Molecular Recognition of Nucleotides - Designing Receptors for the Next Generation of Biosensors - page 56

N N

O

O N NH OH

NH H N N H

O

16 17

O

NH HN NH

N NH

O O OH

N N

Fig. 5 Tweezer receptor 16 for AMP with two tetramethylguanidinio groups ( left ) and ditopic
receptor 17 for TMP with two bicyclic, chiral guanidinium moieties ( right )

At this point it is important to state that great care has to be taken when comparing
different binding constants. Potentiometric measurements represent the binding con-
stant of host and guest in a clearly defined protonation state. However, at a given pH a
mixture of different protonation states always exists. Consequently these K values
only refer to single theoretical protonation states and are therefore not valid for the
macroscopic ensemble but represent rather the maximum possible binding constant
between two single species within the mixture. Without the calculation of an effective
binding constant these data cannot be used to compare binding constants of different
host systems. The above-mentioned examples made use of potentiometry. The effec-
tive binding constant was never calculated except in the work conducted by Garcia-
Espa˜a. This is also why many of the extremely high binding constants must be put
into perspective—the effective binding constants of the macroscopic sample are likely
to be lower by several orders of magnitude. A more appropriate way to obtain
comparable binding constants is to utilize methods such as NMR, UV/Vis, fluores-
cence, or ITC. Whatever the choice may be, it is always strongly advisable to use at
least two different methods to verify one's results.

1.2 Guanidinium-Based Receptors

A statistical evaluation of 3,003 X-ray crystal structures of phosphate binding
proteins revealed that two third of them do not make use of metal ions for phosphate
binding [ 8 ]. More than half of the metal-free proteins use lysine or arginine instead,
especially those with phosphate binding sites located on their surface. Consequently,
ammonium and guanidinium moieties seem to be well suited to bind to phosphate in
natural systems, even when directly opposed to the competitive influence of the
surrounding water molecules and counter ions. While most of the receptors reported
in the literature so far make use of ammonium groups in analogy to lysine, only very
few host systems have been designed yet which utilize the corresponding arginine
analogus—guanidinium moieties—for phosphate recognition.
Wang reported the ditopic receptor 16 with two tetramethylguanidinio moieties
linked via an aromatic scaffold (Fig. 5 )[ 9 ]. With the help of an NMR titration in D 2 O
he was able to show that this rather simple molecule alone was able to bind AMP with
K 10 4 M -1 .

¼ 7