A substrate for the detection of broad specificity α-l- arabinofuranosidases with indirect release of a chromogenic group

Article abstract of DOI:10.1016/j.tetlet.2013.03.136

The synthesis of a compound containing a 4-nitrocatechol bound to two vicinal α-l-arabinofuranosyl moieties through a linker arm was achieved using a sulfate protecting group to facilitate selective alkylation of one aromatic hydroxyl. Several α-l-arabinofuranosidases displaying different selectivities were tested and a simple microtiter plate-based assay was developed. The observed resistance of the compound to α-l- arabinofuranosidase-mediated hydrolysis makes it suitable for the identification of enzymes that are able to accommodate bis-arabinofuranosylated moieties.

Full text of DOI:10.1016/j.tetlet.2013.03.136

Tetrahedron Letters 54 (2013) 3063–3066
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A substrate for the detection of broad specificity
with indirect release of a chromogenic group
a-L-arabinofuranosidases
Vinciane Borsenberger a,b,c, Emmie Dornez d, Marie-Laure Desrousseaux a,b,c, Christophe M. Courtin d,
Michael J. O’Donohue a,b,c, Régis Fauré a,b,c,
⇑
a Université de Toulouse; INSA, UPS, INP; LISBP, 135 Avenue de Rangueil, F-31077 Toulouse, France
b INRA, UMR792 Ingénierie des Systèmes Biologiques et des Procédés, F-31400 Toulouse, France
c CNRS, UMR5504, F-31400 Toulouse, France
d Laboratory of Food Chemistry and Biochemistry, KU Leuven, Kasteelpark Arenberg 20 bus 2463, B-3001 Leuven, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of a compound containing a 4-nitrocatechol bound to two vicinal
a-L-arabinofuranosyl
Received 20 December 2012
Revised 19 March 2013
Accepted 26 March 2013
Available online 11 April 2013
moieties through a linker arm was achieved using a sulfate protecting group to facilitate selective alkyl-
ation of one aromatic hydroxyl. Several -arabinofuranosidases displaying different selectivities were
tested and a simple microtiter plate-based assay was developed. The observed resistance of the com-
pound to -arabinofuranosidase-mediated hydrolysis makes it suitable for the identification of
enzymes that are able to accommodate bis-arabinofuranosylated moieties.
Ó 2013 Elsevier Ltd. All rights reserved.
a
-L
a-L
Keywords:
Carbohydrate
4-Nitrocatechol
AXH-d3
a-L-Arabinofuranosidase
Introduction
attached to a glycoside unit. Although convenient, these types of
substrates are almost certainly poor analogues for the natural sub-
strates of most targeted GHs. This is because the aryl moiety can at
best only approximately imitate a sugar, while the good leaving
group ability of phenolate species,11 such as PNP, promotes hydro-
lysis and can thus reveal GH activities that turn out to be essen-
tially minor when considering an enzyme’s actual substrate
preference.
The increasing penetration of biotechnology into industrial pro-
cesses is driving high-throughput enzyme discovery.1–4 In this con-
text, plant cell wall-degrading glycoside hydrolases (GHs) are
highly sought after for use in sectors such as the food and animal
feed industries, or for the emerging biorefining industry. One
example of such GHs is
a-
L
-arabinofuranosidases (Abfs) that are
active on a variety of complex
L-arabinofuranose-containing poly-
To facilitate the detection of Abfs that act on disubstituted D-
mers, particularly arabinoxylans, which constitute an important
and widespread class of plant polymers.5 When acting on arabin-
Xylp, we set out to design a substrate that is both simple to use,
and whose structure and reactivity might better reflect that of a
natural substrate. To achieve this, we decided to move the aryl
moiety away from the sugar by introducing an alkyl spacer arm be-
tween the two, thus hopefully minimizing the undesirable interac-
tions between the aryl group and the target Abfs. Moreover, to be
able to use the substrate in both liquid and solid media-based as-
says, we designed a compound bearing a 4-nitrocatechol (4NTC)
oxylans, Abfs hydrolyze the
arabinofuranosyl ( -Araf) moieties to main-chain
-Xylp) units, and thus facilitate the action of main-chain-hydro-
lyzing GHs, such as endo-b- -xylanases (Fig. 1). Interestingly, some
Abfs are clearly specific for mono-substituted -Xylp, while others
are highly specific for -Xylp bearing two
-Araf moieties.6–8
a
-1,2 and/or
a
-1,3 bonds that link
L-
L
D
-xylopyranosyl
(D
D
D
D
L
Chromogenic assays are commonly employed in high-through-
put screening, because they provide quick and easy detection of
enzyme activity at the microbial colony level, and are compatible
with automated protocols.9 The most commonly used chromo-
genic substrates for the detection of exo-acting GHs are those com-
posed of indolyl10 or para-nitrophenol (PNP) moieties directly
unit. This was attached to two vicinal
able spacer arm, thus affording a compound that is meant to imi-
tate a disubstituted -Xylp moiety, typical of those found in the
main-chain of highly substituted recalcitrant arabinoxylans. Fur-
thermore, in the case of successful bond hydrolysis and release of
L-Araf moieties via a cleav-
D
both L-Araf units, a diol will be generated, which will be vulnerable
to oxidative cleavage. Therefore, we expected that, in the presence
of NaIO4, an aldehyde could be produced, which would rapidly un-
dergo b-elimination at basic pH,12–15 thus releasing 4NTC, a chro-
⇑
Corresponding author. Tel.: +33 5 6155 9488; fax: +33 5 6155 9400.
