Chemical Models of Protein β-Sheets

Peptide mimics by linear arylamides: a structural and functional diversity test

Hydrogen-bonded oligoamide foldamers represent a large family of peptide mimics. Pioneered by Gellman and Seebach (Appella , J. Am. Chem. Soc. 1996, 118, 13071- 13072; Seebach , Helv. Chim. Acta 1996, 79, 913- 941), aliphatic amino acid-based mimic structures have been extensively studied. Results of these studies have found many useful applications in areas including chemical biology and drug design. This Account describes our efforts in creating arylamide-based foldamers whose compact conformations are stabilized by hydrogen bonding. The aim of our study was to test whether this class of mimic structures is sufficiently rigid to lead to new interesting functions. It was envisioned that, if our approach was workable, it might be developed into a new family of useful soft frameworks for studies toward molecular recognition, self-assembly, and materials science. Three classes of mimic structures, that is, folded or helical, zigzag, and straight oligomers, have been constructed by simply changing the positions of the substituents at the benzene rings in the backbones. Both amide and hydrazide units have been employed to construct the frameworks. In most cases, O...H-N hydrogen bonding was chosen to stabilize the compact conformations. Notably, for the first time the F...H-N hydrogen-bonding pattern has been used to tune the size of the cavity. To test their usefulness, these frameworks have been extensively modified and functionalized. (1)H NMR, UV-vis, fluorescence, circular dichroism, and X-ray diffraction techniques have all been employed to establish the compact structures and their interactions with guest molecules. The properties or functions of the mimic structures have been studied in seven aspects. (1) Acyclic molecular receptors: The amide foldamers can bind amine cations, while the hydrazide foldamers can complex saccharides. (2) Acceleration of anisole hydrolysis: Several folded oligomers are able to bind alkali metal cations and consequently promote the hydrolysis of the nitro-substituted anisole by alkali hydroxides. (3) Facilitation of macrocyclization: The straight and zigzag backbones can be readily functionalized, from which two classes of macrocycles have been prepared. (4) Homoduplex assembly: Zigzag oligomers that are appended with amide units at one side can form stable homoduplexes through the cooperative self-binding of the amide units. (5) Assembly of molecular tweezers: Discrete binding moieties are introduced at the ends of the oligomers, which can bind structurally matched guests. (6) Assembly of nano networks: F...H-N hydrogen-bonded foldamers can stack with fullerenes; thus a mixture of fullerenes with a trifoldamer generates honeycomb-styled nanoarchitectures. (7) Assembly of dynamic [2]catenanes: A preorganized porphyrin tweezer has been synthesized, from which dynamic three-component [2]catenanes have been assembled in high yields. Our results demonstrate that hydrogen-bonding-driven arylamide oligomers are a class of structurally unique mimic structures. The folded oligomers themselves can be used as synthetic receptors for binding different guest molecules, while incorporation of different segments into one system can produce many desired shapes. In addition, all of the rigid frameworks can be readily functionalized at specific sites. We believe that our results have helped to open the door for some new chemistry in molecular recognition, self-assembly, and other related areas.

Exploring beta-sheet structure and interactions with chemical model systems

Beta-sheets consist of extended polypeptide strands (beta-strands) connected by a network of hydrogen bonds and occur widely in proteins. Although the importance of beta-sheets in the folded structures of proteins has long been recognized, there is a growing recognition of the importance of intermolecular interactions among beta-sheets. Intermolecular interactions between the hydrogen-bonding edges of beta-sheets constitute a fundamental form of biomolecular recognition (like DNA base pairing) and are involved protein quaternary structure, protein-protein interactions, and peptide and protein aggregation. The importance of beta-sheet interactions in biological processes makes them potential targets for intervention in diseases such as AIDS, cancer, and Alzheimer's disease. This Account describes my research group's use of chemical model systems to study the structure and interactions of beta-sheets. Chemical model systems provide an excellent vehicle with which to explore beta-sheets, because they are smaller, simpler, and easier to manipulate than proteins. Synthetic chemical models also provide the opportunity to control or modulate natural systems or to develop other useful applications and may eventually lead to new drugs with which to treat diseases. In our "artificial beta-sheets", molecular template and turn units are combined with peptides to mimic the structures of parallel and antiparallel beta-sheets. The templates and turn units form folded, hydrogen-bonded structures with the peptide groups and help prevent the formation of complex, ill-defined aggregates. Templates that duplicate the hydrogen-bonding pattern of one edge of a peptide beta-strand while blocking the other edge have proven particularly valuable in preventing aggregate formation and in promoting the formation of simple monomeric and dimeric structures. Artificial beta-sheets that present exposed hydrogen-bonding edges can form well-defined hydrogen-bonded dimers. Dimerization occurs readily in chloroform solutions but requires additional hydrophobic interactions to occur in aqueous solution. Interactions among the side chains, as well as hydrogen bonding among the main chains, are important in dimer formation. NMR studies of artificial beta-sheets have elucidated the importance of hydrogen-bonding complementarity, size complementarity, and chiral complementarity in these interactions. These pairing preferences demonstrate sequence selectivity in the molecular recognition between beta-sheets. These studies help illustrate the importance of intermolecular edge-to-edge interactions between beta-sheets in peptides and proteins. Ultimately, these model systems may lead to new ways of controlling beta-sheet interactions and treating diseases in which they are involved.

