Enantioselective formation of a dynamic hydrogen-bonded assembly based on the chiral memory concept

In this paper, we report the enantioselective formation of a dynamic noncovalent double rosette assembly 1a(3).(CYA)(6) composed of three 2-pyridylcalix[4]arene dimelamines (1a) and six butylcyanuric acid molecules (BuCYA). The six 2-pyridyl functionalities of the assembly interact stereoselectively with chiral dicarboxylic acids 3a-e via two-point hydrogen-bonding interactions. One of the two enantiomeric assemblies (P- or M-) 1a(3).(CYA)(6) is formed in excess as the result of the complexation of the chiral diacids, resulting in formation of optically active assemblies. The complexations with dibenzoly tartaric acids D-3a and L-3a (3 equivalent), respectively, leading to the formation of diastereomeric assemblies (P)-1a(3).(BuCYA)(6).(D-3a)(3) and (M)-1a(3).(BuCYA)(6).(L-3a)(3) with 90% diastereomeric excess. The diastereomeric excess in (M)-1a(3).(BuCYA)(6).(L-3a)(3) is "memorized" when L-3a is removed by precipitation with ethlylenediamine (EDA). The assembly (M)-1a(3).(BuCYA)(6) is still optically active (90% enantiomeric excess), although none of its individual components are chiral. (M)-1a(3).(BuCYA)(6) has a high kinetic stability toward racemization (E(a) = 119 kJ mol(-)(1), half-life of (M)-1a(3).(BuCYA)(6) is ca. 1 week at 20 degrees C).     

Selective self-organization of guest molecules in self-assembled molecular boxes

This article describes the synthesis and binding properties of highly selective noncovalent molecular receptors 1(3).(DEB)6 and 3(3).(DEB)6 for different hydroxyl functionalized anthraquinones 2. These receptors are formed by the self-assembly of three calix[4]arene dimelamine derivative molecules (1 or 3) and six diethylbarbiturate (DEB) molecules to give 1(3).(DEB)6 or 3(3).(DEB)6. Encapsulation of 2 occurs in a highly organized manner; that is, a noncovalent hydrogen-bonded trimer of 2 is formed within the hydrogen-bonded receptors 1(3).(DEB)6 and 3(3).(DEB)6. Both receptors 1(3).(DEB)6 and 3(3).(DEB)6 change conformation from staggered to eclipsed upon complexation to afford a better fit for the 2(3) trimer. The receptor selectivity toward different anthraquinone derivatives 2 has been studied using 1H NMR spectroscopy, X-ray crystallography, UV spectroscopy, and isothermal microcalorimetry (ITC). The pi-pi stacking between the electron-deficient center ring of the anthraquinone derivatives 2a-c and 2e-g and the relatively electron-poor melamine units of the receptor is the driving force for the encapsulation of the guest molecules. The selectivity of the hydrogen-bonded host for the anthraquinone derivatives is the result of steric interactions between the guest molecules and the calix[4]arene aromatic rings of the host.     

Complexation of phenolic guests by endo- and exo-hydrogen-bonded receptors

This article describes the complexation of phenol derivatives by hydrogen-bonded receptors. These phenol receptors are formed by self-assembly of calix[4]arene dimelamine or tetramelamine derivatives with 5,5-diethylbarbiturate (DEB) or cyanurate derivatives (CYA). The double rosette assemblies 3(3).(DEB)6/(CYA)6 have their phenol-binding functionalities (ureido groups) at the top and at the bottom of the double rosette (exo-receptors). The tetrarosette assemblies 4(3).(DEB)12/(CYA)12 form a cavity with binding sites between the two double rosettes for guest encapsulation (endo-receptors). An intrinsic binding constant Ka of 202 M-1 and 286 M-1 for the binding of 4-nitrophenol to the ureido functionalized exo- and endo-receptors, respectively, was observed. For the exo-receptor a 1:6 stoichiometry was observed while for the endo-receptor 1:4 binding stoichiometry was determined by Job plot and MALDI-TOF MS. The important role that the hydroxy group's acidity plays in the complexation of 4-nitrophenol is clarified by binding studies with different phenol derivatives. The hydrogen-bonded receptors showed a much smaller response towards less acidic phenol derivatives.     

