Novel cage-type cyclophanes bearing L-and D-valine residues and capable of providing a three-dimensionally extended cavity were prepared in order to simulate specific functions of naturally occurring receptors. The cage-type hosts provide relatively apolar microenvironments, which are well shielded from the bulk aqueous phase, for hydrophobic guests. Moreover, the cage-type hosts bearing L-and D-valine residues in their respective bridging segments afford S-and R-helical cavities, respectively.The helically twisted cavities of the hosts furnish conformation-enforced microenvironments for chiral recognition of enantiomeric guests, such as bilirubin-IXα, as well as diastereomeric guests, such as steroid hormones.
Chiral molecular recognition was analyzed on the basis of thermodynamics of the association process. By considering the hypothetical thermodynamic states, the free energy (ΔG°), the enthalpy (ΔH°), and the entropy (ΔS°) changes of the association can be separated into contribution from each recognition group. Experimentally determined values of each AG° for the chiral recognition by functionalized porphyrin indicate that the restriction of molecular motion effectively driven by intermolecular forces plays animportant role in the chiral recognition. Circular dichroism study was proved to be effective toprobe the molecular motion of the guest in the host-guest complex, as exemplified by the splittype induced circular dichroism of functionalized porphyrin-amino acid systems. We proposethat the split type induced circular dichroism is caused by the coupling between the transitionmoments of the carbonyl group and the porphyrin group based on the molecular orbital calculatlons.
A distinct feature of a dipeptide is a straight structure bearing an amino group at one terminal and a carboxyl group at another terminal because a hardly rotatable peptide linkage exits at its center. As a result, dipeptide molecules are arranged in atwo-dimensional layer. On stirring crystalline (R) -phenylglycyl- (R) -phenylglycine (1) and isopropyl phenyl sulfoxide (2 a) in the presence of water, (S) -2 a molecules widened the channel between the peptide layers to be accomodated under asymmetric recognition. The driving forces of this accomodation is hydrogen bonding between H3N+ group and sulfinyl group, phenyl-phenyl edge-to-face interaction, and CH-π-interaction of the isopropyl of 2 a with the phenyl of 1. (R) -Methyl phenyl sulfoxide was stereo-selectively intercalated into crystalline 1. These phenomena were also observed by the peptide (1) -deposited quartz-crystal microbalance. In contrast with 1, (R) - (1-naphthyl) glycyl- (R) -phenylglycine (5) always requires an appropriate guest molecule to maintain a crystalline structure because relatively large naphthyl group disturbs contraction of the peptide layers. Indeed, the dipeptide (5) forms the crystals including a guest molecules (an alcohol, an ether, or sulfoxide) on crystallization from methanol containing the guest. It is noteworthy that the guest exchange is possible by simply stirring the inclusion crystals with other guest molecules in carbon tetrachloride. The layer arrangement in these inclusion crystals was revealed by single-crystal X-ray analysis.
Studies on new aspects of host-guest molecular recognition, which leads to membrane potential changes, are described. Special focus was placed on potential changes induced by host-guest complexation with organic guests at the surface of liquid membranes. The following modes of potentiometric discrimination of organic guests are described. (i) Potentiometric discrimination of organic anions by macrocyclic polyamines, based on electrostatic interactions between protonated polyamine hosts and anionic guests. (ii) Potentiometric discrimination of nucleotides bearing guanine and adenine bases by the hosts having a cytosine residue, based on complementary base pairing in addition to electrostatic interactions. (iii) Potentiometric discrimination of amines by a calix  arene hexaester or a β-cyclodextrin derivative, based on recognition of the steric structures of nonpolar moieties of these guests by inclusion into well-defined cavities of host molecules. Some characteristic aspects of host-guest molecular recognition at membrane surfaces are discussed in comparison with those in homogeneous solutions.
Quartz Crystal Microbalances (QCM) are known to provide very sensitive mass measuring devices because the resonance frequency decreases upon the increase of a given mass on the QCM electrode in a nanogram level. In this review, we describe that the QCM is useful to detect molecular recognition processin situ. For example, when lipid membranes are immobilized on a QCM plate, we could detect the selective adsorption of odor molecules to the lipid matrix from the freqency changes. When dipeptide crystals were immobilized, enantioselective adsorption of amino acid was observed. When the QCM plate was attached hollizontally onto the glycolipid monolayer at the air-water interface, the selective binding of lecithins onto the glycolipid monolayer was observed. When oligonucleotide was immobilized on a QCM, hybridization of DNA double strands were detected from frequency changes.
In the design of functional organic materials, it is important to use molecular interactions. Hydrogen bonding is one of the key interactions for molecular association and recognition in nature. Recently, new types of hydrogen-bonded liquid-crystalline (LC) complexes have been obtained by molecular recognition between complementary components. For example, molecular recognition by hydrogen bonds between carboxylic acid and pyridyl moieties results in the quantitative formation of well-defined structures of mesogens. In the hydrogen-bonded liquid crystals, dynamic nature of hydrogen bonding gives unique LC structures of complexes such as polymer networks, polymer alloys, and one-dimensional polymers. For LC polymer networks, stable liquid-crystalline phases have been induced by the formation of dynamic intermolecular mesogen consisting of polymer side chain and a small molecule. A wide variety of supramolecular mesogenic materials can be obtained by the molecular aggregation via various hydrogen bonds.