Artificial beta-sheets: chemical models of beta-sheets

Chemical models provide tools with which to simplify and study complicated biological systems. Forces and chemical processes that govern the structure, function, and interactions of a biomacromolecule can be explored with a simple, easy-to-study synthetic molecule. Chemical models of beta-sheet structures have helped to elucidate the factors influencing protein structures and functions. Chemical models that mimic beta-sheet quaternary structure and interactions are emerging as valuable tools with which to better understand and control protein recognition and protein aggregation.

Diacid linkers that promote parallel beta-sheet secondary structure in water

We report the development of diacid units that promote formation of a two-stranded parallel beta-sheet secondary structure between peptide segments attached via their N-termini. These linker units are formed by attaching glycine to one carboxyl group of cis-1,2-cyclohexanedicarboxylic acid (CHDA). Parallel sheet formation in water is observed when l-residue strands are attached to the CHDA-Gly unit with either of the two absolute configurations.

A minimalist approach toward protein recognition by epitope transfer from functionally evolved beta-sheet surfaces

New approaches for identifying small molecules that specifically target protein surfaces as opposed to active site clefts are of much current interest. Toward this goal, we describe a three-step methodology: in step one, we target a protein of interest by directed evolution of a small beta-sheet scaffold; in step two, we identify residues on the scaffold that are implicated in binding; and in step three, we transfer the chemical information from the beta-sheet to a small molecule mimic. As a case study, we targeted the proteolytic enzyme thrombin, involved in blood coagulation, utilizing a library of beta-sheet epitopes displayed on phage that were previously selected for conservation of structure. We found that the thrombin-binding, beta-sheet displaying mini-proteins retained their structure and stability while inhibiting thrombin at low micromolar inhibition constants. A conserved dityrosine recognition motif separated by 9.2 A was found to be common among the mini-protein inhibitors and was further verified by alanine scanning. A molecule containing two tyrosine residues separated by a linker that matched the spacing on the beta-sheet scaffold inhibited thrombin, whereas a similar dityrosine molecule separated by a shorter 6 A linker could not. Moreover, kinetic analysis revealed that both the mini-protein as well as its minimalist mimic with only two functional residues exhibited noncompetitive inhibition of thrombin. Thus, this reductionist approach affords a simple methodology for transferring information from structured protein scaffolds to yield small molecule leads for targeting protein surfaces with novel mechanisms of action.

An artificial beta-sheet that dimerizes through parallel beta-sheet interactions

This Article introduces a simple chemical model of a beta-sheet (artificial beta-sheet) that dimerizes by parallel beta-sheet formation in chloroform solution. The artificial beta-sheet consists of two N-terminally linked peptide strands that are linked with succinic or fumaric acid and blocked along one edge with a hydrogen-bonding template composed of 5-aminoanisic acid hydrazide. The template is connected to one of the peptide strands by a turn unit composed of (S)-2-aminoadipic acid (Aaa). 1H NMR spectroscopic studies show that these artificial beta-sheets fold in CDCl3 solution to form well-defined beta-sheet structures that dimerize through parallel beta-sheet interactions. Most notably, all of these compounds show a rich network of NOEs associated with folding and dimerization. The compounds also exhibit chemical shifts and coupling constants consistent with the formation of folded dimeric beta-sheet structures. The aminoadipic acid unit shows patterns of NOEs and coupling constants consistent with a well-defined turn conformation. The present system represents a significant step toward modeling the type of parallel beta-sheet interactions that occur in protein aggregation.