Thermodynamic stability of hydrogen-bonded nanostructures: a calorimetric study

The self-assembly of hydrogen-bonded aggregates (rosettes) in solvent mixtures of different polarity has been studied by calorimetry. The C(50) parameter, the concentration when 50 % of the components are incorporated in the assembly, is used to compare assemblies with different stoichiometry. C(50) for the single rosette 1(3).(BuCYA)(3) (1=N,N-di(4-tert-butylphenyl)melamine; BuCYA=n-butylcyanuric acid) in 1,2-dichloroethane is 25 microM, whereas for double rosettes 2 a(3).(BuCYA)(6) and 2 b(3).(BuCYA) (2=calix[4]arene-dimelamine) it is 0.7 and 7.1 microM, respectively. DeltaG degrees, DeltaH degrees, and TDeltaS degrees values indicate that the thermodynamics of double rosettes reflect the independent assembly of two individual single rosette structures or two rosettes reinforced by additional stabilizing interactions. In more polar solvents the stability of double rosettes decreases. From the correlation of DeltaG degrees with solvent polarity it is predicted that it should be possible to assemble double rosettes in methanol or water. The assembly of 2 b(3).(BuCYA)(6) in 100 % MeOH was proven by (1)H NMR and CD spectroscopy.     

Supramolecular Clippers for Controlling Photophysical Processes through Preorganized Chromophores

A novel supramolecular clipping design for influencing the photophysical properties of functional molecular assemblies, by the preorganization (clipping) of chromophores, is described. Several chromophores end functionalized with molecular recognition units were designed. These molecular recognition units serve as handles to appropriately position these systems upon noncovalent interactions with multivalent guest molecules (supramolecular clippers). Towards this goal, we have synthesized 1,5‐dialkoxynaphthalene (DAN) and naphthalenediimide (NDI) functionalized with dipicolylethylenediamine (DPA) motifs. These molecules could preorganize upon noncovalent clipping with adenosine di‐ or triphosphates, which resulted in preassociated excimers and mixed (cofacial) charge‐transfer (CT) assemblies. Chiral guest binding could also induce supramolecular chirality, not only into the individual chromophoric assembly but also into the heteromeric CT organization, as seen from the strong circular dichroism (CD) signal of the CT transition. The unique ability of this design to influence the intermolecular interactions by changing the binding strength of the clippers furthermore makes it very attractive for controlling the bimolecular photophysical processes.     

Self-assembled nanostructures of oligopyridine molecules

The high potential of self-assembly processes of molecular building blocks is reflected in the vast variety of different functional nanostructures reported in the literature. The constituting units must fulfill several requirements like synthetic accessibility, presence of functional groups for appropriate intermolecular interactions and depending on the type of self-assembly processsignificant chemical and thermal stability. It is shown that oligopyridines are versatile building blocks for two- and three-dimensional (2D and 3D) self-assembly. They can be employed for building up different architectures like gridlike metal complexes in solution. By the appropriate tailoring of the heterocycles, further metal coordinating and/or hydrogen bonding capabilities to the heteroaromatic molecules can be added. Thus, the above-mentioned architectures can be extended in one-step processes to larger entities, or in a hierarchical fashion to infinite assemblies in the solid state, respectively. Besides the organizational properties of small molecules in solution, 2D assemblies on surfaces offer certain advantages over 3D arrays. By precise tailoring of the molecular structures, the intermolecular interactions can be fine-tuned expressed by a large variety of resulting 2D patterns. Oligopyridines prove to be ideal candidates for 2D assemblies on graphite and metal sufaces, respectively, expressing highly ordered structures. A slight structural variation in the periphery of the molecules leads to strongly changed 2D packing motifs based on weak hydrogen bonding interactions. Such 2D assemblies can be exploited for building up host-guest networks which are attractive candidates for manipulation experiments on the single-molecule level. Thus, "erasing" and "writing" processes by the scanning tunneling microscopy (STM) tip at the liquid/solid interface are shown. The 2D networks are also employed for performing coordination chemistry experiments at surfaces.     

Stepwise noncovalent synthesis leading to dendrimer-based assemblies in water

We provide detailed insight into complex supramolecular assembly processes by fully characterizing a multicomponent model system using dynamic light scattering, cryogenic transmission electron microscopy, atomic force microscopy, and various NMR techniques. First, a preassembly of a host molecule (the fifth-generation urea-adamantyl poly(propylene imine) dendrimer) and 32 guest molecules (a water- and chloroform-soluble ureidoacetic acid guest) was made in chloroform. The association constant in chloroform is concealed by guest self-association and is therefore higher than 10(3) M(-1). Via the neat state the single-host complex was transferred to water, where larger dendrimer-based assemblies were formed. The core of these assemblies, consisting of multiple host molecules (on average three), is kinetically trapped upon dissolution in water, and its size is constant irrespective of the concentration. The guest molecules forming the corona of the assemblies, however, stay dynamic since they are still in rapid exchange on the NMR time scale, as they were in chloroform. A stepwise noncovalent synthesis provides a means to obtain metastable dynamic supramolecular assemblies in water, structures that cannot be formed in one step.     

