Synthesis of N3′-P5′-linked phosphoramidate DNA by nonenzymatic template-directed primer extension

Article abstract of DOI:10.1021/ja311164j

A fast and accurate pathway for nonenzymatic RNA replication would simplify models for the emergence of the RNA world from the prebiotic chemistry of the early earth. However, numerous difficulties stand in the way of an experimental demonstration of effective nonenzymatic RNA replication. To gain insight into the necessary properties of potentially self-replicating informational polymers, we have studied several model systems based on amino-sugar nucleotides. Here we describe the synthesis of N3′-P5′-linked phosphoramidate DNA (3′-NP-DNA) by the template-directed polymerization of activated 3′-amino-2′,3′-dideoxyribonucleotides. 3′-NP-DNA is an interesting model because of its very RNA-like A-type duplex conformation and because activated 3′-amino-2′,3′-dideoxyribonucleotides are much more reactive than the corresponding activated ribonucleotides. In contrast to our previous studies with 2′-amino-2′,3′- dideoxyribonucleotides (for which G and C but not A and T exhibit efficient template copying), we have found that all four canonical 3′-amino- 2′,3′-dideoxyribonucleotides (G, C, A, and T) polymerize efficiently on RNA templates. RNA templates are generally superior to DNA templates, and oligo-ribo-T templates are superior to oligo-ribo-U templates, which are the least efficient of the RNA homopolymer templates. We have also found that activation of 3′-aminonucleotides with 2-methylimidazole results in a ca. 10-fold higher polymerization rate relative to activation with imidazole, an observation that parallels earlier findings with ribonucleotides. We discuss the implications of our experiments for the possibility of self-replication in the 3′-NP-DNA and RNA systems.

Full text of DOI:10.1021/ja311164j

Article
pubs.acs.org/JACS
Synthesis of N3′-P5′-linked Phosphoramidate DNA by Nonenzymatic
Template-Directed Primer Extension
Shenglong Zhang, Na Zhang,† J. Craig Blain, and Jack W. Szostak*
Howard Hughes Medical Institute and Department of Molecular Biology and Center for Computational and Integrative Biology,
Massachusetts General Hospital, 185 Cambridge Street, Boston, Massachusetts 02114, United States
S
* Supporting Information
ABSTRACT: A fast and accurate pathway for nonenzymatic
RNA replication would simplify models for the emergence of
the RNA world from the prebiotic chemistry of the early earth.
However, numerous difficulties stand in the way of an
experimental demonstration of effective nonenzymatic RNA
replication. To gain insight into the necessary properties of
potentially self-replicating informational polymers, we have
studied several model systems based on amino−sugar
nucleotides. Here we describe the synthesis of N3′−P5′-
linked phosphoramidate DNA (3′-NP-DNA) by the template-
directed polymerization of activated 3′-amino-2′,3′-dideoxyribonucleotides. 3′-NP-DNA is an interesting model because of its
very RNA-like A-type duplex conformation and because activated 3′-amino-2′,3′-dideoxyribonucleotides are much more reactive
than the corresponding activated ribonucleotides. In contrast to our previous studies with 2′-amino-2′,3′-dideoxyribonucleotides
(for which G and C but not A and T exhibit efficient template copying), we have found that all four canonical 3′-amino-2′,3′-
dideoxyribonucleotides (G, C, A, and T) polymerize efficiently on RNA templates. RNA templates are generally superior to
DNA templates, and oligo-ribo-T templates are superior to oligo-ribo-U templates, which are the least efficient of the RNA
homopolymer templates. We have also found that activation of 3′-aminonucleotides with 2-methylimidazole results in a ca. 10-
fold higher polymerization rate relative to activation with imidazole, an observation that parallels earlier findings with
ribonucleotides. We discuss the implications of our experiments for the possibility of self-replication in the 3′-NP-DNA and RNA
systems.
are subject to faster hydrolysis than NTPs, further impairing
polymerization. An additional increase in reactivity without a
corresponding increase in hydrolysis can be achieved by
replacing the hydroxyl nucleophile of the nucleotide with a
better nucleophile, such as an amino group, as in the 3′-amino-
2′-hydroxy-5′-phosphorimidazolides first studied by Orgel and
co-workers.17,18 These monomers enhance the rate of template-
directed polymerization significantly, but the cyclization rate
also increases,19,20 leading to the accumulation of unreactive
3′−5′-cyclic nucleotides.
