Partial supports of this work from the National Science
Foundation of China (No. 20725206, 20732004 and 21021001),
Program for Changjiang Scholars and Innovative Research
Team in University, the Key Project of Chinese Ministry of
Education in China, and Scientific Fund of Sichuan Province
for Outstanding Young Scientists, and the US National Science
Foundation (CHE-0717995 and ECCS-0708923) are gratefully
acknowledged.
Notes and references
Fig. 5 CV titration profile of TpyCu2+ (1 mM) with various
amounts of L-alanine.
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be used to explain their dramatic difference in fluorescence.
The fluorescence of Tpy is quenched by Cu2+ in the TpyCu2+
complex by either electron or energy transfer from the excited state
of Tpy to the Cu2+ center. The more electron rich Cu(II) center
generated upon coordination of L-histidine to TpyCu2+ should
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more difficult, leading to the large fluorescence enhancement.
However, the Cu(II) centers in the complexes of TpyCu2+ with
other amino acids such as L-alanine should be still electron deficient
enough to accept the electron or energy transferred from the excited
state of Tpy, providing efficient fluorescence quenching for Tpy.
A DFT calculation was conducted on TpyCu(II)–histidine
before.15 Two structures A and B were found to have the lowest
energy, with B slightly lower than A (Fig. 6). It is expected that
the coordination of the amino acids like L-alanine to TpyCu2+
should be similar to A, involving the bonding of an amine and a
carbonyl to the metal center. However, for the interaction of
L-histidine with TpyCu2+, we propose that the structure B
may be more important in order to account for the observed
large difference in the fluorescence response of TpyCu2+
toward L-histidine over the other amino acids. The chelate
coordination of both the more basic imidazole nitrogen (versus
the carbonyl group) and the a-amine group to the Cu(II) center
in B should increase the electron density on the Cu(II) center
and make it harder to be reduced. The structure B has a
resonance form B0 in which the positive charge is delocalized
from the metal center to the imidazole ring to make the metal
center less electron deficient. The more electron rich metal
center in B and B0 should contribute to the inhibition of the
fluorescence quenching of Tpy by Cu(II) in TpyCu(II)–histidine,
giving rise to the observed large fluorescence enhancement. The
chelate coordination of histidine with Cu2+ in B is also
supported by our observation of little fluorescence enhancement
when TpyCuCl2 is treated with a histidine derivative whose
amine group is protected with Boc (see Fig. S12–S14 in ESIw).
In summary, we have discovered that the classical coordination
complex TpyCuCl2 is a highly efficient and selective fluorescent
sensor for histidine in aqueous solution at neutral pH. It is
potentially useful for practical application.
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Fig. 6 Two calculated structures for TpyCu(II)–histidine.
3414 Chem. Commun., 2012, 48, 3412–3414
c
This journal is The Royal Society of Chemistry 2012