Model complexes for the active form of galactose oxidase. Physicochemical properties of Cu(II)- and Zn(II)-phenoxyl radical complexes

Article abstract of DOI:10.1021/ic9910211

Physicochemical properties of the Cu(II)- and Zn(II)-phenoxyl radical complexes of ligand 1H and its 2,4-di-tert-butyl derivative 2H have been examined as models for the active form of galactose oxidase to shed light on the role of the thioether group in

Full text of DOI:10.1021/ic9910211

3708
Inorg. Chem. 2000, 39, 3708-3711
Chart 1
Model Complexes for the Active Form of
Galactose Oxidase. Physicochemical Properties of
Cu(II)- and Zn(II)-Phenoxyl Radical Complexes
Shinobu Itoh,*,† Masayasu Taki,‡ Hideyuki Kumei,‡
Shigehisa Takayama,‡ Shigenori Nagatomo,§
Teizo Kitagawa,*,§ Norio Sakurada,|
Ryuichi Arakawa,*,| and Shunichi Fukuzumi*,‡
Department of Chemistry, Graduate School of Science,
Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku,
Osaka 558-8585, Japan, Department of Material and Life
Science, Graduate School of Engineering, Osaka University,
CREST, Japan Science and Technology Corporation, 2-1
Yamada-oka, Suita, Osaka 565-0871, Japan, Institute for
Molecular Science, Myodaiji, Okazaki 444-8585, Japan, and
Department of Applied Chemistry, Faculty of Engineering,
Kansai University, 3-3-35 Yamate-cho, Suita,
Recent model studies on GAO have provided detailed insights
into the physicochemical properties of the phenolate and the
phenoxyl radical states of the cofactor both in the metal-free
form and in the metal complexes.7-15 Efficient catalytic reactions
for the alcohol oxidation have also been developed by mimick-
ing the enzymatic functions in model systems.10,11,13 However,
few examples have been reported that can reproduce both the
spectroscopic characteristics and the chemical functions of the
active form of GAO (fully oxidized state). Moreover, little
attention has so far been focused on the electronic effects of
the thioether group of the cofactor on the physicochemical
properties and the reactivity of the Cu(II)-phenoxyl radical
species in model systems. In this context, we recently developed
a model complex for the active form of GAO using a cofactor-
containing ligand 1H,16 with which we have successfully
demonstrated the important role of the copper ion in the efficient
two-electron oxidation of alcohols.17 In this paper, we report
detailed characterizations of the Cu(II)- and Zn(II)-phenoxyl
Osaka 564-8680, Japan
ReceiVed August 24, 1999
Introduction
Protein radicals have now been well-recognized to play a
crucial role in several biologically important redox processes.1
Galactose oxidase (GAO, EC 1.1.3.9) is one of the most well-
characterized examples of such systems, where a tyrosyl radical
directly coordinated to the Cu(II) center is the active species in
the aerobic oxidation of D-galactose and primary alcohols to
the corresponding aldehydes (eq 1).2-4 The crystal structure of
(7) (a) Itoh, S.; Hirano, K.; Furuta, A.; Komatsu, M.; Ohshiro, Y.; Ishida,
A.; Takamuku, S.; Kohzuma, T.; Nakamura, N.; Suzuki, S. Chem.
Lett. 1993, 2099. (b) Itoh, S.; Takayama, S.; Arakawa, R.; Furuta, A.;
Komatsu, M.; Ishida, A.; Takamuku, S.; Fukuzumi, S. Inorg. Chem.
1997, 36, 1407.
(8) (a) Whittaker, M. M.; Chuang, Y.-Y.; Whittaker, J. W. J. Am. Chem.
Soc. 1993, 115, 10029. (b) Whittaker, M. M.; Duncan, W. R.;
Whittaker, J. W. Inorg. Chem. 1996, 35, 382.
(9) (a) Halfen, J. A.; Young, V. G., Jr.; Tolman, W. B. Angew. Chem.
1996, 108, 1832; Angew. Chem., Int. Ed. Engl. 1996, 108, 1832. (b)
Halfen, J. A.; Jazdzewski, B. A.; Mahapatra, S.; Berreau, L. M.;
Wilkinson, E. C.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1997,
119, 8217.