Because of the greater monomer reactivity, we have focused
on phosphoramidate-linked oligonucleotides in the search for
an effective sequence-general copying system. To eliminate the
issue of monomer cyclization, we recently explored the use of
2′-amino-2′,3′-dideoxyribonucleoside-5′-phosphorimidazolide
monomers for the rapid synthesis of N2′−P5′-linked
phosphoramidate DNA on DNA, RNA, and locked nucleic
acid (LNA) templates.21 Monomer cyclization is sterically
disfavored as a result of both geometry and ring strain, thus
drastically slowing this side reaction. Polymerization proceeds
rapidly on short homopolymeric G and C templates. However,
INTRODUCTION
■
As part of our ongoing investigations into the chemical origins
of life, we are attempting to construct model protocells capable
of autonomous replication and Darwinian evolution.1−5 Such
protocells would consist of two complementary components: a
self-replicating membrane boundary and a self-replicating
genetic polymer.4,5 Although RNA would seem to be a logical
choice for such a polymer on the basis of the considerable
evidence for an early stage in the evolution of life in which RNA
played the role of both genetic and functional biopolymer, no
effective process for either nonenzymatic or ribozyme-catalyzed
replication of RNA has yet been demonstrated. These
challenges have inspired decades of effort directed toward
improved template-directed RNA-copying chemistry as well as
studies of numerous alternative genetic polymers.6−14 Several
modifications to the structure of the biologically universal
nucleoside triphosphates (NTPs) have been employed to
improve nonenzymatic polymerization rates. Imidazole was first
used in lieu of pyrophosphate as a leaving group to increase the
reactivity, thereby allowing template-directed polymerization to
occur spontaneously.15,16 Although these monomers (ribonu-
cleoside 5′-phosphorimidazolides, ImpNs) can slowly form
phosphodiester-linked RNA, the weakly pairing bases A and U
are copied very poorly. Furthermore, these reactive monomers
Received: November 13, 2012
Published: December 19, 2012
© 2012 American Chemical Society
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Figure 1. An N3′−P5′-linked phosphoramidate DNA (3′-NP-DNA) genetic system. (a) Structures of the activated 3′-amino-2′,3′-
dideoxyribonucleoside-5′-phosphorimidazolide and 5′-phosphor-2-methylimidazolide monomers [3′-NH2-ImpddG (1a), 3′-NH2-2-MeImpddG
(1b), 3′-NH2-7-deaza-ImpddG (1c); 3′-NH2-ImpddC (2a), 3′-NH2-2-MeImpddC (2b); 3′-NH2-ImpddA (3a), 3′-NH2-2-MeImpddA (3b); 3′-
NH2-ImpddT (4a), 3′-NH2-2-MeImpddT (4b)]. (b) General scheme of a nonenzymatic primer extension reaction. A 5′-Cy3-labeled 3′-amino-
terminated DNA primer anneals to a complementary template. 3′-Aminonucleotides form Watson−Crick base pairs on a complementary template
and assemble into the chimeric DNA/3′-NP-DNA product. There is a spare 5′-terminal nucleobase on each template for favorable stacking
interactions. (c) Chemistry of the template-directed copying reaction on a DNA (X = H) or an RNA (X = OH) template using activated 3′-
aminonucleotide monomers. The attacking nucleophile is shown in red.
the 2′-amino-A and -U monomers exhibit poor polymerization.
To overcome this problem, we synthesized the corresponding
diaminopurine and C5-(1-propynyl) uracil nucleotides, which
form a strong base pair. In both the monomer and template
contexts, these nucleotides allow for the effective copying of
homopolymeric templates; however, the copying of mixed-
sequence templates remains very inefficient, possibly because of
poor fidelity.
In light of the above problems with N2′−P5′-linked
phosphoramidate DNA, we have returned to the study of 3′-
amino-2′,3′-dideoxyribonucleotides for the template-directed
synthesis of N3′−P5′-linked phosphoramidate DNA (3′-NP-
DNA). While 3′-NP-DNA has a chemical structure similar to
that of DNA, the 3′-NP-DNA duplex is structurally and
functionally more similar to duplex RNA, as the backbone
conformation in the 3′-NP-DNA duplex is remarkably close to
that of the classical RNA A-form duplex.22 In addition, the
crystal structures of 3′-NP-DNA and RNA duplexes exhibit
similar overall structure, rigidity, and hydration.23 These
properties suggest that 3′-NP-DNA may be a good model for
the study of nonenzymatic template-directed replication.
