A new mechanism of the photochemical oxidation of tryptophan sensitised with the uranyl ion

Article abstract of DOI:10.1016/j.mencom.2009.03.022

It has been found that tryptophan oxidation sensitised with the uranyl ion occurs as a chain reaction of Trp⁺ cation radicals with O₂.

Full text of DOI:10.1016/j.mencom.2009.03.022

Mendeleev
Communications
Mendeleev Commun., 2009, 19, 113–114
A new mechanism of the photochemical oxidation
of tryptophan sensitised with the uranyl ion
Sergey S. Ostakhov, Valerii P. Kazakov* and Irina O. Osina
Institute of Organic Chemistry, Ufa Scientific Center of the Russian Academy of Sciences, 450054 Ufa,
Russian Federation. Fax: +7 3472 356 066; e-mail: chemlum@anrb.ru
DOI: 10.1016/j.mencom.2009.03.022
It has been found that tryptophan oxidation sensitised with the uranyl ion occurs as a chain reaction of Trp + cation radicals with O2.
·
The photooxidation of tryptophan (Trp) used to receive much
attention1 since, along with other aromatic amino acids, such
Trp conversion (mol%)
0
5
10
15
20
25
30
35
as tyrosine, phenylalanine, and cystine, it is one of the main
chromophores determining the photochemical reactions of
proteins. The currently accepted mechanism2 involves ion-radical
photooxidation of Trp in the reaction of its cation radical
40
40
30
1
30
20
+
–
·
·
(Trp ) with the oxygen superoxide ion (O2 ). The main steps
×2000
3
+
·
of tryptophan photooxidation are: formation of a Trp cation
radical and a hydrated electron (eaq) upon photolysis followed
by its reaction with oxygen to give an O2 – superoxide ion. The
reaction of O2 with Trp via an intermediate dioxetane (D)
gives N-formylkynurenin (FK).
20
·
3
2
–
+
·
2
·
10
0
10
0
In an electronically excited state, UO22+ and Trp become
strong oxidising and reducing species, respectively, and can
initiate intracomplex photoinduced electron transfer (PET).4,5
As noted previously,6 the rate of Trp photooxidation increases if
uranyl is selectively excited; this leads to the assumption that a
200
250
300
350
l/nm
400
450 500
Figure 1 Absorption spectra: (1) Trp; (2) UO22+. Plot of f vs. degree of
conversion for Trp photooxidation (3) [c(Trp) = 10–4 mol dm–3, c(UO22+) =
= 10–4 mol dm–3, l = 1 cm, 0.01 M HClO4, 298 K].
chain reaction of Trp + cation radicals with oxygen is possible.7
·
This paper deals with a study of the mechanisms of Trp
protooxidation in complexes with UO22+ upon selective excita-
tion of the latter.†
In the presence of uranyl ion, the j of Trp photooxidation
upon excitation with light Δlexcit. = 240–400 nm increases to
1.5 (j > 1). We failed to strictly fulfill the condition of selectivity
of Trp excitation in the UO22+···Trp complex (Figure 1), since
uranyl partially (35%) absorbs light, and the increase in the
quantum yield is probably due to the simultaneous occurrence
of primary PET photochemical reactions:
By determining the yield of Trp photooxidation, it is possible
to determine, which mechanism, namely, reaction of Trp + with
·
O2 – superoxide ion (j £ 1) or chain reaction of cation radicals
·
with O2 (j > 1), occurs upon selective excitation of the electron
donor (Trp) and acceptor (UO22+). The quantum yield of photo-
oxidation of free Trp [the primary photochemical act is
Trp* ® Trp + + eaq (Δlexcit. = 240–400 nm)] that we determined
·
Trp*···UO22+ ® UO2···Trp ¬® UO2 + Trp
*UO22+···Trp ® UO2···Trp
(1)
(2)
+
+
+
+
·
·
+
+
·
in the substrate conversion range from 5 to 20 mol% is 1.0, in
agreement with other data: j = 0.97 and 1.14 at c(Trp) = 10–3
and 10–2 mol dm–3, respectively.1
On the contrary, the optical excitation selectivity of uranyl
ion is possible, since Trp does not absorb light at lexcit. > 320 nm
(Figure 1). The quantum yield of Trp photooxidation sensitised
with uranyl (lexcit. > 320 nm) is j = 25, i.e., higher than unity,
which indicates that chain processes occur. Like in equation (1), the
cation-radical chain oxidation of Trp photosensitised with uranyl
ion is based on the primary PET process. However, after the
UO2···Trp + ion radical pair has formed, secondary photochemical
reactions occur according to another mechanism (Scheme 1).
