An Alkyne-Appended, Click-Ready PtII Complex with an Unusual Arrangement in the Solid State

Article abstract of DOI:10.1002/anie.201409853

To better understand the range of cellular interactions of Pt^II-based chemotherapeutics, robust and efficient methods to track and analyze Pt targets are needed. A powerful approach is to functionalize Pt^II compounds with alkyne or azide moieties for post-treatment conjugation through the azide-alkyne cycloaddition (click) reaction. Herein, we report an alkyne-appended cis-diamine Pt^II compound, cis-[Pt(2-(5-hexynyl)amido-1,3-propanediamine)Cl₂] (1), the X-ray crystal structure of which exhibits a combination of unusual radially distributed CH/??(Ca??C) interactions, Pt-Pt bonding, and NH:O/NH:Cl hydrogen bonds. In solution, 1 exhibits no Pt-alkyne interactions and binds readily to DNA. Subsequent click reactivity with nonfluorescent dansyl azide results in a 70-fold fluorescence increase. This result demonstrates the potential for this new class of alkyne-modified Pt compound for the comprehensive detection and isolation of Pt-bound biomolecules.

Full text of DOI:10.1002/anie.201409853

Angewandte
Chemie
DOI: 10.1002/anie.201409853
Bioorthogonal Chemistry
An Alkyne-Appended, Click-Ready PtII Complex with an Unusual
Arrangement in the Solid State**
Jonathan D. White, Lindsay E. Guzman, Lev N. Zakharov, Michael M. Haley, and
Victoria J. DeRose*
Abstract: To better understand the range of cellular interac-
tions of PtII-based chemotherapeutics, robust and efficient
methods to track and analyze Pt targets are needed. A powerful
approach is to functionalize PtII compounds with alkyne or
azide moieties for post-treatment conjugation through the
azide–alkyne cycloaddition (click) reaction. Herein, we report
an alkyne-appended cis-diamine PtII compound, cis-[Pt(2-(5-
hexynyl)amido-1,3-propanediamine)Cl2] (1), the X-ray crystal
structure of which exhibits a combination of unusual radially
needed to better understand and thus design Pt drugs and
drug treatments.
To broadly study cellular Pt interactions, we aim to
functionalize Pt complexes with bioorthogonally reactive
handles. Two azide-functionalized PtII compounds have
recently been described. Our group has reported the synthesis
and utility of an azide-functionalized picoplatin derivative for
reaction through copper(I)-catalyzed azide–alkyne cycload-
dition (CuAAC).[5] Furthermore, an extended acridine-func-
tionalized, monofunctional PtII compound has been used in
fluorescence post-labeling experiments.[6] While the utility of
these Pt-azide reagents is clearly demonstrated, Pt-alkyne
reagents capable of click chemistry would provide additional
advantages. These include reduced hydrophilicity (vs. azide
analogues) and access to complementary azide-modified
partner reagents. Bifunctional PtII compounds with accessible
alkyne modifications are, however, rare.
ꢀ
À
distributed CH/p(C C) interactions, Pt Pt bonding, and
À
NH:O/NH:Cl hydrogen bonds. In solution, 1 exhibits no Pt
alkyne interactions and binds readily to DNA. Subsequent
click reactivity with nonfluorescent dansyl azide results in a 70-
fold fluorescence increase. This result demonstrates the
potential for this new class of alkyne-modified Pt compound
for the comprehensive detection and isolation of Pt-bound
biomolecules.
As an alternative to azide-functionalized Pt analogues, we
present herein a Pt-alkyne species for use with “turn-on”
azide-containing fluorophores (Figure 1). We report the syn-
thesis and characterization of alkyne-functionalized Pt com-
P
latinum(II) chemotherapeutics are used ubiquitously in
cancer treatments today. An estimated 50–70% of all regimes
include one of the first- and second-generation FDA-
approved Pt-based drugs cisplatin, carboplatin, and oxalipla-
tin.[1] Drug binding to the purine bases of DNA is an accepted
cause of anticancer activity; however, the effects of binding to
alternative cellular targets remain difficult to study and
poorly understood.[2] Indeed, less than 10% of the Pt (in the
case of cisplatin) accumulates within genomic DNA.[3] It is
known that Pt also binds to RNA, proteins, and other small
molecules (e.g., glutathione) in cells; however, the down-
stream consequences of Pt binding to these alternative targets
are far less understood, especially with respect to how they
relate to cytotoxicity and drug resistance.[4] Novel methods of
analyzing alternative Pt binding and target interactions are
Figure 1. Target DNA bound by compound 1 undergoing click ligation
with the “turn-on” fluorophore dansyl azide.
plex 1 and demonstrate its click reactivity with the “turn-on”
dansyl azide fluorophore in a model Pt-binding and CuAAC
click reaction. Additionally, X-ray structure analysis of
1 reveals a remarkable arrangement within the crystal lattice
[*] J. D. White, L. E. Guzman, Prof. M. M. Haley, Prof. V. J. DeRose
Department of Chemistry & Biochemistry and
Institute of Molecular Biology, University of Oregon
Eugene, OR 97403-1253 (USA)
ꢀ
À
that contains both CH/p(C C) and Pt Pt bonding interac-
tions. The availability of Pt-alkyne reagents permits the future
use of click conjugation to a variety of azide-modified
molecular probes for Pt target elucidation.
