374
A. Penzkofer et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 369–375
10−5
xIA
s
−1, the rate constant of DMIA− and MIA− formation is kIA− =
mass-spectra of Fig. S2 (top part and mass spectra belonging to
tret = 4.93 min and tret = 5.03 min).
(R5):
−
,∞kLF− ≈ 2.5 × 10−6
s
−1 (xIA
−
= 0.039), and the rate constant
,∞
of quinoxaline derivatives formation is kQOs = kLF− − kIA− ≈ 6 ×
10−5 s−1
.
LF + 4H2ON−a→OHQO3 + urea + CO2 + 2CH3OH.
(R5)
5. Conclusions
(see Fig. S2 with tret = 2.83 min). The absorption of QO3 is below
300 nm and its fluorescence is seen in a peak around 305 nm.
The hydrolysis product QO4 is obtained by the reaction process
(R6):
The degradation of lumiflavin in aqueous 1 M, 2 M, and
4 M NaOH at room temperature in the dark was investi-
gated by absorption, fluorescence, and mass spectroscopy.
The hydrolysis of LF− into the quinoxaline derivatives 1,2-
LF + 2H2ON−a→OHQO4 + urea + CO.
(R6)
dihydro-2-keto-1,6,7-trimethyl-quinoxaline-3-carboxylic
acid
(see Fig. S2 with tret = 5.03 min and m/z = 205.0 for [QO4+H]+).
The absorption spectrum and fluorescence spectrum of QO4 is
thought to coincide with those of QO1 because of the same chro-
mophore part.
The formation of the degradation products, i, may be described
by
QO1, 2-methoxy-6,7-dimethyl-quinoxaline-3-carboxylic acid QO2,
methyl-quinoxaline-2-ol QO3, and 3-hydroxy-1,6,7-trimethyl-
1H-quinoxaline-2-one QO4 and into the isoalloxazine derivatives
7,8-dimethyl-isoalloxazine DMIA and methyl-isoalloxazine MIA
was revealed. The rate of degradation increased with NaOH (or
OH−) concentration. The time constant of hydrolysis was found
to be 4.4 h for LF in 4 M NaOH, 13.5 h for LF in 2 M NaOH, 39 h for
LF in 1 M NaOH, and it is expected to be ≥400 h for LF in aqueous
solution at pH 13 (0.1 M NaOH).
ꢅ
dNLF
dt
−
= −
kiNLF− = −kLF− NLF− ,
(2)
i
dNi
dt
= kiNLF− ,
(3)
Acknowledgements
where NLF− is the number density of LF−, and Ni is the number
density of product i.
The authors thank the Deutsche Forschungsgemeinschaft (DFG)
for support in the Graduate College GK 640 “Sensory photorecep-
tors in natural and artificial systems” and in the Research Group
FOR 526 “Sensory Blue Light Receptors”. A.P. is grateful to Profs. F.J.
Gießibl and J. Repp for their kind hospitality.
The solutions are
NLF− = N0 exp(−kLF− t),
(4)
(5)
ki
Ni =
N0[1 − exp(−kLF− t)],
−
kLF
Appendix A. Supplementary data
where N0 is the initial number density of LF−.
Supplementary data associated with this article can be found, in
The degradation rate constant kLF− is obtained from the initial
exponential absorption decrease in Fig. 3. The determined val-
ues are kLF− = 7.2 × 10−6 s−1 for LF in 1 M NaOH, 2.1 × 10−5 s−1
for LF in 2 M NaOH, and 6.3 × 10−5 s−1 for LF in 4 M NaOH. The
constant kLF− increases with NaOH concentration as expected in
a NaOH catalyzed reaction (transition state formation with NaOH
involvement and subsequent NaOH release; see Michaelis–Menten
kinetics [38]). The over-proportional increase of kLF− with NaOH
concentration indicates some transition state activation barrier
decrease with NaOH concentration. The transition state activation
energy, EA, may be estimated by the Arrhenius equation [39–42]:
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ꢀ
ꢁ
kBϑ
h
EA
kLF− ≈
exp
−
(6)
kBϑ
where kB is the Boltzmann constant, ϑ is the temperature, and h is
the Planck constant. Using kLF− = 6.3 × 10−5
s
−1 for LF in 4 M NaOH
a value of EA ≈ 8000 cm−1 is estimated. The barrier height differ-
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kLF
−
−
[NaOH]1 exp(−(EA,1/kBϑ))
[NaOH]2 exp(−(EA,2/kBϑ))
,NaOH1
,NaOH2
=
kLF
ꢀ
ꢁ
EA,2 − EA,1
B ϑ
[NaOH]1
[NaOH]2
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exp
,
(7)
k
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where the rate constant of degradation is set proportional to
the NaOH concentration. The obtained transition state barrier
height differences are EA(1 M NaOH) − EA(2 M NaOH) ≈ 77 cm−1
,
and EA(1 M NaOH) − EA(4 M NaOH) ≈ 160 cm−1
.
The rate constant of LF− degradation is composed of the rate
constants of formation of the hydrolysis products, i.e. kLF− =
kQO1 + kQO2 + kQO3 + kQO4 + kDMIA− + kMIA− = kQOs + kIA− . For LF in
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