(10) Kitajima, N.; Whang, K.; Moro-oka, Y.; Uchida, A.; Sasada, Y. J.
Chem. Soc., Chem. Commun. 1986, 1504.
(11) (a) Wang, Y.; Stack, T. D. P. J. Am. Chem. Soc. 1996, 118, 13097.
(b) Wang, Y.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.; Stack, T.
D. P. Science 1998, 279, 537.
(12) Sokolowski, A.; Mu¨ller, J.; Weyhermu¨ller, T.; Schnepf, R.; Hilde-
brandt, P.; Hildenbrand, K.; Bothe, E.; Wieghardt, K. J. Am. Chem.
Soc. 1997, 119, 8889.
(13) (a) Chaudhuri, P.; Hess, M.; Flo¨rke, U.; Wieghardt, K. Angew. Chem.
1998, 110, 2340; Angew. Chem., Int. Ed. Engl. 1998, 37, 2217. (b)
Chaudhuri, P.; Hess, M.; Weyhermu¨ller, T.; Wieghardt, K. Angew.
Chem. 1999, 111, 1165; Angew. Chem., Int. Ed. Engl. 1999, 38, 1095.
(c) Chaudhuri, P.; Hess, M.; Mu¨ller, J.; Hildenbrand, K.; Bill, E.;
Weyhermu¨ller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 9599.
(14) Ruf, M.; Pierpont, C. G. Angew. Chem. 1998, 110, 1830; Angew.
Chem., Int. Ed. Engl. 1998, 37, 1736.
RCH2OH + O2 f RCHO + H2O2
(1)
galactose oxidase at 1.7 Å resolution has clearly shown that
the tyrosine residue (Tyr272, the precursor of the tyrosyl radical)
is covalently bound to the sulfur atom of the adjacent Cys228
at the R-position of the phenol ring as illustrated in Chart 1.2
Recently, such a phenoxyl radical-copper catalytic motif has
also been found in glyoxal oxidase (GLO) from Phanerochaete
chrysosporium and in the prokaryotic FbfB protein.5,6
† Osaka City University.
‡ Osaka University.
§ Institute for Molecular Science.
| Kansai University.
(1) Stubbe, J.; van der Donk, W. A. Chem. ReV. 1998, 98, 705 and
references therein.
(2) (a) Ito, N.; Phillips, S. E. V.; Stevens, C.; Ogel, Z. B.; McPherson,
M. J.; Keen, J. N.; Yadav, K. D. S.; Knowles, P. F. Nature 1991,
350, 87. (b) Ito, N.; Phillips, S. E. V.; Yadav, K. D. S.; Knowles, P.
F. J. Mol. Biol. 1994, 238, 794.
(3) (a) Whittaker, M. M.; Whittaker, J. W. J. Biol. Chem. 1988, 263, 6074.
(b) Whittaker, M. M.; DeVito, V. L.; Asher, S. A.; Whittaker, J. W.
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W. J. Biol. Chem. 1990, 265, 9610. (d) Whittaker, M. M.; Whittaker,
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Whittaker, J. W. Biochemistry 1998, 37, 8426.
(4) (a) Branchaud, B. P.; Montague-Smith, M. P.; Kosman, D. J.; McLaren,
F. R. J. Am. Chem. Soc. 1993, 115, 798. (b) Wachter, R. M.;
Branchaud, B. P. Biochemistry 1996, 35, 14425. (c) Wachter, R. M.;
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119, 7743.
(5) (a) Kersten, P. J. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 2936. (b)
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Yellowlees, L. J.; Mabbs, F. E.; Davies, J. E. Chem. Commun. 1998,
2465. (b) Halcolm, M. A.; Chia, L. M. L.; Liu, X.; McInnes, E. J. L.;
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J. E. J. Chem. Soc., Dalton Trans. 1999, 1753.
(16) To distinguish the different states of the cofactor moiety of the ligands
more clearly, the symbols of LH, L-, and L• (L ) 1 or 2) are used
to denote the phenol, phenolate, and phenoxyl radical forms, respec-
tively.
10.1021/ic9910211 CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/13/2000

Notes
Inorganic Chemistry, Vol. 39, No. 16, 2000 3709
radical complexes of ligand 1H as well as of its 2,4-di-tert-
butyl derivative 2H16 as a reference to shed light on the role of
the thioether group of the cofactor.