Cation complexation by crown ethers is an important area of study because many biological processes involve the interaction of an alkali or alkaline earth metal cation with a carrier or channel. These ionophores are often complex structures and assessment of interaction is difficult. Use of the simple crown ether structures facilitates these studies and offers other applications as well. The fast atom bombardment mass spectrometry (FAB/MS) technique allows the complexation phenomenon to be assessed largely in the absence of solvent and using relatively small quantities of ligands. Moreover, this technique gives information about complexation when other methods simply cannot be applied due to the complexity of the binding interaction.
This review describes recent developments in the molecular design of crown ethers and analogs, which include lariat ethers, bis (crown ether) s, cryptands, and the corresponding open-chain derivatives. Attention was especially paid to the host compounds for alkali metal and alkaline earth metal cations. The complexation properties were discussed based on the stability constants in methanol, characteristic absorptions in the UV spectrum, the extractabilities, or the transport abilities.
Alkali and alkaline earth metal ions are important media for energy conversion and information transfer in biological system through accumulation and release across the membrane. Two mechanisms are differentiated for the metal ion transport, i.e., carrier and channel, each with high molecular recognition. We have developed a new idea of allosteric binding of metal ion M2 into the molecule organized by complexation of “pre” host with transition metals M1. The resulting metallo-assisted crown ethers showed excellent characteristics for the binding of these typical metal ions. Amphiphilic ion pairs or macrocyclic compound were inserted into bilayer lipid membrane in order to test the artificial ion channel activity. These molecules showed stable current of picoampere level, constant conductance, open-closed transition, cation selectivity, K/Na selectivity and voltage dependence, exactly in the same way as natural ion channels do. Supramolecular assemblage stabilized by helix formation or unimolecular half channel mechanisms explain well the successful observation.
Cyclodextrins (CDs) have been found to recognize the structures of various polymers. α-Cyclodextrin (α-CD) formed complexes with poly (ethylene glycol) (PEG), although, β-CD did not form complexes with PEG. However, β-CD formed complexes with poly (propylene glycol) (PPG), although α-CD did not form complexes with PPG. CDs form complexes not only with hydrophilic polymers but hydrophobic polymers. CDs recognize the molecular weight of polymers. CDs recognize not only main chains but end groups and side chains. Preparation of polyrotaxanes in which many cyclodextrins were trapped in a polymer chain was described. Preparation of polymeric tubes consisting of several cyclodextrins was also described.
For the development of receptor molecules that can precisely recognize saccharide molecules, several phenylboronic acid derivatives that can act as “saccharide interfaces” have been synthe sized. These boronic acid derivatives can bind mono- and/or di-saccharides even in an aqueous media and give some physical signals such as circular dichroism (CD), fluorescence spectra, and so on. In this paper, we explain the complexation abilities of boronic acids with mono- and di-saccharide molecules and the “read-out” mechanism by CD or fluorescence spectra
Cyclodextrins (CDs) capable of accommodating a guest molecule into their hydrophobic cavity are excellent entities for exploring molecular association phenomena. However, the molecular recognition ability of CDs is rather limited owing to their relatively rigid structure. This deficiency can be overcome by introducing pendant residues to CDs. That is hydrophobic pendant residues attached at the primary or the secondary hydroxyl side of CDs acted as hydrophobic caps enhancing hydrophobicity of the CD cavity or spacers narrowing the CD cavity to adapt it for accommodating a small guest. This induced-fit type guest binding usually induces drastic changes in spectroscopic properties of modified residues when they are spectroscopically active. The spectroscopic changes can be used as a measure for molecular recognition and, with regard to this, modified CDs should act as a molecular sensing probe. Selectivity and sensitivity of individual modified CDs upon detecting guests are different from one another, depending on the modified residues. The modified CDs-based sensing probes may be effective in detecting spectroscopically inert substanCes such as bile acids.
Cyclodextrins (CDxs) are cyclic oligosaccharides produced by enzymatic reactions of starch. Since CDxs are composed of the D- (+) -glucopyranose units and have chiral cavities, CDxs can recognize the chiralities of guest molecules. Pyrene dimer as well as pamoic acid becomes optically active when these guest molecules are included in the γ-CDx cavity. Lock and key mechanism may interpret these phenomena. Meanwhile, the conformational enantiomerism of bilirubin is performed by the intermolecular hydrogen bonding between the CO2- groups of bilirubin and the secondary hydroxyl groups of CDx. Peralkylated CDxs well recognize the chiralities of the binaphthyl derivatives. The determined thermodynamic parameters indicate that the complexation of the preferable enantiomer is entropically favorable, which also suggests the lock and key mechanism. Complete opticalresolution of binaphthyl derivatives has been achieved by capillary zone electrophoresis using CDxs as separating agents.