Single-nucleotide extension experiments in which azaoxybenzo-
triazole (OAt)- or 2-methylimidazole (2-MeIm)-activated
deoxyribonucleotide monomers were added to a 3′-amino-
terminated primer have been reported by Richert and co-
workers.24,25 They demonstrated that in the presence of
pyridine as a catalyst, a single activated 2′-deoxyribonucleotide
can be added to a 3′-amino-terminated primer in seconds in
nearly quantitative yield.24 In view of this rapid rate of primer
extension, it should be feasible to synthesize multinucleotide
stretches of 3′-NP-DNA on templates as long as the lifetime of
the activated monomers is longer than the time required for
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Figure 2. Real-time NMR studies of the decay of representative monomers. (a) Activated 3′-NH2-ImpddT 4a (5.0 mM) monitored at δ = −10.58
ppm by real-time 31P NMR spectroscopy over 16 h; reactions were performed at 4 °C in a solution of 100 mM HEI, 100 mM MES-CAPS-HEPES
buffer (pH 7.5), and 150 mM NaCl with 10.0 mM phosphate buffer (δ = 0 ppm) as an internal reference. (b) Real-time 31P NMR spectra of
●
▲
activated 4a (−10.58 ppm, red ) showing the increase of cyclized product 5 (2.96 ppm, green ) over time, with phosphate buffer as a reference
(blue ). (c) Activated 3′-NH2-2-MeImpddT 4b (5.0 mM) monitored over time by 31P NMR spectroscopy as above. (d) The slower decay of 5.0
■
mM activated 4b in the absence of HEI. The curves in the decay diagrams are shown for illustrative purposes. Half-times were calculated from curve
fitting to peak integration values. More detailed NMR data are presented in the Supporting Information.
template copying. Here we present the results of our studies of
template-directed polymerization with two differently activated
sets of 3′-amino monomers on a series of short homopolymer
templates with a range of helical geometries and sugar
conformations: DNA, RNA, and LNA.
monomers exhibit enhanced reactivity, rapidly polymerizing to
form N3′−P5′-linked phosphoramidate DNA (Figure 1b,c).
Monomer Cyclization. Monomer cyclization and hydrol-
ysis are undesired side reactions in all nonenzymatic template-
copying reactions. Understanding the kinetics of these side
reactions is crucial to optimization of the primer extension
reaction conditions. Generally, cyclization is faster at higher
temperatures, while primer extension is faster at lower
temperatures, presumably because of enhanced monomer
binding to the template.26,27 To obtain some insight into the
rates of these side reactions relative to that of template-directed
polymerization, we used real-time 31P NMR spectroscopy to
examine monomer cyclization and hydrolysis. We first
examined 3′-NH2-ImpddT and 3′-NH2-2-MeImpddT as
representative examples of the two families of activated
monomers. Under all of the conditions we tested for both
kinds of activated 3′-amino monomers, hydrolysis products
were negligible compared with the accumulation of 3′−5′-
cyclized monomers (Figure 2 and Figure S1 in the Supporting
Information).
RESULTS
■
Synthesis of Monomers and Template-Copying
Chemistry. We synthesized both the 3′-NH2-ImpddN (1a−
4a) and 3′-NH2-2-MeImpddN (1b−4b) monomers for each of
the four standard nucleobases (N = G, C, A, and T for 1−4,
respectively). The structures of these activated monomers are
shown in Figure 1a, and the detailed synthetic procedures are
described in the Supporting Information. During nonenzymatic
template-directed synthesis, these monomers form Watson−
Crick base pairs with complementary bases on the template
strand and participate in a chemical (nonenzymatic) primer-
extension reaction. As a result of the combined effects of the
good imidazole leaving group and the enhanced nucleophilicity
of the 3′-amino compared to the normal 3′-hydroxyl, these
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Figure 3. Nonenzymatic primer-extension reactions using 3′-NH2-ImpddG and 3′-NH2-2-MeImpddG as monomers. (a) Primer-extension reaction
scheme showing a 5′-Cy3-labeled 3′-amino-terminated DNA primer annealed to a complementary template. The 3′-NH2-ImpddG or 3′-NH2-2-
MeImpddG monomers participate in a chemical extension reaction that extends the primer by four nucleotides on the complementary template,
forming a chimeric DNA/3′-NP-DNA polymer product. The red line indicates newly formed phosphoramidate bonds. (b) High-resolution PAGE
analysis of the primer-extension products on the indicated templates. The primer-extension reaction mixtures contained 0.1 μM Cy3-labeled 3′-
amino-terminated DNA primer, 0.5 μM template, 100 mM MES-CAPS-HEPES (pH 7.5), 150 mM NaCl, and 100 mM HEI. The reaction was
initiated by addition of 5.0 mM 3′-NH2-ImpddG or 3′-NH2-2-MeImpddG. Arrows indicate the primer and full-length product. (c) High-resolution
MS analysis of the primer-extension products from a reaction of 25 pmol of 5′-Cy3-labeled 3′-amino-terminated primer extended on a d(C)4 DNA
template for 12 h followed by ethanol precipitation. (d) Monoisotopic mass for the chimeric DNA/3′-NP-DNA of Cy3-labeled 19-mer full-length
primer + 4 product: calculated mass, 6448.3328 Da; observed mass, 6448.3045 Da; error, 4.0 ppm. More detailed LC−MS data are presented in
Figures S10 and S11 and Table S1 in the Supporting Information.