Tetrahydrofuran Calpha-tetrasubstituted amino acids: two consecutive beta-turns in a crystalline linear tripeptide

The synthesis of tetrahydrofuran Calpha-tetrasubstituted amino acids (TAAs) and their effect on the conformation in small peptides are reported. The synthesis starts from the protein amino acid methionine, which is protected at the C and N terminus and converted into the corresponding sulfonium salt by alkylation. Simple base treatment in the presence of an aryl aldehyde leads to the formation of tetrahydrofuran tetrasubstituted Calpha-amino acids in a highly diastereoselective (trans/cis ratio up to 97:3) reaction with moderate to good yields (35-78%) depending on the aldehyde used. Palladium-catalyzed coupling reactions allow a subsequent further functionalization of the TAA. The R,S,S-TAA-Ala dipeptide amide adopts a beta-turn type I conformation, whereas its S,R,S isomer does not. The R,S,S-Gly-TAA-Ala tripeptide amide shows in the solid state and in solution a conformation of two consecutive beta-turn type III structures, stabilized by i+3-->i intramolecular hydrogen bonds.

Preparation, isolation, and characterization of Nalpha-Fmoc-peptide isocyanates: solution synthesis of oligo-alpha-peptidyl ureas

The N(alpha)-Fmoc-peptide isocyanates 3a-q, 4a-c, and 5a-c were prepared by the Curtius rearrangement of N(alpha)-Fmoc-peptide acid azides in toluene under thermal, microwave, and ultrasonic conditions. All the N(alpha)-Fmoc-oligo-peptide isocyanates made were isolated as stable crystalline solids with 71 to 94% yield and were fully characterized by 1H NMR, 13C NMR, and mass spectroscopy. Their utility for the synthesis of oligo-alpha-peptidyl ureas 7a-f and 8a-c by the divergent coupling approach was demonstrated. The coupling of N(alpha)-Fmoc-dipeptide isocyanates with amino acid ester or with N,O-bis(trimethylsilyl)amino acids resulted in N(alpha)-Fmoc-tripeptidyl urea ester and acids containing one each of peptide bond and urea bond. The divergent approach is extended to the synthesis of tetrapeptidyl ureas by the 2 + 2 strategy using bis-TMS-peptide acid as an amino component. To incorporate urea bonds in adjacent positions, N(alpha)-Fmoc-peptidyl urea isocyanates 9a-d were prepared and employed in the synthesis of three tetrapeptidyl ureas 10a-b and 11 containing one peptide bond and two urea bonds in series from the N-terminal end. The protocol was then employed for the synthesis of five urea analogues 13-15, 18, and 21 of [Leu5]enkephalin containing urea bonds at the 2, 3, 4 positions as well as at the 2, 4 and 2, 3, 4 positions. The analogue 2l was made by the convergent synthesis by the N --> C terminal chain extension. Finally, two urea analogues 22 and 23 of repeat units of bioelasto polymers, namely Val-Pro-Gly-Val-Gly-OH and Pro-Gly-Val-Gly-Val-OH, were synthesized incorporating the urea bond by the concomitant isocyanate generation and urea bond formation under thermal conditions.

Helical molecular duplex strands: multiple hydrogen-bond-mediated assembly of self-complementary oligomeric hydrazide derivatives

Careful examination of the X-ray structure of a ditopic hydrazide derivative 7 led to the concept that with malonyl groups as interhydrazide linkers hydrogen-bonding-mediated molecular duplex strands might be obtained. Complexation studies between 7, 8, and 9 confirmed this hypothesis. Two quadruple hydrogen-bonded heterodimers formed, in which spectator repulsive secondary electrostatic interaction was found to play an important role in determining the stability of the complexes. Extensive studies on 1-4 indicated that the hydrogen-bonding mode could persist in longer oligomeric hydrazide derivatives with chain extension from monomer to tetramer. Molecular duplex strands via two to fourteen interstrand hydrogen bonds were obtained. In addition to affecting the stability of the duplex strands, spectator repulsive secondary electrostatic interaction also played an important role in determining dynamic behavior of the duplex strands as exemplified by variable temperature (1)H NMR experiments. IR studies confirmed stronger hydrogen bonding in the longer oligomers. The assemblies of 1-4 on HOPG were also studied by STM technology. Molecular mechanical calculations further revealed double-helical structures for the longer oligomers. The results provide new opportunities for development of polymeric helical duplexes with well-defined structures.

