd}.)\\ =1.030/0.481=2.14\end{array}$

^a ${\text{Abs}}_{\text{final}}^{{\text{PQQH}}_2}$ (obsd.) value was obtained by subtracting the contribution of ${\text{Abs}}_{\text{final}}^{\text{Vit C}}$ (calcd.) from ${\text{Abs}}_{\text{final}}^{\text{total}}$ (obsd.).

Ouchi A, Nakano M, Nagaoka S, Mukai K. Kinetic study of the antioxidant activity of pyrroloquinolinequinol (PQQH2, a reduced form of pyrroloquinolinequinone) in micellar solution. J. Agric. Food Chem. 2009;57:450–456.10.1021/jf802197d

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