†
The quantum yield of Trp photooxidation was determined according
to a published procedure.13 Aerated Trp solutions (0.01 mol dm–3 HClO4)
kept at constant temperature (298 K) were irradiated in a quartz cell
using a DKSSh-1000 xenon lamp (LOS-2 illuminator) through UFS-5
(Δlexcit. = 240–400 nm) and SZS-17 optical filters (lexcit. > 320 nm). The
amount of absorbed light was determined using a ferrioxalate dosimeter
prepared by a reported procedure;14 its quantum yield and fraction of
absorbed light vary insignificantly in the irradiation wavelength range
studied.13 Since wide-band optical filters were used for irradiation rather
than a monochromatic light source, the emission spectrum of the lamp
and the transmittance spectrum of the optical filter were taken into
account in determinations of the absorbed light intensity. The concentra-
tion of Trp that underwent the reaction was monitored spectrophoto-
metrically using a Specord M-40 spectrophotometer. Analytical grade
D-tryptophan was recrystallised from twice-distilled water and dried
in vacuo. Analytical grade UO2(NO3)2·6H2O was used without preliminary
purification. The concentrations of Trp and UO22+ were 10–4 mol dm–3.
+
·
+
+
·
·
The Trp cation radical adds oxygen to give the TrpO2
cation peroxy radical. The TrpO2 peroxy biradical is formed in
the reaction of TrpO2 + with Trp; this reaction also gives a new
·
Trp + cation radical (chain growth). The TrpO2 peroxy biradical
·
undergoes cyclisation to dioxetane D, decomposition of which
results in FK.
The difference between the mechanisms of Trp photooxida-
tion is probably due to the specifics of the formation of the
coordination sphere of UO22+···Trp complexes upon selective
– 113 –
© 2009 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2009, 19, 113–114
oxidation quantum yield in the case of selective Trp excitation
O
O
does not depend on the concentration of the product formed, the
j value increases linearly from 8 to 36 when the Trp conversion
changes from 6 to 30 mol% in the case of uranyl excitation
(Figure 1).
O
O
Trp
Trp + O2
Trp + Trp
Trp
chain growing
Thus, uranyl ion-photosensitised oxidation of tryptophan
NH2
+
NH2
·
involves the chain reaction (Scheme 1) of Trp cation radicals
3
CH2 CH COOH
O
CH2 CH COOH
with O2 rather than a reaction of the substrate cation radical
with O2 – superoxide ion.
·
O
O
O
N
N
H
This study was supported by the Russian Foundation for Basic
Research (project nos. 08-03-00147-a and 08-03-99008-r-ofi),
the Federal Agency for Science and Innovations (project
no. 02.513.12.0050) and the Russian Academy of Sciences
(project no. OKh-01).
H
D
O
NH2
CH2 CH COOH
N
H
O
References
1
I. I. Sapezhinskii, Biopolimery: kinetika radiatsionnykh i fotokhimi-
cheskikh prevrashchenii (Biopolymers: Kinetics of Radiation and
Photochemical Transformations), Nauka, Moscow, 1988, p. 214 (in
Russian).
FK
UO2···Trp
UO22 ···Trp
chain extension
2
3
4
J. Eriksen, C. S. Foot and C. S. Parker, J. Am. Chem. Soc., 1977, 99,
6455.