We measured the rate of cyclization of both activated
monomers under the same conditions we used for template-
directed primer-extension reactions: 100 mM 2-(N-
morpholino)ethanesulfonic acid/N-cyclohexyl-3-aminopropa-
nesulfonic acid/4-(2-hydroxyethyl)-1-piperazineethanesulfonic
acid (MES-CAPS-HEPES) buffer (pH 7.5) and 100 mM NaCl
in the presence of 100 mM 1-(2-hydroxyethyl)imidazole
(HEI), a catalyst for primer-extension reactions,3,21,28 at 4 °C.
The half-time for the decay of the activated monomer 3′-NH2-
ImpddT (δ = −10.58 ppm) was 2.3 h under these conditions
(Figure 2a,b and Figure S2 in the Supporting Information),
while for 3′-NH2-2-MeImpddT, the half-time was 1.2 h (Figure
2c and Figure S3 in the Supporting Information). Under the
same conditions, 3′-NH2-2-MeImpddA and 3′-NH2-2-
MeImpddG cyclize with similar half-times of 1.1 and 1.3 h,
respectively (Figure S4 in the Supporting Information). Since,
as we previously reported, HEI enhances the rate of
nonenzymatic polymerization,3 we suspected that HEI might
also accelerate monomer cyclization. To probe the role of HEI
in this side reaction, we followed the decay of 3′-NH2-2-
MeImpddT without HEI and found that the half-time of
activated monomer decay was 26 h (Figure 2d and Figure S5 in
the Supporting Information). Thus, HEI catalyzes cyclization as
well as polymerization of activated monomers.
MES-CAPS-HEPES buffer with 100 mM HEI (Figure S6 in the
Supporting Information). The pKa of the ammonium in the
27
+
protonated 3′-NH3 -ddT is 7.7, and the pKa of protonated
imidazole is 7.05. Therefore, the slower monomer decay at the
higher pH probably results from a balance between the higher
equilibrium concentration of the free 3′-amino group (a
nucleophile) and the lower equilibrium concentration of the
protonated imidazolyl moiety (resulting in a poorer leaving
group); these factors would apply equally to primer extension.
A pH above 9 is likely to be suboptimal for primer extension,
given that G and T have pKa values between 9 and 10. We
therefore performed all of the primer-extension reactions in
MES-CAPS-HEPES buffer (pH 7.5) with HEI at 4 °C, unless
otherwise indicated.
Primer-Extension Reactions with 3′-NH2-ddN Mono-
mers on Homopolymeric DNA, RNA, and LNA Tem-
plates. We examined the ability of 3′-NH2-ImpddNs and 3′-
NH2-2-MeImpddNs to copy a series of DNA, RNA, and LNA
homopolymer templates, as illustrated schematically in Figure
1b. The use of short homopolymer templates avoids the
complexities associated with poor fidelity and post-mismatch
synthesis and focuses on the rate of primer extension in a
relatively homogeneous context. In each experiment, we used a
primer ending in a 3′-NH2 group, which was extended by four
additional nucleotides complementary to the template
oligonucleotide. Following the four template nucleotides, we
included an additional residue to provide favorable stacking
interactions. Reaction products were analyzed by polyacryla-
In addition to temperature and small-molecule catalysts, pH
can change the decay rate of activated 3′-NH2 monomers. The
half-time for the decay of activated 3′-NH2-ImpddT increased
to 6.3 h when the pH was increased from 7.5 to 9.3 in 100 mM
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Figure 4. Nonenzymatic primer-extension reactions using 3′-NH2-ImpddC and 3′-NH2-2-MeImpddC as monomers. (a) General reaction scheme
for the nonenzymatic primer-extension reaction. The red line indicates newly formed phosphoramidate bonds. (b) High-resolution PAGE analysis of
the primer-extension products on the indicated templates. Left panel: Primer-extension reactions were carried out under conditions similar to those
previously described except that the buffer pH was 8.5 and the reaction was initiated by addition of 5.0 mM 3′-NH2-ImpddC. Right panel: Primer-
extension reactions were carried out as previously described, and the reaction was initiated by addition of 5.0 mM 3′-NH2-2-MeImpddC. Arrows
indicate the primer and full-length product. (c) High-resolution MS analysis of the primer-extension products from a reaction of 25 pmol of 5′-Cy3-
labeled 3′-amino-terminated primer extended on a d(G)4 DNA template for 12 h followed by ethanol precipitation. (d) Monoisotopic mass for the
chimeric DNA/3′-NP-DNA of Cy3-labeled 19-mer full-length primer + 4 product: calculated mass, 6288.3083 Da; observed mass, 6288.2864 Da;
error, 3.5 ppm. More detailed LC−MS data are presented in Figure S12 and Table S2 in the Supporting Information.
mide gel electrophoresis (PAGE) and liquid chromatography−
mass spectrometry (LC−MS).