On‐Surface Self‐Assembly of a C3‐Symmetric π‐Conjugated Molecule Family Studied by STM: Two‐Dimensional Nanoporous Frameworks

The on‐surface self‐assembled behavior of four C ₃‐symmetric π‐conjugated planar molecules ( Tp , T12 , T18 , and Ex ) has been investigated. These molecules are excellent building blocks for the construction of noncovalent organic frameworks in the bulk phase. Their hydrogen‐bonded 2D on‐surface self‐assemblies are observed under STM at the solid/liquid interface; these structures are very different to those in the bulk crystal. Upon combining the results of STM measurements and DFT calculations, the formation mechanism of different assemblies is revealed; in particular, the critical role of hydrogen bonding in the assemblies. This research provides us with not only a deep insight into the self‐assembled behavior of these novel functional molecules, but also a convenient approach toward the construction of 2D multiporous networks.     

Fluorescent assemblies: Synergistic of amphiphilic molecules and fluorescent elements

In recent years, fluorescent assemblies based amphiphilic molecules have gained attention as unique and powerful materials for multiple applications that cover sensors, optoelectronics and bioimaging because of amphiphilic molecules self-assembly with outstanding flexibility and diversity spanning assembly structure from micelles, vesicles and nano-assemblies to gels. Weak and noncovalent interactions are important driving force for assemblies. The combination of the structural characteristics of self-assembly and the fluorescent properties of the fluorescent building element render the fluorescent material versatility and their easy-to-tune properties. Amphiphilic molecules can be used as building elements to co-assemble with dye molecules, aggregation-induced emission (AIE) gens, fluorescent nanoparticles and new amphiphilic molecules containing fluorescent groups can also be designed and prepared with self-assembly capability. Concomitantly, the improvement of fluorescence performance including fluorescence intensity, quantum yield, stability and controllability during assembly proved outstanding properties of fluorescence assemblies. These promising fluorescent assemblies are by far not exhaustive in construction method and mechanism explanation but foreshadow their more potential applications. Here, we will understand deeper the fluorescent assemblies and inspire future developments and applications employing this emerging fluorescence soft materials.     

Growth of individual hydrogen-bonded nanostructures on gold monolayers

The growth of individual nanometer-sized (3.4 +/- 1.4 nm) hydrogen bonded assemblies 1(2) x (DEB)6 on gold monolayers was achieved through an exchange reaction between single isolated calix[4]arene dimelamine 2 (1.1 +/- 0.2 nm) embedded in hexanethiol monolayers and double rosette hydrogen bonded assembly 1(3) x (DEB)6 in solution. The growth process was monitored by tapping mode atomic force microscopy (TM-AFM).     

Dynamic combinatorial libraries based on hydrogen-bonded molecular boxes

This article describes two different types of dynamic combinatorial libraries of host and guest molecules. The first part of this article describes the encapsulation of alizarin trimer 2a3 by dynamic mixtures of up to twenty different self-assembled molecular receptors together with the amplification and selection of the best binder. Receptors (1a-d)3.(DEB)6 are formed by the self-assembly of six diethyl barbiturate (DEB) and calix[4]arene dimelamine derivatives 1a-d by using hydrogen bonds. The largest amplification factor (2.8) for a host assembly (1a3.(DEB)6) was observed after the addition of 2a to four-component library 1a(n).1b(3-n).(DEB)6 (n=0-3). Addition of 2a to twenty-component library 1a(n).1b(m).1c(o).1d(3-(n+m+o)).(DEB)6 (n, m, o=0-3; (n+m+o)相似文献   

Molecular Architectonics-guided Design of Biomaterials

The quest for mastering the controlled engineering of dynamic molecular assemblies is the basis of molecular architectonics. The rational use of noncovalent interactions to programme the molecular assemblies allow the construction of diverse molecular and material architectures with novel functional properties and applications. Understanding and controlling the assembly of molecular systems are daunting tasks owing to the complex factors that govern at the molecular level. Molecular architectures depend on the design of functional molecular modules through the judicious selection of functional core and auxiliary units to guide the precise molecular assembly and co-assembly patterns. Biomolecules with built-in information for molecular recognition are the ultimate examples of evolutionary guided molecular recognition systems that define the structure and functions of living organisms. Explicit use of biomolecules as auxiliary units to command the molecular assemblies of functional molecules is an intriguing exercise in the scheme of molecular architectonics. In this minireview, we discuss the implementation of the principles of molecular architectonics for the development of novel biomaterials with functional properties and applications ranging from sensing, drug delivery to neurogeneration and tissue engineering. We present the molecular designs pioneered by our group owing to the requirement and scope of the article while acknowledging the designs pursued by several research groups that befit the concept.     

