A Ca2+-, Mg2+-, and Zn2+-Based Dendritic Contractile Nanodevice with Two pH-Dependent Motional Functions

Article abstract of DOI:10.1002/anie.201506474

A contractile dendritic motional device is reported where metal ions with biological importance - Ca²⁺ (the main regulatory and signaling species of the natural muscles), Mg²⁺, and Zn²⁺ - initiate two kinds of motional functions. The first motional function is the metal-ion-induced contraction of a linear strand into a Z-shaped dinuclear complex, and the second one is the change of the height of Z-shaped complexes via transmetalation. By means of the pH-dependent counterligand tren, the two motional features of the machine can depend on alternate additions of acid and base. An optical response is associated with the conversion of the linear form (which is yellow) into the metalated Z-shaped one (which is red).

Full text of DOI:10.1002/anie.201506474

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International Edition: DOI: 10.1002/anie.201506474
German Edition: DOI: 10.1002/ange.201506474
Molecular Machines
A Ca2+-, Mg2+-, and Zn2+-Based Dendritic Contractile Nanodevice with
Two pH-Dependent Motional Functions
Adrian-Mihail Stadler,* Lydia Karmazin, and Corinne Bailly
˘
In memory of Areta Macelaru
amplitude, along with a shape change of the whole
dendrimer (Figure 1a).
2) A reversible jack-like motion between almost planar and
three-dimensional Z-shaped units, consisting of a height
change induced by the displacement of Ca2+ by Zn2+
(Figure 1b).
Abstract: A contractile dendritic motional device is reported
where metal ions with biological importance—Ca2+ (the main
regulatory and signaling species of the natural muscles), Mg2+,
and Zn2+—initiate two kinds of motional functions. The first
motional function is the metal-ion-induced contraction of
a linear strand into a Z-shaped dinuclear complex, and the
second one is the change of the height of Z-shaped complexes
via transmetalation. By means of the pH-dependent counter-
ligand tren, the two motional features of the machine can
depend on alternate additions of acid and base. An optical
response is associated with the conversion of the linear form
(which is yellow) into the metalated Z-shaped one (which is
red).
T
he contraction of muscle fibers is induced by Ca2+, which is
the main regulatory and signaling species in the machines of
everyday life, the muscles.[1] Mg2+ also plays important
biological roles[2] including formation of Mg-ATP and influ-
encing muscle contraction. In parallel, much interest has been
focused on artificial nanomachines, including chemical molec-
ular machines.[3] Nevertheless, Ca2+- and Mg2+-based syn-
thetic molecular machines are poorly represented to date. We
report herein a pH-modulable contractile dendrimer, in
which contraction/extension motions are induced, through
a mechanism that differs from that of natural muscles, by
metal ions of biological importance (Ca2+, Mg2+, and Zn2+).
Moreover, while most molecular machines can perform
(generally reversibly) only one type of controlled motion, the
dynamic device reported herein can successively accomplish
two[4] different kinds of controlled motional functions:
1) A metal-ion-induced reversible contraction[5] of a linear
strand into a Z-like dinuclear complex, which includes the
motion of one end with respect to another with significant
Figure 1. Representation of the two types of motional functions of the
molecular machine[3] reported herein. a) Type 1: reversible contraction;
b) Type 2: reversible jack-like motion.
[*] Dr. A.-M. Stadler
Institut de Science et d’IngØnierie SupramolØculaires (UMR 7006)
CNRS and UniversitØ de Strasbourg
8 AllØe Gaspard Monge, 67000 Strasbourg (France)
and
Institut für Nanotechnologie (INT)
Karlsruhe Institut für Technologie (KIT)
76344 Eggenstein-Leopoldshafen (Germany)
E-mail: mstadler@unistra.fr
In this way the nature of the motional function is
controlled. One can also control the occurrence of the
motion by acid and base triggers, by means of a pH-dependent
enveloping ligand that releases metal ions on protonation.
This multiple modulation of the motion is of importance for
functional nanomaterials with controllable mechanical prop-
erties. In addition, this device is amongst the few types of
dendritic[6] mechanical molecular machines[3j,6a–f] reported to
date.
Dr. L. Karmazin, C. Bailly
Service de Radiocristallographie, UniversitØ de Strasbourg
1, rue Blaise Pascal, 67000 Strasbourg (France)
Supporting information for this article (including synthetic details,
characterization of new compounds, and CIF files) is available on the
WWW under http://dx.doi.org/10.1002/anie.201506474.
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Ligand 1 (Figure 2) was synthesized in 16 steps (for an
overview of the synthesis, see Scheme 1; for details see the
Supporting Information (SI) pp. S2–S11, S47–S75) and it
consists of a heterocyclic part and two second-generation
FrØchet-type dendrons,[7] which amplify the heterocyclic core
and improve the solubility. Due to its design based on the
choice and placement of the hydrazone-connected[8a] 2,5- and
2,6-disubstituted pyrazine units, and to the anti orienta-
tion[4b, 9] of N(sp2) atoms, the core of dendrimer 1 adopts
a linear conformation, as suggested by its ROESY NMR
spectrum (SI p. S9) and by the X-ray structures (Figure 3a) of
model molecules 2a and 2b (SI pp. S1, S41–S46).
Figure 2. Structural formulae of ligand 1 and of the corresponding
complexes M21. The 4+ charge of the complexes is not represented;
À
distances noted on the formulae are rounded centroid-to-centroid C C
distances; the sizes of the whole dendrimers were estimated roughly.
See also SI p. S40.
Figure 3. X-ray structures of a) molecules 2a and 2b (which are