We next evaluated the activity of 3′-NH2-ImpddC and 3′-
NH2-2-MeImpddC monomers in copying G4 DNA and RNA
templates, each with a terminal 5′-T/U (Figure 4a). We
observed efficient copying of these templates by the two
differently activated C monomers (Figure 4b), but the reactions
were somewhat slower than the copying of C4 templates by G
monomers. Again, the 2-methylimidazolides exhibited faster
primer extension: 3′-NH2-2-MeImpddC copied the r(G)4 RNA
template in 20 min, reaching 85% completion, while the 3′-
NH2-ImpddC took more than 4 h to achieve 80% completion.
The identity of the N + 4 gel bands was further confirmed
through LC−MS (Figure 4c,d). We observed less primer
extension on the DNA template using both 3′-NH2-ImpddC
and 3′-NH2-2-MeImpddC, possibly because of competing G-
quadruplex formation of the template strands and/or weaker
stacking of C monomers on the DNA primer template. Control
extension reactions using 3′-NH2-2-MeImpddC on non-
complementary templates showed only trace primer + 1
product and no full-length primer + 4 product at 2 h (Figure S8
in the Supporting Information).
In general, A:T base pairs are problematic in nonenzymatic
primer-extension reactions.2,21 We therefore asked whether the
superior performance of activated 3′-NH2 G and C monomers
would extend to the efficient copying of complementary
templates by 3′-NH2 A and T monomers. Remarkably, 3′-NH2-
2-MeImpddT monomer (10 mM) resulted in excellent primer
extension on homopolymeric A4 DNA and RNA templates. We
observed ∼80% full-length product on the RNA template in 20
min (Figure 5b), similar to the copying of the G4 template by
3′-NH2-2-MeImpddC. The correct mass was observed for the
Since G is generally the most efficiently incorporated
monomer in primer-extension experiments, we began by
examining the copying of DNA and RNA C4 templates by
template-directed primer extension in the presence of 5 mM 3′-
NH2-ImpddG at 4 °C (Figure 3b). In both cases, the primer +
1, primer + 2, and primer + 3 intermediates were visible after 5
min, and the reaction had reached >92% completion (primer +
4 product) within 10 min. To test the idea that the
incorporation of G monomers might be limited by the
competing formation of G-quartets, we prepared the analogous
7-deaza-G activated monomer 1c. Although this monomer
cannot form G-quartets, we did not observe any enhancement
of the rate of template-directed primer extension (Figure S7 in
the Supporting Information). On the basis of previous reports
of faster and more regiospecific template copying with G
activated with 2-methylimidazole rather than imidazole,29 we
prepared the corresponding 3′-NH2-2′,3′-dideoxy-5′-(2-
methyl)phosphor-imidazolide. We observed that 3′-NH2-
MeImpddG resulted in faster and more complete copying of
both the d(C)4 and r(C)4 templates, reaching essentially
quantitative yield (>98%) in less than 5 min on both templates
(Figure 3b). The identity of the full-length primer + 4 product
was confirmed by LC−MS analysis (Figure 3c,d). On all of the
noncomplementary templates, primer extensions with the 2-
methylimidazolide monomer gave only trace N + 1 products
after 2 h, and we observed no longer products (Figure S8 in the
Supporting Information).
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Figure 5. Nonenzymatic primer-extension reactions using 3′-NH2-ImpddT and 3′-NH2-2-MeImpddT as monomers. (a) General reaction scheme
for the nonenzymatic primer-extension reaction. The red line indicates newly formed phosphoramidate bonds. (b) High-resolution PAGE analysis of
the primer-extension products on the indicated templates. Primer-extension reactions were carried out as previously described; the reaction was
initiated by the addition of 10 mM 3′-NH2-ImpddT or 3′-NH2-2-MeImpddT. Arrows indicate the primer and full-length product. (c) High-
resolution MS analysis of the primer-extension products from a reaction of 30 pmol of 5′-Cy3-labeled 3′-amino-terminated primer extended on an
r(A)4 RNA template for 12 h followed by ethanol precipitation. (d) Monoisotopic mass for the chimeric DNA/3′-NP-DNA of Cy3-labeled 19-mer
full-length primer + 4 product: calculated mass, 6348.3070 Da; observed mass, 6348.2936 Da; error, 2.1 ppm. More detailed LC−MS data are
presented in Figure S13 and Table S3 in the Supporting Information.
N + 4 product (Figure 5c,d), confirming its identity. As
observed before, RNA is a superior template, and the copying
efficiency was lower on the d(A)4 template. Notably, a
significant portion of N + 3 product remained unreacted,
possibly because of the weak stacking of the pyrimidine
monomers. Primer extension with 5 mM 3′-NH2-2-MeImpddT
was much slower than when the concentration was 10 mM
(Figure S9 in the Supporting Information), suggesting that
monomer binding may be cooperative and that the template is
not saturated with T monomer at 5 mM and may not be
saturated even at 10 mM. Control extension reactions on the
C4 and T4 noncomplementary templates showed only trace
primer + 1 product but no full-length primer + 4 product at 2 h.