E-mail: derose@uoregon.edu
Dr. L. N. Zakharov
CAMCOR—Center for Advanced Materials Characterization in
Oregon, University of Oregon
Compound 1 was designed to mimic the difunctional
reactivity and cis geometry of cisplatin. Reduction of tert-
butyl 2-azidopropane-1,3-diyldicarbamate with tin(II) chlo-
ride yielded the Boc-protected amine,[7] which was treated
with 5-hexynoic acid to form the peptide-coupled product
(Scheme 1). The Boc-protected amide was reacted with cis-
[Pt(DMSO)2Cl2] after deprotection to afford 1. Crystals were
deposited from the crude reaction mixture upon the addition
Eugene, OR 97403-1443 (USA)
[**] We thank the National Science Foundation (CHE-1153147) for
support of this research. L.E.G. acknowledges the University of
Oregon McNair Scholars Program. We thank Alexander J. Kendall
for his contribution in illustrating the scorpion-themed artwork.
Supporting information for this article (including experimental
details) is available on the WWW under http://dx.doi.org/10.1002/
anie.201409853.
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actions has rarely been described.[13] The crystal structure of
1 also includes a Pt–Pt distance of 3.34 ꢀ, which is indicative
of Pt–Pt orbital overlap.[15] Hydrogen bonds involving the
carbonyl oxygen and halide acceptors with the amine ligand
donors are also observed. In all, three very different types of
intermolecular interactions in the crystal structure of 1 pro-
vide for an unusual solid-state arrangement.
To confirm its target binding and subsequent click
reactivity, 1 was bound to a 5’-GG-3’ DNA hairpin sequence,
followed by click conjugation with the dansyl azide fluoro-
phore (Figure 3). A suspension of 1 in 20% v/v DMF was
added to folded hairpin DNA in two-fold molar excess and
allowed to react at 378C for 18 h. After removal of unbound
1 with Sephadex G25 resin, the 1-bound DNA was reacted
with dansyl azide (1 equiv) with CuSO4 and sodium ascorbate
in 0.6% v/v CH3CN. Fluorescence of the 1-bound DNA
strand is revealed only upon click conjugation, since
unreacted dansyl azide is nonfluorescent.[16] Despite wide-
spread use of the dansyl group in fluorescent labeling and
detection methods,[17] to our knowledge, the ability of dansyl
azide to become a “turn-on” fluorophore upon azide–alkyne
cycloaddition has not been reported. This turn-on fluores-
cence capability of 1 with azide-containing fluorophores
demonstrates its potential utility for cellular localization
studies, as well as other imaging studies.
While the usual click reaction between azides and alkynes
produces the thermodynamically stable 1,2,3-triazole, click
reactions with sulfonyl azides may produce additional prod-
ucts upon subsequent rearrangement and elimination of N2.
Upon Cu-catalyzed formation of the sulfonyl triazole, rear-
rangement and elimination of N2 may lead to the formation of
stable N-sulfonyl amides.[18] Owing to the reduced stability of
N-sulfonyl triazoles and the use of aqueous CuSO4 as the
catalytic Cu source,[18c] we hypothesize that the product
formed in the reaction between 1-bound DNA and dansyl
azide is the N-sulfonyl amide conjugated species. To confirm
the fluorescent properties of the N-sulfonyl amide, dansyl N-
sulfonyl amide was synthesized through click ligation with
Scheme 1. Synthesis of PtII complex 1. Boc=tert-butoxycarbonyl,
EDC=1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, DMSO=di-
methyl sulfoxide, DBU=1,8-diazabicyclo[5.4.0]undec-7-ene,
DMF=N,N-dimethylformamide.