E-mail address: regis.faure@insa-toulouse.fr (R. Fauré).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.136

3064
V. Borsenberger et al. / Tetrahedron Letters 54 (2013) 3063–3066
mogenic moiety that can be monitored in liquid medium and thus
used to assay enzyme activity.16,17
1 h by adding a 1.2 equiv of CSA at the end of the coupling reaction,
thus affording 4 in 57% yield.19 Nevertheless, although acetone was
employed as the co-solvent in order to prevent the loss of the iso-
propyledene protecting group, fully deprotected material 9 was
obtained in 12% yield.20 Classic esterification of 4 with benzoyl
chloride in pyridine provided the adequately protected acetal,
which was quantitatively hydrolyzed in 80% aqueous AcOH, afford-
ing 5 in 82% overall yield from 4.21 An inverse procedure of
Schmidt glycosylation22 was employed to glycosylate the diol moi-
ety using an excess of the activated sugar donor 623 with a catalytic
amount of BF3.Et2O, which gave the desired compound 724 in 94%
yield. Benzoyl protecting groups were removed using sodium
methanolate and finally, 825 was obtained in a 40% overall yield
in six steps.
Results and discussion
The synthesis is described in scheme 1. In order to obtain the
target compound 8 as one regioisomer, it was essential to find a
method to selectively alkylate the 4NTC at one hydroxyl position
only. As direct alkylation of 4NTC under alkaline conditions with
an equimolar amount of protected halide 218 resulted mainly in
the production of the O-1,O-2 disubstituted 4NTC (80%) and very
little of the O-1 mono-substituted species (7%), we adopted a reg-
ioselective protection strategy. Esterification of 4NTC using ben-
zoyl chloride resulted in the production of a poorly soluble solid,
which appeared to be a mixture of O-1 and O-2 benzoylated
4NTC. Attempts to separate the two isomers by selective recrystal-
lization failed, because the benzoyl underwent spontaneous intra-
molecular migration between the aromatic cis hydroxyls, resulting
in a relatively fast equilibrium between the two regioisomers. Fi-
nally, we turned to the commercially available dipotassium salt
of 4-nitrocatechol sulfate 1, and proceeded to a halide coupling un-
der anhydrous conditions. Using a small excess of bromo derivative
2 in the presence of a proton sponge only produced the desired
product in low yield (<2%). Therefore, we turned to its more reac-
tive iodo version 3,18 which successfully afforded the aryl ether.
Next, the sulfate protecting group being more labile than the acetal
moiety, was necessary to swap for a benzoyl protecting group.
Assuming that in situ deprotection would greatly simplify the cou-
pling reaction work-up, the acid labile sulfate was removed over
The principle of the enzymatic assay is detailed in scheme 2.
After enzymatic release of the L-Araf units by Abfs, the free diol
can be removed by oxidative cleavage using sodium periodate.