R. F. Vasil’ev and Yu. B. Tsaplev, Usp. Khim., 2006, 75, 1103 (Russ.
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Gainetdinova and G. Kh. Akhmadeeva, Radiokhimiya, 2006, 48, 399
[Radiochem. (Engl. Transl.), 2006, 48, 447].
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Gainetdinova and G. Kh. Akhmadeeva, Radiokhimiya, 2006, 48, 403
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Trans. 1, 1975, 20, 2055.
Scheme 1
excitation of the metal ion and ligand, which is also inherited
by the UO2···Trp + ion-radical pair. It is well known that uranyl
+
·
forms complexes with amino acids8 and UO22+ excitation is
accompanied by an abrupt increase (by a few orders of magni-
tude) in their stability9 owing to an increase in the contribution
of 5f orbitals to donor–acceptor interactions. For adamantylidene-
adamantane, which does not form complexes with UO22+, uranyl-
photosensitised oxidation10–12 occurs with j = 0.55 < 1, which
rules out the possibility of chain processes. Hence, adaman-
tylideneadamantane-1,2-dioxetane is formed in the reaction of
5
6
7
8
9
the substrate cation radical with O2 – superoxide ion.
·
G. M. Sergeev and I. A. Korshunov, Radiokhimiya, 1973, 15, 621 (in
Russian).
S. S. Ostakhov, V. P. Kazakov, D. D. Afonichev and V. V. Rikova, Radio-
khimiya, 1995, 37, 503 (in Russian).
It can be assumed that, during selective intracomplex Trp···UO22+
photoexcitation of the amino acid [equation (1)], oxidation occurs
+
+
·
in the solvate-separated ion-radical pair UO2 + Trp and the
uranoyl ion is oxidised to UO22+ due to the transfer of an electron
10 G. L. Sharipov, V. P. Kazakov and G. A. Tolstikov, Khimiya i khemi-
luminestsentsiya 1,2-dioksetanov (Chemistry and Chemiluminescence
of 1,2-Dioxetanes), Nauka, Moscow, 1990, p. 287 (in Russian).
11 A. I. Voloshin, G. L. Sharipov, V. P. Kazakov and G. A. Tolstikov, Izv.
Akad. Nauk SSSR, Ser. Khim., 1987, 2837 (Bull. Acad. Sci. USSR, Div.
Chem. Sci., 1987, 36, 2634).
12 V. P. Kazakov, S. S. Ostakhov, A. S. Alyab’ev and I. O. Osina, Khim.
Vys. Energ., 2006, 40, 291 [High Energy Chem. (Engl. Transl.), 2006,
40, 248].
13 M. Ya. Mel’nikov and V. L. Ivanov, Eksperimental’nye metody khimicheskoi
kinetiki. Fotokhimiya (Research Methods of Chemical Kinetics. Photo-
chemistry), Izd. MGU, Moscow, 2004 (in Russian).
14 C. A. Parker, Photoluminescence of Solutions, Elsevier, Amsterdam–
London–New York, 1968, p. 198.
–
+
·
·
to oxygen to give O2 , which reacts with Trp to give FK. On
the contrary, uranyl ion-photosensitised [equation (2)] cation-
radical chain oxidation of Trp (Scheme 1) occurs in an intra-
complex way and FK remains in the coordination sphere of UO22+,
as follows from the shifts in the excitation and fluorescence
spectra.6
However, both mechanisms of Trp photooxidation in the
presence of the uranyl ion are not mutually exclusive. In case of
cooperative photoexcitation of Trp and UO22+ (lexcit. > 220 nm),
j = 2.7 [by an order smaller than j (*UO22+) but two times
higher than j (*Trp)], which reflects the competition of the two
oxidation processes.
The above j values for Trp photooxidation were obtained at
20 mol% amino acid conversion. However, while the photo-
Received: 9th July 2008; Com. 08/3172
– 114 –