Results and Discussion
Ligand 1H was prepared from 4-tert-butylphenol by acid-
catalyzed sulfenylation with dimethyl disulfide followed by a
Mannich reaction with bis[2-(2-pyridyl)ethyl]amine and paraform-
aldehyde as employed for the synthesis of the 4-methyl
derivative of 1H.7b Another ligand 2H was obtained in one step
by the Mannich reaction of 2,4-di-tert-butylphenol, paraform-
aldehyde, and bis[2-(2-pyridyl)ethyl]amine. Treatment of the
deprotonated ligand 1- 16 with CuCl2 in methanol led to the
formation of a copper(II) complex that immediately crystallized
as a dimer, [CuII (1-)2](PF6)2, when NaPF6 was added to the
2
solution.18 The Zn(II) complex of 1- was also isolated as a
dimer, [ZnII (1-)2](PF6)2, in a similar manner using Zn(ClO4)2‚
2
6H2O instead of CuCl2.18 These dimeric Cu(II) and Zn(II)
complexes of 1- can be converted into the corresponding
monomeric complexes, [CuII(1-)(AcO-)] and [ZnII(1-)-
(CH3CN)]PF6, in CH3CN by the coordination of the external
ligand (AcO-) and the solvent, respectively.18 In contrast to the
case of ligand 1-, the Cu(II) and Zn(II) complexes of ligand
2H were isolated as mononuclear complexes, [CuII(2-)-
(CH3CN)]PF6 and [ZnII(2-)(CH3CN)]PF6, when the deproto-
nated ligand 2- 16 was treated with CuCl2 and Zn(ClO4)2‚6H2O,
respectively, in CH3OH followed by the addition of NaPF6.18
All mononuclear phenolate complexes, [CuII(1-)(AcO-)],
[ZnII(1-)(CH3CN)]PF6, [CuII(2-)(CH3CN)]PF6, and [ZnII(2-)-
(CH3CN)]PF6, exhibited a reversible redox couple for the one-
electron oxidation of the phenolate at 0.14, 0.51, 0.44, and 0.50
V vs Ag/AgNO3, respectively.18 The large difference in redox
potential between the first complex and the other three could
be attributed mainly to the difference in the external ligand:
AcO- vs CH3CN.
Figure 1. (A) UV-vis spectra of [CuII2(1-)2](PF6)2 (dotted line) (2.5
× 10-4 M) and [CuII(1•)(NO3)]+ (solid line) in CH3CN at 25 °C. Inset:
Resonance Raman spectrum of [CuII(1•)(NO3)]+ in CH3CN at -30 °C.
(B) UV-vis spectra of [ZnII(1-)(CH3CN)](PF6)2 (dotted line) (5.0 ×
10-4 M) and [ZnII(1•)(NO3)]+ (solid line) in CH3CN at 25 °C. Inset:
Resonance Raman spectrum of [ZnII(1•)(NO3)]+ in CH3CN at -30 °C.