Oligo(p‐phenylene‐ethynylene)‐Derived Super‐π‐Gelators with Tunable Emission and Self‐Assembled Polymorphic Structures

Linear π‐conjugated oligomers are known to form organogels through noncovalent interactions. Herein, we report the effect of π‐repeat units on the gelation and morphological properties of three different oligo(p‐phenylene‐ethynylene)s: OPE3 , OPE5 , and OPE7 . All of these molecules form fluorescent gels in nonpolar solvents at low critical gel concentrations, thereby resulting in a blue gel for OPE3 , a green gel for OPE5 , and a greenish yellow gel for OPE7 . The molecule–molecule and molecule–substrate interactions in these OPEs are strongly influenced by the conjugation length of the molecules. Silicon wafer suppresses substrate–molecule interactions whereas a mica surface facilitates such interactions. At lower concentrations, OPE3 formed vesicular assemblies and OPE5 gave entangled fibers, whereas OPE7 resulted in spiral assemblies on a mica surface. At higher concentrations, OPE3 and OPE5 resulted in super‐bundles of fibers and flowerlike short‐fiber agglomerates when different conditions were applied. The number of polymorphic structures increases on increasing the conjugation length, as seen in the case of OPE7 with n=5, which resulted in a variety of exotic structures, the formation of which could be controlled by varying the substrate, concentration, and humidity.     

Noncovalent Assembly of a Fifteen-Component Hydrogen-Bonded Nanostructure

A total of 72 hydrogen bonds are formed in the spontaneous association of calix[4]arene tetramelamine and barbituric acid derivatives to give nanosized assemblies of the type represented in the picture. These consist of 15 components that assemble in a completely diastereoselective sense: of the eight possible diastereomers only the all-staggered diastereoisomer is obtained.     

How to Make Weak Noncovalent Interactions Stronger

By employing noncovalent interactions, chemists have constructed a variety of molecular aggregates with well‐defined structures and fascinating properties. In fabricating stable and large molecular assemblies, noncovalent interactions with high binding strength are needed. This Concept summarizes some strategies to modify and optimize the structures of building blocks for making weak noncovalent interactions stronger. The strategies include: 1) Preorganization of binding sites; 2) spatial confinement effects; 3) multivalent enhancement; 4) synergistic binding with multiple forces. Examples of the fabrication of supramolecular architectures by utilizing these strategies are presented and discussed. Guidance is offered in the construction and fabrication of stable molecular assemblies and supramolecular materials.     

Interconversion between [5]pseudorotaxane and [3]pseudorotaxane by pasting/detaching two axle molecules

An acceptor-donor-acceptor-type linear molecule 1(2+) containing one electron-rich naphthoxy (NP) unit and two monocharged viologen (MCV) units was synthesized. Through the noncovalent interaction of cucurbit[8]uril (CB[8]) with one NP and one MCV in 1(2+), we first obtained a [2]pseudorotaxane ([1(2+)]?CB[8]), and the excess CB[8] included simultaneously the two bare MCV units of two [2]pseudorotaxanes to form a [5]pseudorotaxane ([1(2+)](2)?[CB[8]](3)). Its transformation to [3]pseudorotaxane was achieved through detaching the two axle molecules in the presence of acid, and then the addition of base may result in a reversible switch between two different pseudorotaxanes. This novel methodology elongating reversibly linear molecules by noncovalent interactions will benefit the development of stimuli-responsive functional molecular devices.     

Theoretical and Crystallographic Study of Lead(IV) Tetrel Bonding Interactions

The ability of tetrahedral lead(IV) to establish noncovalent σ-hole tetrel bonding interactions with electron-rich atoms (ElRs; anions and Lewis bases) has been studied at the PBE0-D3/def2-TZVPD level of theory. An analysis of the Cambridge Crystallographic Database (CSD), which is a convenient storehouse of geometric information, has been performed to investigate the existence of tetrel bonding interactions involving tetrahedral lead(IV) derivatives. Several examples of tetrel bonding interactions that are crucial in crystal packing, ranging from 0D to 2D assemblies, have been found. In addition to the energetic and theoretical study of several XPb(CH₃)₃⋅⋅⋅ElR complexes (X=F, CN, CF₃, and CH₃), Bader's theory of atoms in molecules has also been used to further analyze and characterize the noncovalent interactions described herein.     