Macrocyclic beta-sheet peptides that mimic protein quaternary structure through intermolecular beta-sheet interactions

This paper reports the design, synthesis, and characterization of a family of cyclic peptides that mimic protein quaternary structure through beta-sheet interactions. These peptides are 54-membered-ring macrocycles comprising an extended heptapeptide beta-strand, two Hao beta-strand mimics [JACS 2000, 122, 7654] joined by one additional alpha-amino acid, and two delta-linked ornithine beta-turn mimics [JACS 2003, 125, 876]. Peptide 3a, as the representative of these cyclic peptides, contains a heptapeptide sequence (TSFTYTS) adapted from the dimerization interface of protein NuG2 [PDB ID: 1mio]. 1H NMR studies of aqueous solutions of peptide 3a show a partially folded monomer in slow exchange with a strongly folded oligomer. NOE studies clearly show that the peptide self-associates through edge-to-edge beta-sheet dimerization. Pulsed-field gradient (PFG) NMR diffusion coefficient measurements and analytical ultracentrifugation (AUC) studies establish that the oligomer is a tetramer. Collectively, these experiments suggest a model in which cyclic peptide 3a oligomerizes to form a dimer of beta-sheet dimers. In this tetrameric beta-sheet sandwich, the macrocyclic peptide 3a is folded to form a beta-sheet, the beta-sheet is dimerized through edge-to-edge interactions, and this dimer is further dimerized through hydrophobic face-to-face interactions involving the Phe and Tyr groups. Further studies of peptides 3b-3n, which are homologues of peptide 3a with 1-6 variations in the heptapeptide sequence, elucidate the importance of the heptapeptide sequence in the folding and oligomerization of this family of cyclic peptides. Studies of peptides 3b-3g show that aromatic residues across from Hao improve folding of the peptide, while studies of peptides 3h-3n indicate that hydrophobic residues at positions R3 and R5 of the heptapeptide sequence are important in oligomerization.

Synthetic molecular motors and mechanical machines

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Luminescent pyrimidine hydrazide oligomers with peptide affinity

The modular synthesis of pyrimidine oligohydrazides and their peptide binding ability are reported. Ethylene glycol substituents ensure water solubility of the compounds. The pattern of hydrogen bond donors and hydrogen bond acceptors resembles the functionalities of a peptide backbone, and intramolecular hydrogen bonds restrict conformational mobility. The pyrimidine heterocycles show emission at 423 nm if either excited with light of 320 nm or by a FRET process from a nearby Trp residue. This property is useful for the luminescent detection of interactions with peptides and proteins.

One-Armed Artificial Receptors for the Binding of Polar Tetrapeptides in Water: Probing the Substrate Selectivity of a Combinatorial Receptor Library

We have recently developed a new class of one-armed artificial receptors 1 for the binding of the polar tetrapeptide N-Ac-D-Glu-L-Lys-D-Ala-D-Ala-OH (EKAA) 2 in water using a combined combinatorial and statistical approach. We have now further probed the substrate selectivity of this receptor library 1 by screening a second tetrapeptide substrate (3) with the inverse sequence N-Ac-D-Ala-D-Ala-L-Lys-D-Glu-OH (AAKE). This "inverse" substrate is also efficiently bound by our receptors, with K(ass) approximately 6000 M(-1) for the best receptors, as determined both by a quantitative on-bead binding assay and by UV and fluorescence titration studies in free solution. Hence, the inverse tetrapeptide 3 is in general bound two to three times less efficiently than the "normal" peptide 2 (K(ass) approximately 17,000 M(-1)), even though the complexation mainly involves long-range electrostatic interactions and both the receptor and substrate are rather flexible. Molecular modeling and ab initio calculations have been used to rationalize the observed substrate selectivity and to analyze the various binding interactions within the complex.