Figure 1A shows the spectral changes observed upon addition
of an equimolar amount of (NH4)2[CeIV(NO3)6] (CAN) per
copper ion to a CH3CN solution of [CuII (1-)2](PF6)2 (2.5 ×
2
10-4 M), where the absorption band at 522 nm due to the
phenolate complex immediately disappears together with the
concomitant appearance of new absorption bands at 415 nm (ꢀ
) 1790 M-1 cm-1) and 867 nm (550 M-1 cm-1) at 25 °C. A
similar spectrum is obtained upon the oxidation of [ZnII(1-)-
(CH3CN)]PF6 by CAN under the same experimental conditions,
as shown in Figure 1B [418 nm (1250 M-1 cm-1) and 887 nm
(510 M-1 cm-1) in CH3CN at 25 °C]. The resulting spectra are
very similar in shape to those of the phenoxyl radical species
of 2-(methylthio)-p-cresol (λmax ) 400 and 830 nm)7,8a and of
the active forms of GAO [444 nm (5194 M-1 cm-1) and 800
(2430 M-1 cm-1) and 666 nm (140 M-1 cm-1) for the Zn(II)
complex; see Figures S12 and S13 in the Supporting Informa-
tion]. It is interesting to note that both Cu(II) and Zn(II)
complexes of the phenoxyl radical 1• exhibit very broad and
strong absorption bands above 850 nm (λmax ) 867 and 887
nm), while those of 2• and of the other phenoxyl radical species
thus far reported do not show such characteristic absorption
bands above 800 nm,9,11,12,13a,b except for one example recently
reported by Halcolm et al.15 On the basis of the spectroscopic
and theoretical examinations of the metal-free phenoxyl radical
species of cofactor model compounds, we have attributed the
broad absorbance above 800 nm to the intramolecular charge
transfer from the benzene ring to the methylthio group in the
nm (3211 M-1 cm-1)] and GLO [448 nm (5700 M-1 cm-1
)
and 851 nm (4300 M-1 cm-1)],5b indicating that the character-
istic absorption bands obtained upon the CeIV oxidation are due
to the phenoxyl radical species (1•)16 of the ligand. The Cu(II)
and Zn(II) complexes of 2• 16 can also be generated by CeIV
oxidation of the corresponding phenolate compounds. The
phenoxyl radical complexes display two characteristic absorption
bands around 410 and 670 nm [411 nm (2440 M-1 cm-1) and
675 nm (300 M-1 cm-1) for the Cu(II) complex and 409 nm
phenoxyl radical group.7 The larger ꢀ values (∼5000 M-1 cm-1
)
of the lower energy absorption bands of GAO and GLO as com-
pared to those of our model compounds (ꢀ ) ∼500 M-1 cm-1
)
may indicate that an interligand charge transfer from Tyr272•
to Tyr495- also contributes to the characteristic UV-NIR
features of the enzymes as suggested by Whittaker et al.19
(17) Itoh, S.; Taki, M.; Takayama, S.; Nagatomo, S.; Kitagawa, T.;
Sakurada, N.; Arakawa, R.; Fukuzumi, S. Angew. Chem. 1999, 111,
2944; Angew. Chem., Int. Ed. Engl. 1999, 38, 2774.
(18) Details of the synthetic procedures, X-ray structure determinations,
and spectroscopic and electrochemical characterizations for the
phenolate complexes are provided in the Supporting Information.
(19) McGlashen, M. L.; Eads, D. D.; Spiro, T. G.; Whittaker, J. W. J.
Phys. Chem. 1995, 99, 4918.

3710 Inorganic Chemistry, Vol. 39, No. 16, 2000
Notes
Figure 4. Hyperfine coupling constants (G) determined from the
computer simulations for (A) [ZnII(1•)(NO3)]+ and (B) [ZnII(2•)(NO3)]+.
theoretical calculations.7 The ESI-MS spectra in Figure 2 each
exhibit only a set of prominent peaks with a mass and an isotope
distribution pattern being consistent with Cu(II) and the Zn(II)
complexes of the phenoxyl radical with one nitrate ion as a
coordinated counteranion coming from CAN: [CuII(1•)(NO3)]+
and [ZnII(1•)(NO3)]+.
Figure 2. Experimental (bottom) and calculated (top) peak envelopes
in the positive-ion electrospray mass spectra of [CuII(1•)(NO3)]+ (left)
and [ZnII(1•)(NO3)]+ (right) in CH3CN.
A CH3CN solution of [CuII(1•)(NO3)]+ was ESR silent at 77
K, which is consistent with magnetic coupling between the S
) 1/2 Cu(II) ion and the S ) 1/2 phenoxyl radical, although the
type of coupling (antiferromagnetic or ferromagnetic) is unclear
at present. In contrast, the solution ESR spectrum of [ZnII(1•)-
(NO3)]+ recorded at -80 °C in C2H5CN (Figure 3A) exhibits
a well-resolved isotropic signal at g ) 2.0052, which is very
close to those of the cofactor radical (2.0055) in the enzyme
active site and of the phenoxyl radicals derived from our cofactor
model compounds (2.0052-2.0060).3c,7b [ZnII(2•)(NO3)]+ also
affords a well-resolved ESR signal at g ) 2.0038 under the
same experimental conditions (Figure 3B). Hyperfine coupling
constants (hfc’s) determined from the computer simulations for
[ZnII(1•)(NO3)]+ and [ZnII(2•)(NO3)]+ (SIMs in Figure 3) are
indicated in Figure 4 (parts A and B, respectively). It is obvious
that the spin density at the benzylic methylene group in 1• (aH
) 2.43 G) is significantly small as compared to that of 2• (aH
) 7.19 G), whereas a relatively large amount of the spin
delocalizes into the methylthio group in 1• (aMe ) 3.72 G). The
spin delocalization into the methylthio group in 1• is reflected
by the larger g value of [ZnII(1•)(NO3)]+ (g ) 2.0052) due to
the large spin-orbit coupling on the sulfur atom as compared
to that of [ZnII(2•)(NO3)]+ (g ) 2.0038).