After quenching of the excess oxidant with ethylene glycol, the
aldehyde 10 undergoes a b-elimination under basic conditions,
releasing free 4NTC that displays a characteristic red color above
pH 11.16 Importantly, the pH of the oxidative step must be care-
fully controlled in order to avoid premature release of 4NTC, which
would result in its oxidation and consequent lowered sensitivity of
the assay. To determine ideal operating conditions, compound 9
was incubated with 10 equiv of sodium periodate in buffered solu-
tions, in the range pH 3–8.26 At pH 5, the linker arm remained sta-
ble, thus protecting 4NTC from oxidation while allowing complete
cleavage of the free diol moiety within 10 min. Longer exposure
(1 h) to periodate did not bring about other changes. Further incu-
Figure 1. A generic scheme of arabinoxylan structures. The main-chain is composed of b-1,4-linked
residues. Depending on the exact botanic origin, other side-chain decorations are also possible, but these are not shown. Potential enzymatic cleavage sites are indicated by
wavy lines (Abf, -arabinofuranosidase; AXH-d3, double substituted xylan -1,3- -arabinofuranosidase; Xyn, endo-b- -xylanase; Xyl, b-
-xylosidase).5
D-Xylp subunits, decorated with a-1,2 and/or a-1,3-bounded L-Araf
a-
L
a
L
D
D
Scheme 1. Reagents and conditions: (a) 1.5 equiv of 3, 18-C-6, DMF, 50 °C, 23 h; (b) CSA 1.2 equiv, acetone, 1 h, 57% over two steps; (c) BzCl 3.7 equiv, pyridine, DCM, 7 h; (d)
AcOH, 82% over two steps; (e) 4.4 equiv of 6, BF3.Et2O, DCM, 3 h, 94%; (f) MeONa 2.3 equiv, MeOH, DCM, 39 h, 90%.

V. Borsenberger et al. / Tetrahedron Letters 54 (2013) 3063–3066
3065
Scheme 2. Reactions involved in the Abf activity assay.
Figure 2. Hydrolysis of 8 in 96-well microtiter plates. A1: compound 8 (negative
control), A2: compound 9 (positive control), B1-9: discontinuous monitoring of
enzyme-mediated hydrolysis of 8 by an Abf. Each well contains a 50 pmol of
substrate to which 500 pmol of periodic acid, 50
lL ethylene glycol and 150 lL of
1 M Na2CO3 are added successively.30
bation (10 min) in the presence of ethylene glycol was sufficient to
ensure complete quenching of unreacted sodium periodate. Finally,
the addition of a 1 M Na2CO3 solution prompted b-elimination of
the aldehyde. The reaction was complete within 5 min at 0 °C, thus
the presence of free 4NTC, which is red at highly basic pH could be
monitored at OD505nm. In our tests, the final concentration of free
4NTC corresponded to a quantitative conversion of the starting
material. Importantly, when submitted to this assay compound 8
remained perfectly stable, with no detectable increase of absor-
bance at 505 nm being observed (Fig. 2).
Figure 3. Time-dependent hydrolysis of 8 by various Abfs, revealing selectivity
differences (4, TxAbf; d, AbfB; s, AbfA; and j, AXH-d3).30 Reactions were
discontinuously monitored at 505 nm.
served that TxAbf does indeed release L-Araf from disubstituted D-
Xylp moieties in arabinoxylo-oligosaccharides provided that these
are located at a terminal position (unpublished data).
To further validate the assay, compound 8 was submitted to the
hydrolytic action of several Abfs that display different specificities.
Two of the Abfs used belong to the family GH43 of the CAZy clas-
sification27 and are known to selectively release
a-L-Araf from D-
Conclusions
Xylp units, with AbfA (GenBank accession no. BAF39204.1),28 only
being able to act on mono-substituted units, and AXH-d3
In summary, we have shown that sulfate is a good protecting
group to selectively alkylate one hydroxyl of a catechol moiety, be-
cause it does not migrate and is easily removed, although the sol-
ubility issue that arises from having an ionic component in organic
mixture might limit its scope. Nevertheless, this procedure has
afforded a new chromogenic substrate that can be used to detect
(AAO67499.1),6 specifically, removing
a-1,3-bonded L-Araf from
disubstituted
D-Xylp moieties. The other Abfs employed, AbfB
(BAF40305.1)28 and TxAbf (CAA76421.2),29 are family GH51 mem-
bers. These present broader specificities and have the potential to
hydrolyze doubly-substituted D
-Xylp units to a small extent.16,28
Duplicate or triplicate experiments with discontinuous monitoring
were performed over 1 h in order to observe enzymatic hydrolysis
of 8, operating at the optimum pH and temperature of the Abf
a specific class of Abfs that selectively release
L-Araf from disubsti-
tuted -Xylp moieties. This substrate, which we believe better imi-
D
tates natural osidic linkages than conventional chromogenic
substrates, might be used in combination with an Abf that displays
being used.30 At regular intervals, 50
lL aliquots were removed
and added to cold (0 °C) periodate solution (pH 2) in 96-wells
microtiter plates and kept at 0 °C throughout, followed by treat-
ment using the previously established procedure. Hence, samples
were exposed to the oxidative cleavage for 10 min, before another
10 min quenching with ethylene glycol, after which absorbance
was measured at 505 nm. As shown in Fig. 3, AbfB and TxAbf suc-
cessfully removed both sugars from compound 8, while AbfA and
AXH-d3 failed to produce any free 4NTC. Interestingly, 1H NMR
analysis of the AXH-d3-mediated reaction indicated that the en-
tight selectivity for monosubstituted D-Xylp moieties, thus provid-
ing the means to detect 4NTC, the ultimate product of the two-en-
zyme cascade reaction, or alone to detect Abfs that have broader
substrate specificity, being able to release
L-Araf from both mono-
and disubstituted -Xylp moieties. This molecule would enable the
D
selection of ‘GH51-like’ enzymes of high catalytic efficiency.