Mass spectrometric study of gas‐phase ions of acid β‐glucosidase (Cerezyme) and iminosugar pharmacological chaperones

The effect on the conformations and stability of gas‐phase ions of Cerezyme, a glycoprotein, when bound to three small‐molecule chaperones has been studied using intact ESI MS, collision cross section and MS/MS measurements. To distinguish between the peaks from apo and small‐molecule complex ions, Cerezyme is deglycosylated (dg‐Cer). ESI MS of dg‐Cer reveals that glycosylation accounts for 8.5% of the molecular weight. When excess chaperone, either covalent (2FGF) or noncovalent (A and B iminosugars), is added to solutions of dg‐Cer, mass spectra show peaks from 1:1 chaperone–enzyme complexes as well as free enzyme. On average, ions of the apoenzyme have 1.6 times higher cross sections when activated in the source region of the mass spectrometer. For a given charge state, ions of complexes of 2FGF and B have about 30% and 8.4% lower cross sections, respectively, compared to the apoenzyme. Thus, binding the chaperones causes the gas‐phase protein to adopt more compact conformations. The noncovalent complex ions dissociate by the loss of charged chaperones. In the gas phase, the relative stability of dg‐Cer with B is higher than that with the A, whereas in solution A binds enzyme more strongly than B. Nevertheless, the disagreement is explained based on the greater number of contacts between the B and dg‐Cer than the A and dg‐Cer (13 vs. 8), indicating the importance of noncovalent interactions within the protein–chaperone complex in the absence of solvent. Findings in this work suggest a hypothesis towards predicting a consistent correlation between gas‐phase properties to solution binding properties. Copyright © 2014 John Wiley & Sons, Ltd.     

Robust Ordered Bundles of Porous Helical Nanotubes Assembled from Fully Rigid Ionic Benzene‐1,3,5‐tricarboxamides

Size‐controlled and ordered assemblies of artificial nanotubes are promising for practical applications; however, the supramolecular assembly of such systems remains challenging. A novel strategy is proposed that can be used to reinforce intermolecular noncovalent interactions to construct hierarchical supramolecular structures with fixed sizes and long‐range ordering by introducing ionic terminals and fully rigid arms into benzene‐1,3,5‐tricarboxamide (BTA) molecules. A series of similar BTA molecules with distinct terminal groups and arm lengths are synthesized; all form hexagonal bundles of helical rosette nanotubes spontaneously in water. Despite differences in molecular packing, the dimensions and bundling of the supramolecular nanotubes show almost identical concentration dependence for all molecules. The similarities of the hierarchical assemblies, which tolerate certain molecular irregularities, can extend to properties such as the void ratio of the nanotubular wall. This is a rational strategy that can be used to achieve supramolecular nanotubes in aqueous environments with precise size and ordering at the same time as allowing molecular modifications for functionality.     

Enantioselective noncovalent synthesis of hydrogen-bonded double-rosette assemblies

The noncovalent synthesis of enantiomerically pure hydrogen-bonded assemblies (M)- and (P)-1(3).(CA)(6) is described. These dynamic assemblies are of one single handedness (M or P), but do not contain any chiral components. They are prepared by using the "chiral memory" concept: the induction of supramolecular chirality is achieved through initial assembly with chiral barbiturates, which are subsequently replaced by achiral cyanurates. This exchange process occurs quantitatively and without loss of the M or P handedness of the assemblies. Racemization studies have been used to determine an activation energy for racemization of 105.9+/-6.4 kJ mol(-1) and a half-life time to racemization of 4.5 days in benzene at 18 degrees C. Kinetic studies have provided strong evidence that the rate-determining step in the racemization process is the dissociation of the first dimelamine component 1 from the assembly 1(3).(CA)(6). In addition to this, it was found that the expelled chiral barbiturate (RBAR or SBAR) acts as a catalyst in the racemization process. Blocking the dissociation process of dimelamines 1 from assembly 1(3).(CA)(6) by covalent capture through a ring-closing metathesis (RCM) reaction produces an increase of more than two orders of magnitude in the half-life time to racemization.