Figure 3. (A) Solution ESR spectrum (EXP) of [ZnII(1•)(NO3)]+ in
CH3CH2CN at -80 °C (microwave frequency 9.213 GHz, modulation
frequency 100 kHz, modulation amplitude 0.5 G, microwave power 5
mW) and its computer simulation spectrum (SIM). (B) Solution ESR
spectrum (EXP) of [ZnII(2•)(NO3)]+ in CH3CH2CN at -80 °C (micro-
wave frequency 9.210 GHz, modulation frequency 100 kHz, modulation
amplitude 0.63 G, microwave power 5 mW) and its computer simulation
spectrum (SIM).
It has been reported that hfc values for benzylic methylene
protons of phenoxyl radicals can be estimated by the angle-
dependent McConnell-type relationship aC-H ) FC1B cos2 θ,
where aC-H is the hfc of the methylene proton, FC1 is the spin
density at the C1 position, B is a constant equal to 162 Hz, and
θ is the dihedral angle defined in Figure 8 of Babcock’s paper.21
Formation of the M(II)-phenoxyl radical species was con-
firmed by resonance Raman spectroscopy (RR; insets of Figure
1), electrospray ionization mass spectroscopy (ESI-MS; Figure
2), and ESR spectroscopy (Figure 3) of the solutions resulting
from the CeIV oxidation of [CuII (1-)2](PF6)2 and [ZnII(1-)-
2
(CH3CN)]PF6. In the RR spectra, both Cu(II) and Zn(II)
complexes of 1• display two prominent peaks at 1512 and 1589
cm-1 and at 1517 and 1595 cm-1, respectively, which are
characteristic vibrational peaks of metal-coordinated phenoxyl
Thus, the ratio of the hfc values of two methylene protons (aH1
H2) is proportional to cos2 θ1/cos2 θ2. If one accepts an
assumption that the dihedral angles of benzylic methylene
protons do not change upon one-electron oxidation, the aH1
/
a
radicals9b,12,20 and of activated GAO (1487, 1595 cm-1 5b
and
)
/
GLO (1486, 1591 cm-1).5b These two bands have been assigned
to the modes ν7a′ and ν8a′, which predominantly include C-O
stretching and Cortho-Cmeta stretching, respectively.20 The higher
energy of the C-O stretching of the phenoxyl radical (ν7a′) as
a
H2 ratio can be calculated as 136 by using the dihedral angles
obtained in the X-ray structure of [ZnII (1-)2](PF6)2 (θ1(av)
)
2
25.1°, θ2(av) ) 85.6°).18 Such a large value of aH1/aH2 is
consistent with the large difference between aH1 and aH2 (2.43
mT vs ∼0 mT) determined from the computer simulation of
compared to that of phenolate (ν7a ∼ 1250 cm-1 9b
)
clearly
the ESR spectrum (Figure 3). Such a good correlation for aH1
/
indicates that the C-O bond of the phenoxyl radical species
has a partial double bond character, as we have already
suggested on basis of the results of ESR measurements and
aH2 indicates the validity of our assumption, suggesting that the
phenoxyl radical remains metal-bound. The existence of strong
(20) Schnepf, R.; Sokolowski, A.; Mu¨ller, J.; Bachler, V.; Wieghardt, K.;
(21) Babcock, G. T.; El-Deeb, M. K.; Sandusky, P. O.; Whittaker, M. M.;
Hildebrandt, P. J. Am. Chem. Soc. 1998, 120, 2352.
Whittaker, J. W. J. Am. Chem. Soc. 1992, 114, 3727.