models for the linear part of ligand 1) and b) complexes Zn23a, Ca23b,
and Mg23b (which are models for the core of dendritic complexes
M21). Anions and water and solvent molecules were omitted for
clarity; distances are centroid-to-centroid distances; heights are
approximate, rounded values. See also SI pp. S26–S34 and S41–S46.
On reaction with metal ions (Ca2+, Mg2+, Zn2+ as their
triflates), the two 2,6-dihydrazonepyrazine units produce two
bent or helical metalated[10] motifs that are connected,
through the central 2,5-disubstituted pyrazine unit, like the
two halves of the letter Z (Figure 3b). The Z-shape of the
metalated ionic core is suggested by ROESY NMR spectra
(SI pp. S14, S19, S22, S25) and by the X-ray structures of
model compounds Zn23a, Ca23b, and Mg23b (charges are
omitted for simplicity; see Figure 3b and SI pp. S26–S34). For
the formation of Zn21 complex 3–4 equiv of Zn triflate were
added. While Ca21 and Mg21 form upon addition of 4–5 equiv
of the metal ion, higher quantities (up to 12–18 equiv) were
added to make sure the equilibrium was shifted. The
Scheme 1. Overview of the synthesis of compounds 1, 3a, and 3b (see
SI pp. S2–S11, S47–S80).
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complexation produces a color change from yellow (1) to red
(M21). This metal-ion-induced linear-into-Z conversion cor-
responds to a motion of contraction of an average amplitude
of about 17 Š.
A subsequent extension (M21!1; Figure 4) was per-
formed via complexation of the M2+ ion by the tren ligand[4b]
N(CH2CH2NH2)3, a competing ligand that coordinates more
strongly than 1 (1.1–1.3 equiv tren per metal ion). At this
stage, the pH-dependence of tren can render the molecular
machine[3] responsive to pH changes. Indeed, addition of
triflic acid (theoretically 3 equiv of acid per equiv of tren, but
in practice one adds about 70–80% of the required quantity of
acid, and the final amount is determined by NMR titration)
results in protonation of tren and formation of the complex
M21 (contraction, Figure 4). Addition of base (Et3N, 1.1–
1.5 equiv per equiv of acid) produces the free ligand
1 (extension). The process was followed by NMR spectros-
copy (Figure 4 and SI pp. S35, S36, S38).
This first motional function is completed by a second one
generated by transmetalation. The ionic radii[8b] (in Š) of the
metal ions used herein are: 0.72 (Mg2+), 0.74 (Zn2+) and 1.00
(Ca2+). The Z-shaped complexes induced by Mg2+ and Zn2+
have, according to X-ray structures of model complexes
(Figure 3b), a height of about 5–6 Š, while that induced by
Figure 5. a) Sequence of reactions corresponding to the reversible
jack-like motions between Ca21 and Zn21. b) 1H NMR spectra
(400 MHz, CD3CN) corresponding to the sequence from (a). See also
SI p. S37.
Ca2+ has a height of about 1 Š. Here, the quantitative
replacement of Ca2+ by Zn2+ (molar ratio Ca/Zn = 2–2.5) has
an effect on the nanomechanical properties of the complex.
Through this transmetalation, a jack-like motion occurs
where the height of the heterocyclic core changes with an
amplitude of about 4–5 Š (Figures 1b, 3b, and 5). After
displacement of Ca2+ by Zn2+, complex Ca21 was restored by
addition of tren. Subsequent addition of acid, then of base
(Figure 5) as described before, modulated the second type of
motional function. Zn2+ can also displace Mg2+ in the pH-
modulable way (SI p. S39) but, according to X-ray structures
of models (Figure 3b), without remarkable change of the
height. In the mixtures containing Ca2+ or Mg2+, degradation
of ligand 1 occurs over time.
In conclusion, we have developed an unprecedented
nanomechanical Ca2+-, Mg2+-, and Zn2+-based dendritic
switch-type motional device[3] with two levels of control of
the motional function. One level of control concerns the
mode of motion: a) a change in length (average amplitude of
about 17 Š) through reversible metal-ion-induced contrac-
tion of a linear to a Z-shaped structure accompanied by
different optical responses associated with the extended
(yellow) and contracted (red) states and b) a change in the
height of the Z-shaped complexes (amplitude of about 4–5 Š)
through transmetalation. The second level of control concerns
the triggering of the motion, where the use of a pH-dependent
Figure 4. a) Sequence of reactions corresponding to reversible contrac-
tion/extension motions between 1 and M21 (M=Ca2+, Mg2+, Zn2+).
Tren is used as competing pH-dependent ligand, and triflic acid and
triethylamine are used as pH triggers. b) 1H NMR spectra (400 MHz;
CDCl3 for ligand 1 and CD3CN for complex Ca21) corresponding to the
sequence from (a) for M=Ca2+. See also SI pp. S35, S36, and S38.
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For simultaneous rotation and reversible contraction, see: c) H.
counter-ligand (tren) makes both above motional functions
responsive[11] to alternate additions of acid and base, that is,
controllable via pH changes. This novel molecular device
offers potential for the design of artificial molecular mus-
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Experimental Section
The CIF files for this article have been deposited at the Cambridge
Crystallographic Data Centre, and they have been assigned the
following deposition numbers: CCDC 1041754 (2a), 1062153 (2b),
1041755 (Zn23a), 1047076 (Ca23b), and 1050603 (Mg23b).
Acknowledgements
We thank Prof. Jean-Marie Lehn, Prof. Jean-Pierre Sauvage,
Prof. Jack Harrowfield, and Dr. Juan Ramirez for helpful
discussions, and Dr. HØl›ne Nierengarten, StØphanie Kouaho,
and MØlanie Lebreton for mass spectrometry analyses.
Keywords: calcium · contraction · dendrimers ·
molecular machines · transmetalation
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Received: July 14, 2015
Published online: October 14, 2015
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