of water (see the Supporting Information). While alkyne–Pt
complexes comprise a diverse, well-known field, PtII com-
plexes in which a terminal alkyne moiety is attached through
organic linkers and not directly bound to the Pt metal are far
less common. These usually incorporate bulkier aromatic or
exchange-inert ligands around the Pt coordination sphere to
discourage Pt–alkyne interactions.[8] De Sousa et al. reported
a moderately unhindered propargyl-containing chloroquino-
line derivative coordinated to PtII.[9] By contrast, Parker and
co-workers reported on the incompatibility of cis-[Pt(NH3)2-
(OH2)2]2+ with a small alkyne-containing O,O’-bidentate
ligand in aqueous solution.[10] It was assumed that the
insoluble brown solid that formed was a result of PtII-
catalyzed polymerization reactions with the alkyne. In the
case of 1, formation of the desired complex through a route
avoiding the highly reactive Pt–H2O-containing species was
desired and proved successful. Despite concerns that 1 might
yet undergo unwanted side reactions in solution, such as Pt-
catalyzed hydration or hydroamination across the triple bond,
no such reactivity of 1 was observed over several hours (in d7-
DMF) by 1H, 13C and 195Pt NMR spectroscopy (see the
Supporting Information).
The X-ray crystal structure of 1 reveals a network of six
ꢀ
CH/p(C C) bonds with the terminal alkyne group acting as
both hydrogen-bond donor and
hydrogen-bond
acceptor
(Figure 2).[11,12] It has been shown
that the acidity of the terminal
alkyne proton and electron-rich
p orbitals of the alkyne group give
rise to a relatively weak hydro-
gen-bonding interaction. In the
ꢀ
case of 1, the CH/p(C C) hydro-
gen bonds form a radially sym-
metric wheel or spoke-type
arrangement, with alkyne–hydro-
gen distances of 2.83 ꢀ and bond
angles of less than 1808, which are
within the range of previously
ꢀ
reported values for CH/p(C C)
interactions.[13,14] While Steiner,
Boese, and others have reported
several “zig-zag” formations in
Figure 2. a) Crystal structure arrangement of 1 showing the radial distribution of alkyne “tails.” b) An
alternate view of the crystal structure showing Pt–Pt stacking. Selected bond lengths (ꢀ) and angles
À
À
ꢀ
(8): Pt Cl 2.3137(11), 2.3028(9); Pt N 2.042(3), 2.047(3); C C 1.174(9); Pt···Pt 3.3367(5), H(N)···O (as
the solid state,[13c,14]
a cyclic
À ꢀ
shown) 2.14(3); C C C 178.0(8). NH:O/NH:Cl hydrogen bonds are also present (not shown). See the
arrangement of CH/p(C C) inter- Supporting Information and deposited structure[11] for detailed crystallographic information.
ꢀ
2
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chemie
bound 1 undergoes covalent ligation with dansyl azide,
demonstrated here to be a convenient turn-on fluorophore
upon alkyne–azide cycloaddition. Together, these experi-
ments highlight the unique ability of 1 to report on the target
binding and localization of Pt compounds, thus advancing
high-throughput and comprehensive studies on the cellular
distributions, pathways, and mechanisms of action of metal-
lodrugs.
Received: October 7, 2014
Published online: && &&, &&&&
Keywords: bioorthogonal chemistry · click chemistry ·
.
fluorescent probes · metallodrugs · platinum
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characterized by 1H and 13C NMR, mass spectrometry,
fluorescence spectroscopy, and UV/Vis spectroscopy (see
the Supporting Information). Importantly, the dansyl N-
sulfonyl amide exhibits a fluorescence quantum yield of 0.35
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reaction with methyl propargyl ether, an approximately 70-
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In summary, we report the unusual, bifunctional alkyne-
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ꢀ
1 exhibits CH/p(C C) interactions in a rare six-fold radially
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methyl propargyl ether, revealing an approximately 70-fold fluorescence
increase.
29.893(4) ꢀ, c = 7.6143(9) ꢀ, V= 5892.7(12) ꢀ3, Z = 18, 1cald
=
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Communications
R1 = 0.0221, wR2 = 0.0446 and GOF = 1.051 for 3158 reflections
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4
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Bioorthogonal Chemistry
Poised for click: The alkyne-functional-
ized probe Scorpio-Pt, developed to
J. D. White, L. E. Guzman, L. N. Zakharov,
M. M. Haley,
investigate the cellular interactions of Pt-
À
based therapeutics, exhibits no Pt alkyne
V. J. DeRose*
&&&& — &&&&
interactions and binds readily to DNA.
Subsequent click reactivity with fluoro-
genic dansyl azide results in a 70-fold
fluorescence increase. The PtII compound
exhibits an unusual solid-state arrange-
An Alkyne-Appended, Click-Ready PtII
Complex with an Unusual Arrangement in
the Solid State
ꢀ
À
ment, with CH/p(C C) interactions, Pt
Pt bonding, and NH:O/NH:Cl hydrogen
bonds.
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5
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