Notes
Inorganic Chemistry, Vol. 39, No. 16, 2000 3711
Packard 8453 photodiode array spectrophotometer. ESR spectra were
recorded on a JEOL JES-ME-2X spectrometer, and the g values were
determined using an MnII marker as a reference. Computer simulations
of the ESR spectra were carried out by using ESRaII version 1.01
(Calleo Scientific Publishing Co.) on a Macintosh personal computer.
Electrospray ionization mass spectroscopy (ESI-MS) and electrochemi-
cal measurements were performed as reported previously.7b,18
magnetic coupling in the Cu(II)-phenoxyl radical species also
indicates that there is a direct metal-phenoxyl radical interac-
tion.
In summary, one-electron oxidations of the Cu(II)- and
Zn(II)-phenolate complexes of ligand 1H afford relatively
stable phenoxyl radical complexes, which exhibit very charac-
teristic UV-NIR features similar to those exhibited by the active
forms of the native enzymes. Comparison of the spectroscopic
characteristics (UV-vis and ESR) of the Cu(II) and Zn(II)
complexes of 1• to those of the corresponding complexes of 2•
indicates that the methylthio group of 1• exerts an electron-
sharing conjugative effect, thus stabilizing the radical form of
the cofactor, as has been demonstrated in model studies of the
metal-free radicals.7 Such an important role for the thioether
group (electron-sharing conjugative effect) has also been
predicted by ab initio theory and demonstrated by high-
frequency ESR studies of model radicals.22 It should be noted,
however, that such an electronic effect of an alkylthio group is
not always observed in other model systems,9,11,13a suggesting
that the molecular geometry of a complex is also very important
to the enhancement of this effect. The smaller ꢀ values for the
NIR features of the model complexes as compared to those for
the active forms of the native enzymes may indicate a strong
contribution from Tyr272• f Tyr495- interligand charge
transfer in addition to intramolecular charge transfer from the
benzene ring to the alkylthio group in the phenoxyl radical group
itself in the enzymatic systems.
Visible Resonance Raman Measurements. The 413.1 nm line of
a Kr+ laser (model 2060, Spectra Physics) was used as the exciting
source. Visible resonance Raman scattering was measured with a CCD
detector (model CCD3200, Astromed) attached to a 1 m single
polychromator (model MC-100DG, Ritsu Oyo Kogaku). The slit width
and slit height were set to 200 µm and 10 mm, respectively.
Wavenumber ranges per channel were 0.9 (Kr+ laser) and 0.5 cm-1
(dye laser). The laser power used was 2.5 mW at the sample point. All
measurements were carried out at -40 °C with a spinning cell (1000
rpm). Raman shifts were calibrated with indene, and the accuracy of
the peak positions of the Raman bands was (1 cm-1
.
Acknowledgment. This study was financially supported in
part by a Grant-in-Aid for Scientific Research on Priority Areas
(Molecular Biometallics, Electrochemistry of Ordered Interfaces,
and Creation of Delocalized Conjugated Electronic Systems)
from the Ministry of Education, Science, Sports, and Culture
of Japan and by a Research Fellowship from the Japan Society
for the Promotion of Science for Young Scientists (No. 02410).
Supporting Information Available: Textual details of the synthetic
procedures, X-ray structure determinations, and spectroscopic and
electrochemical characterizations, tables of X-ray experimental details,
bond distances, and bond angles, figures showing ORTEP diagrams,
UV-vis, EI-MS, and ESR spectra, and cyclic voltammograms (Figures
S1-S11), and X-ray crystallographic files, in CIF format, for the
phenolate complexes and UV-vis spectra for the Cu(II) and Zn(II)
complexes of 2• (Figures S12 and S13). This material is available free
of charge via the Internet at http://pubs.acs.org.
Experimental Section
General Procedures. Syntheses of the ligands and the complexes
were performed similarly to those reported previously.7b,18 UV-vis
spectra were recorded on a Hewlett-Packard 8452A or a Hewlett-
(22) Gerfen, G. J.; Bellew, B. F.; Griffin, R. G.; Singel, D. J.; Ekberg, C.
A.; Whittaker, J. W. J. Phys. Chem. 1996, 100, 16739.
IC9910211