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N
N
O
2
U
O
O
OH
OH
O
O
HO
HO
O
O
OH
HO
HO
OH
19
Fig. 10 Chiral uranyl-salophen anion receptor 19.
A situation is different when one uses Lewis acid for modification of
anion binding pocket. Such approach is presented in the work of Cort
and Schia
no
25
who investigated hybrid glucose-uranyl-salophen anion
receptor 19 (Fig. 10). Owing to incorporation of two unprotected glucose
units, receptor 19 was soluble in water, what allowed for determination of
binding constants. Receptor 19 bound fluoride anion in water with
K
ass
=
115 M
1
. The Lewis acid-base interaction is strong enough to
overcome energetic penalty associated with dehydratation of small
fluoride anion. Salophen derivative 19 showed also substantial binding
anities for phosphates such as HPO
4
2
and P
2
O
7
2
as well as bio-
logically important anions AMP
2
, ADP
3
, and ATP
4
. Interestingly, the
association constant for adenosine monophosphate (AMP
2
, K
ass
=
83)
was six times lower than for that for HPO
4
2
(K
ass
=
480 M
1
). For all other
phosphates, the association constants were higher than 10
4
M
1
.
3 Sugar decorated anion receptors in chiral recognition
As already mentioned, an anion receptor exhibiting chiral recognition
must itself be chiral. One of the ways to accomplish this is by incorpor-
ation of chiral molecule into the host backbone. In this section we will
show application of anion receptors decorated with sugars in chiral
recognition processes.
A colorimetric anion sensor 20 was reported as effective in chiral rec-
ognition
26
(Fig. 11). Compound 20 upon addition of anions, displayed a
large batochromic shift (
B
145 nm) in UV/VIS spectra. This change can be
attributed to azophenol deprotonation, which can cause photoinduced
charge transfer. The chiral recognition ability of receptor 20 was meas-
ured by UV/VIS titrations using Benesi-Hildebrand approximation
(Table 2). Anion receptor 20 exhibited a good enantioselectivity for N-
Boc-protected a-amino acids with preference for
D
-amino acids and
enatioselectivity up to 3.6 for alanine. Enantiomers DNB-protected amino
acids were recognized with moderate enantioselectivity (1.10-2.55). Host
compound 20 was also examined with chiral carboxylate such as
naproxen and 2-phenylpropionic acid. UV/VIS titration experiments
showed K
R
/K
S
equal 1.86 and 2.95, respectively.
Hyun and coworkers
27
reported fluorescent anion sensors 21 and 22
for recognition of N-Boc or 3,5-dinitrobenzoyl (DNB) protected a-amino
acids (Fig. 12). Amino acid such as alanine (Ala), valine (Val), threonine
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