Acknowledgments
zyme had removed one L-Araf, hence proving that the substrate
This work was supported by Région Midi-Pyrénées Grant DAER
Recherche 10008500 (V.B.). The NMR work was performed at the
successfully mimics the recalcitrant bis-arabinofuranosylated mo-
tif (unpublished data). Moreover, regarding the GH51 enzymes, the
enzyme concentration needed to hydrolyze 50% of 8 was at least
Laboratory for BioSystems
& Process Engineering (Toulouse,
France) using equipment of MetaToul (Toulouse Center for Meta-
bolomics), which is supported by grants from the Région Midi-
Pyrénées, the European Regional Development Fund, SICOVAL,
the Centre National de la Recherche Scientifique (CNRS), and the
Institut National de la Recherche Agronomique (INRA). High-reso-
lution mass spectra (HRMS) analyses were performed by the
CRMPO (University of Rennes, France).
10-fold higher than that required to hydrolyze PNP a-L-arabinofu-
ranoside under identical conditions, indicating that catalytic effi-
ciency on 8 was greatly diminished, and thus confirming that
this compound is well-suited to the identification of enzymes dis-
playing high catalytic efficiency on both mono-substituted and
disubstituted D
-Xylp moieties. Interestingly, using 1H NMR we ob-

3066
V. Borsenberger et al. / Tetrahedron Letters 54 (2013) 3063–3066
and then, cooling it to ꢀ20 °C for 1 h, which results in the formation of a white
Supplementary data
precipitate. Filtration and drying, provided 29 mg (0.12 mmol, 12%) of white
crystals. 1H NMR (500 MHz, CD3OD) d 7.77 (dd, 1H, J = 8.9, 2.8), 7.64 (d, 1H,
J = 2.8), 7.08 (d, 1H, J = 8.9), 4.34 (ddd, 1H, J = 9.7, 8.1, 5.4), 4.30 (dt, 1H, J = 9.7,
5.6), 3.90 (dtd, 1H, J = 9.0, 5.4, 3.6), 3.57 (dd, 1H, J = 11.1, 6.0), 3.55 (dd, 1H,
J = 11.1, 5.8), 2.14 (dddd, 1H, J = 14.4, 8.1, 6.2, 3.6), 1.89 (ddt, 1H, J = 14.4, 9.0,
5.3); 13C NMR (126 MHz, CD3OD) d 154.2, 148.2, 142.8, 117.2, 112.5, 111.2,
70.2, 67.5, 67.3, 33.8; HRMS (ESI) calcd C10H13NO6Na [M+Na]+: 266.0641
found: 266.0641 (0 ppm).
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
03.136.
References and notes
21. (S)-4-O-(2-O-benzoyl-4-nitrophenyl)-1,2,4-butanetriol 5: 1H NMR (500 MHz,
CDCl3) d 8.22–8.18 (m, 3H), 8.11 (d, 1H, J = 2.7), 7.68 (t, 1H, J = 7.5),), 7.54 (t, 2H,
J = 7.5), 7.11 (d, 1H, J = 9.1), 4.28 (dd, 2H, J = 6.5, 5.3), 3.80–3.75 (m, 1H), 3.51
(dd, 1H, J = 11.1, 3.3), 3.37 (dd, 1H, J = 11.1, 7.0), 2.17 (s, 2H), 1.89–1.79 (m, 2H);
13C NMR (126 MHz, CDCl3) d 164.6, 156.1, 139.9, 134.4, 130.5, 129.0, 128.5,
123.5, 119.4, 112.5, 68.9, 66.6, 66.4, 32.2; HRMS (ESI) calcd for C17H17NO7Na
[M+Na]+: 370.0903 found: 370.0905 (1 ppm).
