Enantioselective sulfoxidation reaction catalyzed by a G-quadruplex DNA metalloenzyme

Article abstract of DOI:10.1039/c6cc03016e

Enantioselective sulfoxidation reaction is achieved for the first time by a DNA metalloenzyme assembled with the human telomeric G-quadruplex DNA and Cu(ii)-4,4′-bimethyl-2,2′-bipyridine complex, and the mixed G-quadruplex architectures are responsible for the catalytic enantioselectivity and activity.

Full text of DOI:10.1039/c6cc03016e

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Enantioselective sulfoxidation reaction catalyzed by G-quadruplex
DNA metalloenzyme
Mingpan Cheng,ab Yinghao Li,ab Jun Zhou,a Guoqing Jia,a Sheng-Mei Lu,a Yan Yang a and Can Li*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/chemcom
Enantioselective sulfoxidation reaction is achieved for the first
time by DNA metalloenzyme assembled with human telomeric G-
quadruplex DNA and Cu(II)-4,4′-bimethyl-2,2′-bipyridine complex,
and the mixed G-quadruplex architectures are responsible for the
catalytic enantioselectivity and activity.
The potential catalysis function of DNA has been explored in
the past decade. Two distinct strategies are developed for
constructing DNA catalysts --- i) in vitro selection from
synthetic DNA libraries1,2 and ii) anchoring transition metal
complexes as cofactors to naturally occurring double stranded
(ds) DNAs.3,4 The former focuses on the transformation of
nucleic acids and peptides, meanwhile asymmetric catalysis of
small molecules is frequently applied in exploring the catalytic
function of the latter.
DNA is usually a double helix in vivo, but single stranded
guanine-rich DNAs can fold into alternative structural forms
known as G-quadruplex.5,6 These four helix DNA architectures
possess advantages of abundant and plastic conformations.7
Therefore, G-quadruplex DNAs have been introduced as chiral catalyze enantioselective oxygen transfer reaction by G-
scaffolds for asymmetric Diels-Alder reaction with Cu(II)- quadruplex•heme metalloenzymes resulted in racemic
bipyridine,8,9 terpyridine10 and porphyrin11 complexes as products.16-18 The reason was that the big planar aromatic
cofactors. Another new type of ligand-free DNA porphyrin ligand was stacked to outer G-tetrad of G-
metalloenzyme constructed by directly coordinating metal ion, quadruplex, namely, the active sites were not surrounded by
such as Cu(II), with G-quadruplex DNA has been found to be G-quadruplex scaffold.19
active for asymmetric C-C bond formation reactions.12-14
The copper ion is frequently involved in fundamental redox
However, it is not clear whether the G-quadruplex DNA based process,20 and its coordinating ligands such as bipyridines and
chiral catalysis function can be extended to the other type phenanthrolines21 can interact with dsDNA via groove binding
reactions.
and intercalation, respectively. We anticipate that these small
, CuL1
Biological oxidation reaction is one of the most important sized Cu(II)-bidentate ligand complexes (Scheme
1
-5)
processes in nature. Most of these reactions are performed by would be the appropriate cofactors to the G-quadruplex DNA
highly evolved assemblies of proteins and cofactors, such as for asymmetric oxidation reactions. Herein we report the
metalloporphyrin-containing enzymes.15 Early tempts to enantioselective sulfoxidation reaction catalyzed by human
telomeric G-quadruplex DNA and Cu(II) complexes with H2O2
as the oxidant and up to 77% ee was achieved. To the best of
our knowledge, this is the first report on DNA based
enantioselective sulfoxidation reaction.
A natural 21mer human telomeric sequence (5′-(G3T2A)3G3-
3′, HT21) was utilized as the chiral scaffold, and thioanisole (1a
)
was chosen as the model substrate. Firstly, five achiral
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bidentate ligands, which can be categorized into sequence (cgmHT21) was added instead of HT21. The only
phenanthroline type (1,10-phenanthroline, L1; 5,6-dimethyl- difference between cgmHT21 and HT21 is that a single central
1,10-phenantiroline, L2) and bipyridine type (2,2′-bipyridine, guanine (G14) base from the third G-tract is replaced by
L3
bipyridine, L5) were investigated and the results were depicted quadruplex backbone evidenced by CD spectra (Fig. S3), which
in Table
. CuL1 and CuL2 complexes combined with HT21 is in accordance with previous result.26 Along with this
;
4,4′-bimethyl-2,2′-bipyridine, L4
;
4,4′-bimethoxy-2,2′- thymine (T14). This mutation leads to the collapse of G-
1
catalyze the reaction with extremely low conversions (15% and unwinding of the ordered G-quadruplex to disordered state,
7%, respectively) and nearly zero enantioselectivities. In the conversion is restored to about 50% of the wild-type level.
contrast, the bipyridine type CuL3-5 complexes combined with More importantly, the enantioselectivity of sulfoxides drops to
HT21 lead to higher conversions, and more importantly, the near-zero (Table 1, Entry 11). These results emphasize that
products exhibit chiral selectivity. Notably, CuL4 complex with maintaining the integrity of quadruplex helix backbone is
methyl group at 4,4′-position of bipyridine gives the product essential to chiral control in sulfoxidation reaction.
with the full conversion and 56% ee. The methyl group of CuL4
G-quadruplex RNA based catalysts were capable of