However the r(G)4 RNA template (Figure S8 in the Supporting
Information) yielded significant levels of primer + 1, primer +
2, and primer + 3 products and even a trace of the primer + 4
product. This result suggests that the formation of G:T wobble
base pairs will be a significant problem in the 3′-NP-DNA
system. As before, the imidazolides were less active than the 2-
methylimidazolides. It took more than 4 h for the 3′-NH2-
ImpddT monomer to copy the same r(A)4 RNA template,
reaching 80% full-length product (Figure 5b). On the d(A)4
DNA template, only half of the primer was extended to full-
length primer + 4 product in 12 h, compared to ∼50%
conversion in 20 min for 3′-NH2-2-MeImpddT.
template the primer-extension reaction and a terminal 5′-A for
favorable stacking interactions (Figure 6). The LNA template,
with a rigid C3′-endo sugar conformation, was the most
efficient template, yielding 71% N + 4 full-length product and
24% N + 5 product with 3′-NH2-2-MeImpddA in 10 min
(Figure 6c). The DNA template was less efficient, taking more
than 1 h to achieve 54% full-length product. As in other base
and backbone contexts, 3′-NH2-ImpddA was less active than
the 2-methylimidazolide: it took longer than 10 min to copy the
LNA template and more than 12 h to copy the DNA template
(Figure 6b). Surprisingly, we found that the r(U)4 RNA
template was copied less efficiently than d(T)4 DNA template
by both 3′-NH2 monomers, although 3′-NH2-2-MeImpddA
was still better than 3′-NH2-ImpddA, leading to 40% full-length
product at 1 h. We suspected that the relatively poor copying of
the r(U)4 RNA template might have been due to the weaker
stacking interactions of U compared with T, resulting in a more
disordered RNA template. Therefore, we also examined the
activity of r(T)4 RNA as a template. When the monomer was
3′-NH2-ImpddA, the r(T)4 RNA template resulted in 52% full-
length product at 12 h, compared with 10% full-length product
on the standard r(U)4 RNA template. The r(T)4 RNA template
was also superior to the d(T)4 DNA template, which had only
reached 35% full-length product at the same time point. A
similar enhancement was also observed with the 3′-NH2-2-
MeImpddA monomer, where the r(T)4 RNA template reached
67% full-length product in 1 h, compared with 40% full-length
product on the standard r(U)4 RNA template (Figure 6c). The
observation that T is significantly better than U in the template
suggests that other simple nucleobase modifications that would
further accelerate polymerization might be identified. Control
extension reactions using 3′-NH2-2-MeImpddA on non-
Encouraged by the efficient template copying observed using
the activated 3′-aminonucleotides of guanosine, cytidine, and
thymidine, we proceeded to assess the activated adenosine
nucleotides by using 10 mM 3′-NH2-ImpddA and 3′-NH2-2-
MeImpddA in primer-extension reactions. In addition to DNA
and RNA templates, we also examined an LNA template in this
sequence context. Each template had a T4 or U4 sequence to
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Figure 6. Nonenzymatic primer-extension reactions using 3′-NH2-ImpddA and 3′-NH2-2-MeImpddA as monomers. (a) General reaction scheme
for the nonenzymatic primer-extension reaction. The red line indicates newly formed phosphoramidate bonds. (b, c) High-resolution PAGE analysis
of the primer-extension products on the indicated templates. Primer-extension reactions were carried out as previously described; the reaction was
initiated by addition of 10 mM 3′-NH2-ImpddA or 3′-NH2-2-MeImpddA. Arrows indicate the primer and full-length product. (d) High-resolution
MS analysis of the primer-extension product from the reaction of 30 pmol of 5′-Cy3-labeled 3′-amino-terminated primer extended on a d(T)4 DNA
template for 12 h followed by ethanol precipitation. (e) Monoisotopic mass for the chimeric DNA/3′-NP-DNA of Cy3-labeled 19-mer full-length
primer + 4 product: calculated mass, 6384.3532 Da; observed mass, 6384.3454 Da; error, 1.2 ppm. More detailed LC−MS data are presented in
Figure S14 and Table S4 in the Supporting Information.
complementary templates showed only trace N + 1 product
and did not generate any full-length N + 4 product at 2 h
(Figure S8 in the Supporting Information).
For the d(T)4 template, we observed the expected mass of
the primer + 4 product by LC−MS (Figure 6d,e; lowest
monoisotopic mass, 6384.3454 Da; calculated mass 6384.3532
Da; error, 1.2 ppm). Besides the desired N + 4 products, we
also detected incomplete N + 3 products as well as N + 5
products (Figure S14 and Table S4 in the Supporting
Information).
lack of a requirement for divalent cations and the need for only
low monomer concentrations, warrants the further develop-
ment of the 3′-aminonucleotide/phosphoramidate DNA
system.