22. Schmidt, R. R.; Toepfer, A. Tetrahedron Lett. 1991, 32, 3353–3356.
23. Du, Y.; Pan, Q.; Kong, F. Carbohydr. Res. 2000, 329, 17–24.
24. (S)-1,2-O-bis-(2,3,5-tri-O-benzoyl-a-L-arabinofuranosyl)-4-O-(2-O-benzoyl-4-
1. Wang, M.; Si, T.; Zhao, H. Bioresour. Technol. 2012, 115, 117–125.
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1885–1888.
17. Poraj-Kobielska, M.; Kinne, M.; Ullrich, R.; Scheibner, K.; Hofrichter, M. Anal.
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18. Tanis, S. P.; Raggon, J. W. J. Org. Chem. 1987, 52, 819–827.
19. Procedure for the synthesis of (S)-4-O-(2-hydroxy-4-nitrophenyl)-1,2-O-
isopropylidene-1,2,4-butanetriol 4: to a solution of 4-nitrocatechol sulfate
dipotassium salt 1 (312 mg, 1.0 mmol) and proton sponge (135 mg, 0.5 mmol)
in anhydrous DMF (10 mL) under an N2 atmosphere, was added (S)-1-iodo-3,4-
nitrophenyl)-1,2,4-butanetriol 7: 1H NMR (500 MHz, CDCl3) d 8.16-8.12 (m,
2H), 8.04–7.98 (m, 9H), 7.96–7.93 (m, 2H), 7.91–7.87 (m, 2H), 7.85 (dd, 1H,
J = 9.1, 2.7), 7.61 (tt, 1H, J = 7.5, 1.3), 7.58–7.41 (m, 10H), 7.39–7.31 (m, 4H),
7.22–7.31 (m, 6H), 6.71 (d, 1H, J = 9.1), 5.61 (s, 1H), 5.56 (d, 1H, J = 5.0), 5.50 (d,
1H, J = 4.4), 5.48 (d, 1H, J = 1.0), 5.47 (d, 1H, J = 1.2), 5.24 (s, 1H), 4.76 (dd, 1H,
J = 11.9, 3.6), 4.74 (dd, 1H, J = 12.2, 3.9), 4.63 (dd, 1H, J = 11.9, 4.9), 4.60–4.54
(m, 2H), 4.41–4.37 (m, 1H), 4.30–4.24 (m, 1H), 4.21–4.15 (m, 2H), 3.89 (dd, 1H,
J = 10.6, 3.7), 3.53 (dd, 1H, J = 10.6, 5.9), 2.09–2.03 (2H, m); 13C NMR (126 MHz,
CDCl3) d 166.3, 166.3, 165.8, 165.6, 165.5, 165.4, 164.2, 155.9, 141.1, 139.9,
134.1, 133.8, 133.6, 133.6, 133.2, 133.2, 130.4, 130.0, 130.0, 129.9, 129.9, 129.8,
129.8, 129.8, 129.2, 129.2, 129.2,129.0, 128.9, 128.7, 128.6, 128.6, 128.5, 128.4,
128.4, 123.2, 119.4, 111.8, 106.0, 104.5, 82.3, 82.1, 82.0, 81.4, 78.2, 77.8, 70.9,
68.7, 65.6, 64.0, 63.8, 31.8; HRMS (ESI) calcd C69H57NO21Na [M+Na]: 1258.3321
found: 1258.3318 (0 ppm).
25. (S)-1,2-O-bis-(a-L-arabinofuranosyl)-4-O-(2-O-benzoyl-4-nitrophenyl)-1,2,4-
butanetriol 8: 1H NMR (500 MHz, D2O) d 7.73 (dd, 1H, J = 9.0, 2.8), 7.59 (d, 1H,
J = 2.8), 6.98 (d, 1H, J = 9.0), 5.23 (d, 1H, J = 1.6), 5.07 (d, 1H, J = 1.6), 4.27–4.16
(m, 3H), 4.14 (dd, 1H, J = 3.5, 1.7), 4.10 (dd, 1H, J = 3.6, 1.8), 4.09 (td, 1H, J = 5.9,
3.3), 3.99–3.92 (m, 3H), 3.89–3.86 (m, 1H), 3.84 (dd, 1H, J = 12.3, 3.3), 3.73 (dd,
1H, J = 12.3, 5.6), 3.73 (dd, 1H, J = 12.3, 5.6), 3.63 (dd, 1H, J = 11.1, 6.0), 3.52 (d,
2H, J = 4.2), 2.17–2.08 (m, 1H), 2.07–2.0 (m, 1H); 13C NMR (126 MHz, D2O) d
152.9, 144.9, 140.6, 117.6, 111.4, 110.3, 107.9, 106.7, 83.7, 83.6, 81.4, 81.0, 76.4,
76.2, 72.8, 69.9, 65.6, 61.1, 60.5, 30.5; HRMS (ESI) calcd C20H29NO14 [M+Na]+:
530.1486 found: 530.1482 (1 ppm).