replaced by H or CH3O forming CuL3 and CuL5 complexes give catalyzing the oxidation of thioanisole17 and the G-quadruplex
the product with lower enantioselectivities (22% and 39% ee, RNA•CuL4 complex was also investigated here. The RNA used
respectively) and conversions.
in this work shares the sequence of HT21 and is named as
HT21 sequence adopts mixed conformations (including rHT21. CD spectra reveal that rHT21 folds into a characteristic
antiparallel, parallel and hybrid forms) in K+ solution and a parallel conformation (positive peak at 265 nm and negative
three layered antiparallel conformation in the presence of Na+ peak at 240 nm, Fig. S3) in K+ solution.27 Though thioanisole
(Fig. S2).22-25 Hence control experiments were carried out in can be converted by RNA based catalyst, no preference for
MOPS buffer containing K+, Na+ or no ion. HT21 alone shows generating any enantiomer of sulfoxides is observed (Table 1,
no catalytic activity in the sulfoxidation reaction (Table 1, Entry 12).
Entries 1, 2). In the presence of CuL4 alone, 1a is almost
completely converted and racemic phenyl methyl sulfoxides (b) molar ratio between CuL4 and HT21, (c) reaction
2a) are produced (Table 1, Entry 6). Interestingly, K+-stabilized temperature and (d) buffer pH were optimized (Fig. S4). CD
Then, reaction conditions including (a) K+ concentration,
(
HT21 assembled with CuL4 gives full conversion and 56% ee titration spectra show that the conformation of HT21
(Table 1, Entry 7). When K+ is replaced by Na+ or removed, transforms from a random coil into an antiparallel type with
conversions are decreased dramatically and racemic products the addition of K+, further, into a stable mixed type23 by
are obtained (Table 1, Entries 9, 10). These results reveal that increasing the K+ concentration (Fig. S5). Correspondingly,
the K+ plays a vital role in inducing the G-quadruplex both conversions and enantioselectivities are increasing
architecture for chiral control.
To test whether the quadruplex helix backbone is presence of 150 mM K+ (Fig. S4a). The optimal molar ratio
indispensable, single base mutation experiment was between HT21 and CuL4 is five (Fig. S4b), albeit the binding
performed (Table 1, Entry 11). A central guanine mutation stoichiometry is unknown (vide infra). The HT21•CuL4 catalyst
continuously and going up to the highest values in the
a
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shows the best results at 15 oC and pH of 7.0 (Figs. S4c and
S4d). The following experiments were conducted under the
optimal conditions ([K+] = 150 mM, [CuL4]/[HT21] = 5, Temp. =
15 oC, pH = 7.0).
Small molecular ligands usually bind to G-quadruplex
architectures at terminal G-tetrads or grooves which can be
distinguished by biochemical characterization methods.28-33
Very recently, the CuL4 combined with dsDNA via groove
binding was identified.21 So far there is no identification of the
binding mode between G-quadruplex DNA and Cu(II)-
bipyridine. Compared to the CD profiles of HT21 alone, tiny
(a)
peak intensity changes between 250-275 nm and no ICD signal
of CuL4 are induced beyond 310 nm after the addition of CuL4
(Fig. S2a). Moreover, by adding HT21 into CuL4 solution, no
wavelength shifts, and even no absorbance changes at 307 nm
are observed in UV/vis absorption spectra (Fig. S6). What’s
more, the melting temperatures of HT21 in the absence or
presence of CuL4 are almost identical (Fig. S7). Taking the
above spectral characterization together, the binding affinity
between CuL4 and HT21 is too weak to identify their
interaction mode.
(b)
(c)
(d)
Given the above results that the CuL4 is not able to
disturb the structure of HT21 G-quadruplex, the G-quadruplex
architectures induced by K+ can represent the main structure
of HT21•CuL4 catalyst. It is well known that human telomeric
sequences are extremely polymorphic, especially for the
truncated HT21 sequence, in K+ solution.34 Phan’s and Yang’s
groups independently found that appropriate choices of
extending the flanking nucleotides at HT21 G-tetrad core can
overcome the conformational heterogeneity.35-38 The effect of
the G-quadruplex DNA sequence on the catalytic performance
of the complex was investigated by addition of flanking
deoxynucleosides. Both addition of 5′ and 3′ terminus flanking
sequences decrease the enantioselectivities obviously, but 5′
addition has a lower conversion than 3′ (Table 2, Entries 1-6).
Among these sequences, the HT21+T and TA+HT21 sequences
can fold into a single basket-antiparallel-type G-quadruplex
with two G-tetrad layers and hybrid-1 quadruplex in the K+
solution, based on the CD spectra and literatures (Fig. 1a-
c).35,39 In addition, the TA+HT21+TT sequence adopts
a
hybdrid-2 conformation (Fig. 1d).37 Reaction outcomes show
that the distinct antiparallel quadruplex affords higher
enantio-selectivity (35% ee) and conversion than the two
hybrid conformations, and the hybrid-2 one shows the lowest
catalytic activity (59% conv.) (Table 2, Entries 2, 4 and 7).
Finally, the loops modification of base types or sequence
orders caused the notable decrease of both conversions and
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In this communication, we present DNA based
enantioselective sulfoxidation reaction and up to 77% ee was
obtained, which is the highest enantioselectivity for DNA
based oxidation reaction to date. The mixed G-quadruplex
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