90
Notes
Chem. Pharm. Bull. 53(1) 90—94 (2005)
Vol. 53, No. 1
Interaction between Quaternary Ammonium Ions and Dipeptides:
Positive Anion Allosteric Effect
Kazuaki ITO,* Kayo NAGASE, Naoya MOROHASHI, and Yoshihiro OHBA
Department of Chemistry and Chemical Engineering, Faculty of Engineering, Yamagata University; 4–3–16 Jyonan,
Yonezawa 992–8510, Japan. Received June 25, 2004; accepted October 1, 2004
The binding properties of dipeptides possessing aromatic residues towards quaternary ammonium ions have
1
been investigated by H-NMR spectroscopy. The intermolecular hydrogen bonding between exchangeable pro-
tons (OH and NH) of aromatic residues of dipeptides and the counter anion of ammonium ion is the primary
force. After the formation of the intermolecular hydrogen bonding, two aromatic residues of dipeptides can pro-
vide p-base cavity to interact with the quaternary ammonium moiety.
Key words cation–p interaction; hydrogen bonding; allosteric effect; quaternary ammonium ion; peptide
Noncovalent interactions are of fundamental importance in fect of the aromatic rings of 1a during formation of the com-
determining the highly dimensional structures and funda- plex as shown in Fig. 2. Especially, higher induced chemical
mental properties of molecular systems in biology, chemistry, shifts of the Nꢁ–CH2 and Nꢁ–CH3 protons of 2a were ob-
and materials sciences. Relatively strong non-covalent bond- served, suggesting that the ammonium moiety is located
ing such as salt bridge and hydrogen bonding has been exten- close to the aromatic surface of 1a.5—12) The same phenome-
sively investigated. In contrast, weak noncovalent bonding non was also observed in the mixture of Boc-Trp-Trp-OMe
such as p–p, CH–p and cation–p interaction is now known (1b) and 2a in CDCl3. In contrast, the chemical shifts of 2a
to play important roles in the stabilization of the com- scarcely changed in the presence of Boc-Phe-Phe-OMe (1c),
plexes1—3) but is not well understood, because the weak non- indicating that no complexation occurs between 1c and 2a.
covalent bond is usually cancelled behind the strong nonco- Although Boc-Tyr-Phe-OMe (1d) and Boc-Trp-Phe-OMe
valent bonding. However, there is much attention on (1e) having a phenol or an indole group can interact with am-
cation–p interaction in a biological event such as recognition monium ion (2a), the induced chemical shifts are smaller
of biological relevant quaternary ammonium ions, for exam- than those of 1a and 1b. We also observed a similar induced
ple, the neurotransmitter acetylcholine, by receptors and en- chemical shift pattern in the complexation of peptides (1)
zyme.4) Therefore, several research groups have devoted con- with other quaternary ammonium salts (2b, c) as shown in
siderable effort to the synthesis of receptor molecules to Table 1.
investigate binding properties of quaternary ammonium
Interestingly, the induced chemical shifts were dramati-
cally reduced by the replacement of counter anion of ammo-
cations.5—10)
In spite of many reports of synthetic receptors for binding nium from chloride (2a) to iodide (2d). Another important
1
quaternary ammonium ions, we are unaware of any on observation was that the H-NMR signals for exchangeable
acyclic compounds acting as a receptor.11) Recently, we re- protons (NH and OH) of aromatic amino acid residues of 1
ported a study on the recognition of quaternary ammonium were considerably downfield-shifted and extensively broad-
ion using acyclic phenol-formaldehyde dimer as a receptor, ened upon addition of 2.12,14) These results could be ex-
and found that quaternary ammonium cation is entrapped in plained by hypothesizing an important role for the anion that
the p cavity which is constructed of two phenol units; its interacts with the OH or NH groups of 1. This hypothesis is
counter ion participates in the recognition process by coordi- supported by the lack of efficiency shown by Boc-Phe-Phe-
nation via hydrogen bonding of the two hydroxyl groups.12)
As part of this study, we investigated the interaction be- prevents any participation of the anion in the binding event.
OMe (1c) of which the absence of the exchangeable proton
tween quaternary ammonium ions and dipeptides having the
We estimated the stoichiometry of the complex (1–2)
aromatic amino acid residues tyrosine and tryptophane, using the Job plot method.15) The 1 : 1 stoichiometry of the
which have exchangeable protons (OH and NH) and aro- complexes (1–2) was confirmed by the plot, which contains a
matic surfaces.
maximum at the molar ratio of 0.5 in all cases (Fig. 3). The
Dipeptides (1) were prepared in good yield by the conden- binding affinities of peptide (1) to quaternary ammonium
sation reactions of N-Boc L-amino acid with L-amino acid ions (2) were determined by nonlinear least-squares fitting
methyl ester monohydrochloride in the presence of N-methyl method of the saturation curves obtained from 1H-NMR titra-
morphorine in CHCl3 using DCC (1,3-dicyclohexylcarbodi- tion (Fig. 4).16) During the titration by varying concentrations
imide) as a condensation reagent according to the literature of 1 and 2 in CDCl3, the signals for the protons of ammo-
(Fig. 1).13)
nium moiety of 2 are gradually upfield-shifted.17) The ob-
The interaction behavior of dipeptides (1) and quaternary tained association constants (Ka) of the complexes (1–2) indi-
1
ammonium ions (2) was studied by using H-NMR spec- cated that 2 was bound by tyrosine peptides (1a, d) more effi-
troscopy at 20 °C. When benzyl trimethyl ammonium chlo- ciently than tryptophane peptides (1b, e). It is also clear that
ride (2a) was added into a CDCl3 solution of Boc-Tyr-Tyr- the association constants are strongly dependent on the na-
OMe (1a) at the ratio of 1a : 2aꢀ1 : 1 ([1]ꢀ[2]ꢀ10 mM), all ture of the anion of 2. Although Boc-Tyr-Tyr-OMe (1a) and
peaks of 2a moved to a higher field due to the ring current ef- Boc-Tyr-Phe-OMe (1d) had similar association constants, the
∗ To whom correspondence should be addressed. e-mail: itokazu@yz.yamagata-u.ac.jp
© 2005 Pharmaceutical Society of Japan