In contrast to RNA monomers (e.g., 2-MeImpN’s), which
degrade largely by hydrolysis, cyclization is the main reason for
decay of activated 3′-NH2 monomers. The similar rates of
cyclization for the 2-methylimidazole-activated 3′-NH2 A, G,
and T monomers suggests that differences in the 3′-NH2
nucleophilicity or sugar conformation cannot account for the
observed differences in the rates of polymerization of these
nucleotides. Fortunately, in most cases template-directed
polymerization is faster than monomer cyclization. Under our
typical extension conditions with 2-methylimidazolides, most of
the primer-extension reactions proceeded to completion on
homopolymer templates within 20 min, which is less than one-
third of the half-life of a 2-methylimidazolide monomer under
the same conditions. These experiments suggest that despite
competing cyclization, it should be feasible to carry out longer
primer-extension reactions, perhaps involving up to 10−15
DISCUSSION
■
Our observation of rapid, efficient nonenzymatic template-
directed synthesis of short tracts of 3′-NP-DNA using activated
3′-aminonucleotides is an encouraging step toward the
demonstration of a chemically self-replicating genetic polymer.
The most striking advantage of this system relative to
previously studied nucleic acids is that all four canonical
nucleotides exhibit robust primer-extension synthesis on
complementary templates. This property, together with the
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nucleotides, using activated 3′-amino monomers. Periodic
replacement of the reaction solution with fresh monomers, as
has been reported using immobilized RNA,30 may allow for the
nonenzymatic copying of even longer templates.
might be realized with this polymer. However, in order to reach
this goal, the fidelity of template-directed copying must be
improved. Our observation of extensive polymerization of T
and rT on a G4 template suggests that G:T wobble pairing in
particular must be disfavored; a possible means of accomplish-
ing this would be to replace T (or rT) with the corresponding
2-thionucleotide, as previously suggested.36 We are currently
pursuing these and other avenues in our efforts to demonstrate
efficient and accurate self-replication of phosphoramidate
nucleic acids.
The use of 2-methylimidazole rather than imidazole as a 5′
activating group for ribonucleotides results in remarkably
enhanced rates and 3′ versus 2′ regiospecificity for template-
directed primer extension in an all-RNA system.29 We observed
a similar 5−10-fold enhancement of the rate of polymerization
using 3′-aminonucleotides activated with 2-methylimidazole
versus imidazole. The reason for this enhancement is still
unknown, although many effects, including higher pKa, steric
effects, stacking, and sugar/phosphate conformation, could
make contributions. Other leaving groups, especially benzo-
triazole esters such as the −OAt group, have been reported to
lead to significantly improved rates of primer extension
compared with 2-methylimidazole-activated DNA and RNA
monomers, although the magnitude of these effects varies
considerably for different nucleotides.24,25,31
The activated 3′-NH2-ddA and 3′-NH2-ddT nucleotides
performed unexpectedly well in copying their homopolymeric
DNA, RNA, or LNA templates. This is particularly surprising
because our previous experiments with activated 2′-amino
monomers showed very poor polymerization of 2′-NH2-
ImpddA and 2′-NH2-ImpddT under identical conditions,21
even though 2′-NH2-ImpddG and -C behave in a qualitatively
similar manner to 3′-NH2-ImpddG and -C. In the analogous
RNA copying reactions using activated ribonucleotides, AA or
TT steps on the template resulted in virtually complete
inhibition of replication.32 The improved incorporation of A
and T monomers in the 3′-amino system versus the 2′-amino
system points to previously unsuspected nucleobase-specific
subtleties in the details of the reaction mechanism. One
possibility is that an electron-donating 2′-amino substituent
may affect the nucleobase pKa values or sugar puckering in a
way that further weakens A:T base pairing, whereas a 3′-amino
substituent, being further from the glycosidic linkage, might not
have this effect.33,34 Another example of a nucleobase-specific
effect is our observation that an r(T)4 template is significantly
better than an r(U)4 template in directing the polymerization of
activated 3′-amino A monomer. Presumably this improvement
reflects stronger or altered stacking interactions that cause the
r(T)4 template to be more ordered.
MATERIALS AND METHODS
■
Synthesis of 3′-NH2-ImpddNs and 3′-NH2-2-MeImpddNs.
The synthesis and characterization of 3′-NH2-ImpddNs (including 3′-
NH2-7-deaza-ImpddG) and 3′-NH2-2-MeImpddNs are described in
the Supporting Information.
Real-Time NMR Studies of the Decay of 3′-NH2 Monomers.