26. All experiments were carried in the presence of 2 equiv of L-Ara in order to
account for the consumption of oxidant by the free sugar in the enzymatic
hydrolysis of 8.
27. Cantarel, B. L.; Coutinho, P. M.; Rancurel, C.; Bernard, T.; Lombard, V.;
Henrissat, B. Nucleic Acids Res. 2009, 37, D233–D238.
O-isopropylidene-3,4-butanediol 3 (250 lL or 388 mg, 1.5 mmol) by a syringe.
The reaction mixture was stirred for 23 h at 50 °C, then allowed to reach room
temperature, and poured into a solution of camphor sulfonic acid (274 mg,
1.2 mmol) in acetone (70 mL). Solvents were removed under reduced pressure
and the residue was taken in a saturated NaHCO3 solution, extracted with ethyl
acetate. The combined organic extracts were washed in turn with water and
brine, dried (MgSO4), filtered, and evaporated. The product was isolated by
flash column chromatography using a gradient of acetone in toluene (10–20%),
which provided 161 mg (0.57 mmol, 57%) of 4 as white crystals: 1H NMR
(500 MHz, (CD3)2CO) d 8.56 (1H, s), 7.79 (ddd, 1H, 8.9, 2.8, 0.6), 7.67 (dd, 1H,
J = 2.8, 1.1), 7.18 (d, 1H, J = 8.9), 4.39–4.28 (m, 3H), 4.11 (dd, 1H, J = 8.1, 6.0),
3.65 (dd, 1H, J = 8.1, 6.9), 2.15–2.3 (m, 2H), 1.35 (s, 3H), 1.29 (s, 3H); 13C NMR
(126 MHz, (CD3)2CO) d 153.5, 147.7, 142.6, 117.1, 112.4, 110.9, 109.3, 73.9,
28. Lagaert, S.; Pollet, A.; Delcour, J. A.; Lavigne, R.; Courtin, C. M.; Volckaert, G.
Biochem. Biophys. Res. Commun. 2010, 402, 644–650.
29. Debeche, T.; Cummings, N.; Connerton, I.; Debeire, P.; O’Donohue, M. J. Appl.
Environ. Microbiol. 2000, 66, 1734–1736.
30. In a typical experiment, discontinuous enzyme assays were performed in
triplicate in buffered conditions (50 mM sodium acetate, pH 6.0) in the
presence of 8 (1 mM) and 1 g Lꢀ1 of BSA. Reactions were incubated at 30 (AXH-
d3, 22
28
g mLꢀ1) for 60 min. Aliquots (50
with 50 L of cooled (0 °C) 10 mM NaIO4 solution (pH 2) in 96-well microtiter
plates. After keeping 10 min at 0 °C throughout, 50 L of ethylene glycol were
added to each of the wells and incubated, followed by 150 L of 1 M Na2CO3
l l l
g mLꢀ1, and AbfB, 36 g mLꢀ1), 50 (AbfA, 97 g mLꢀ1), or 60 °C (TxAbf,
l
lL) were removed every 6 min and mixed
l
l
l
69.9, 67.4, 34.0, 27.3, 26.0; HRMS (ESI) calcd for
306.0954 found: 306.0952 (0 ppm).
C
13H17NO6Na [M+Na]+:
after 10 min. The optical densities at 505 nm were recorded for all wells on a
VersaMax microplate reader (molecular devices). Control reactions containing
all reactants except enzyme were prepared and incubated for 1 h at 30, 50, or
60 °C.
20. (S)-4-O-(2-hydroxy-4-nitrophenyl)-1,2,4-butanetriol 9 was obtained after the
extraction step and before the chromatography purification of 4. It can be
isolated from the crude mixture by triturating the yellow oil in toluene (4 mL)