3′-NH2-ImpdT (5.0 mM) in a 500 μL aqueous solution containing
150 mM NaCl, 100 mM HEI, 100 mM MES-CAPS-HEPES buffer
(pH 7.5), 10% D2O, and 10.0 mM sodium phosphate buffer as a
reference was studied by 31P NMR spectroscopy at 161.8 MHz on a
Varian NMR spectrometer (Oxford AS-400). Spectra were collected
every 1 h at 4 or 25 °C. More details of the NMR studies are described
in the Supporting Information.
Nonenzymatic Primer-Extension Reactions. Template-copying
reactions contained 0.1 μM Cy3-labeled 3′-amino-terminated
primer,3,21 0.5 μM template oligonucleotide, 150 mM NaCl, 100
mM HEI, 100 mM MES-CAPS-HEPES buffer (pH 7.5), and 3′-NH2-
ImpddN or 3′-NH2-2-MeImpddN at the indicated concentrations.
Reactions were initiated by addition of the activated monomer and
incubated at 4 °C. Aliquots were removed and stopped at indicated
time points by addition of three volumes of formamide and heating to
95 °C for 10 min, followed immediately by ethanol precipitation on
dry ice. Stopped reactions were resuspended in 8.0 M urea and heated
to 95 °C for 5 min. Samples were analyzed by electrophoresis on 7.0
M urea, 17% polyacrylamide sequencing gels. Reaction products were
visualized by fluorescence imaging on a Typhoon 9410 Phosphor-
Imager using the Cy3 fluorophore filter set. Product quantification and
analysis were performed using ImageQuant TL software (GE
Healthcare Life Sciences).
LC−MS Studies of Products from Nonenzymatic Primer-
Extension Reactions. Primer-extension products analyzed by LC−
MS were prepared by extending 25−30 pmol of the 5′-Cy3-labeled 3′-
amino-terminated DNA primer at 4 °C for 12 h on a complementary
DNA or RNA template under conditions similar to those described
previously.21 Reactions were stopped by addition of three volumes of
8.0 M urea and heating to 95 °C for 10 min, followed immediately by
ethanol precipitation on dry ice. Samples were dried by Speedvac and
redissolved in 40 μL of LC-grade water; 35 μL aliquots were injected
for analysis on an Agilent Q-TOF LC−MS instrument. More detailed
LC−MS procedures are provided in the Supporting Information.
The kinetics of primer extension using activated 3′-
aminonucleotides is unusual in that the full-length (primer +
4) product often began to accumulate at early time points, at
which considerable unused primer remained, and only low
levels of intermediates (primer + 1, primer + 2, and primer + 3)
were seen. Such kinetics could result from a highly cooperative
primer-extension process or, alternatively, could reflect
polymerization of monomers either on or off the template
followed by addition of di-, tri-, or tetranucleotides to the
primer. More detailed kinetic analysis may allow these models
to be distinguished from one another.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, characterization data, Figures S1−
S14, and Tables S1−S4. This material is available free of charge
via the Internet at http://pubs.acs.org.
Templates that are more constrained toward an A-type
helical conformation and 3′-endo sugar pucker (i.e., LNA >
RNA > DNA) are more favorable for 3′-amino monomer
polymerization, consistent with our previous observations using
2′-amino monomers.21,35 Since N3′−P5′-linked phosphorami-
date DNA is very similar to RNA in terms of overall duplex
structure, rigidity, and hydration, we anticipate that 3′-NP-
DNA may also function as an efficient template, suggesting the
tantalizing possibility that successive rounds of self-replication
AUTHOR INFORMATION
■
Corresponding Author
szostak@molbio.mgh.harvard.edu
Present Address
†High Magnetic Field Laboratory, Hefei Institutes of Physical
Science, Chinese Academy of Sciences, Hefei 230031, P. R.
China.
931
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(31) Vogel, S. R.; Deck, C.; Richert, C. Chem. Commun. 2005, 4922−
4924.
(32) Zielinski, M.; Kozlov, I. A.; Orgel, L. E. Helv. Chim. Acta 2000,
Notes
The authors declare no competing financial interest.
83, 1678−1684.
ACKNOWLEDGMENTS
■
(33) Patra, A.; Paolillo, M.; Charisse, K.; Manoharan, M.; Rozners, E.;
Egli, M. Angew. Chem., Int. Ed. 2012, 51, 11863−11866.
(34) Williams, A. A.; Darwanto, A.; Theruvathu, J. A.; Burdzy, A.;
Neidigh, J. W.; Sowers, L. C. Biochemistry 2009, 48, 11994−12004.
(35) Zhang, N.; Zhang, S.; Szostak, J. W. J. Am. Chem. Soc. 2012, 134,
3691−3694.
We are grateful to the members of our laboratory for helpful
discussions and comments on the manuscript. This research
was funded in part by Grant CHE-0809413 from the NSF.
J.W.S. is an Investigator of the Howard Hughes Medical
Institute.
(36) Szostak, J. W. J. Syst. Chem. 2012, 3, 2.
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