A New Tool in Peptide Engineering: A Photoswitchable Stilbene-type β-Hairpin Mimetic

Peptide secondary structure mimetics are important tools in medicinal chemistry, as they provide analogues of endogenous peptides with new physicochemical and pharmacological properties. The development, synthesis, photochemical investigation, and conformational analysis of a stilbene-type beta-hairpin mimetic capable of light-triggered conformational changes have been achieved. In addition to standard spectroscopic techniques (nuclear Overhauser effects, amide temperature coefficients, circular dichroism spectroscopy), the applicability of self-diffusion measurements (longitudinal eddy current delay pulsed-field gradient spin echo (LED-PGSE) NMR technique) in conformational studies of oligopeptides is demonstrated. The title compound shows photoisomerization of the stilbene chromophore, resulting in a change in solution conformation between an unfolded structure and a folded beta-hairpin.

Three-component synthesis of alpha,beta-cyclopropyl-gamma-amino acids

[reaction: see text] The multicomponent coupling of alkenylzirconocenes with N-diphenylphosphinoyl imines provides rapid access to functionalized C-cyclopropylalkylamides which have been readily transformed into alpha,beta-cyclopropyl-gamma-amino acids. These novel scaffolds are thus accessible in ca. 8 steps from commercially available alkynes.

Supramolecular Chelation Based on Folding

Crystallographic analysis revealed that pyridine-palladium complexation is a good geometric match to the m-phenylene ethynylene (mPE) repeat unit and thus could serve as a reversible linking group to join oligomer segments together. A series of pyridine-terminated mPE oligomers were then synthesized and found to coordinate with palladium dichloride to give complexes effectively twice the length of the free oligomers. A quantitative analysis of these coordination equilibria by isothermal calorimetry found the ability of the pyridine end-group to form a coordination complex corresponded with their ability to fold. Oligomers that were able to form complexes of sufficient length to fold showed positive cooperativity based on experimental determination of their association constants with a palladium ion. We suggest that the additional free energy of complexation for the folded oligomers is analogous to chelation by multidentate ligands, but here the "multidentate ligand" is held together by supramolecular rather than covalent bonds.

Bis(amino acid) derivatives of 1,4-diamino-2-butyne that adopt a C2-symmetric turn conformation

1,4-Diamino-2-butyne was prepared from 1,4-dichloro-2-butyne via 1,4-diazido-2-butyne. Bis(amino acid) derivatives of 1,4-diamino-2-butyne having the general structure (Boc-Xxx-NHCH2C[triple bond])2 (Xxx = Ala, Phe and Met) were prepared and examined by 1H NMR spectroscopy. Using chemical shift, coupling constant and DMSO titration data it is found that these compounds adopt a C2-symmetric turn conformation featuring two intramolecular hydrogen bonds.

Stereoselective synthesis of conformationally constrained ω-amino acid analogues from pyroglutamic acid

Bicyclic lactams derived from pyroglutamic acid provide a useful scaffold for synthesis of conformationally restricted analogues of lysine, ornithine and glutamine, as well as an Ala-Ala dipeptide analogue. Amino alcohol and carboxylic acid derivatives are accessible from a common intermediate. In this strategy, the bicyclic lactam system not only controls, but also facilitates the determination of the stereochemistry of the synthetic intermediates.

Luminescent Crown Ether Amino Acids: Selective Binding toN-terminal Lysine in Peptides

Crown ether amino acids (CEAAs) with a luminescent phthalic ester or phthalimide moiety have been prepared. Simple peptide chemistry covalently tethers the macrocycles to give ditopic ammonium-ion binders. The binding events of both crown ether groups are monitored independently by changes of their specific emission properties. The affinity of the bis-CEAA to bis-ammonium ions is distance dependent, which allows distinguishing between isomeric small peptides containing a lysine residue in different positions.

Prevention of Alzheimer's Disease-associated Aβ Aggregation by Rationally Designed Nonpeptidic β-Sheet Ligands

A new concept is introduced for the rational design of beta-sheet ligands, which prevent protein aggregation. Oligomeric acylated aminopyrazoles with a donor-acceptor-donor (DAD) hydrogen bond pattern complementary to that of a beta-sheet efficiently block the solvent-exposed beta-sheet portions in Abeta-(1-40) and thereby prevent formation of insoluble protein aggregates. Density gradient centrifugation revealed that in the initial phase, the size of Abeta aggregates was efficiently kept between the trimeric and 15-meric state, whereas after 5 days an additional high molecular weight fraction appeared. With fluorescence correlation spectroscopy (FCS) exactly those two, i.e. a dimeric aminopyrazole with an oxalyl spacer and a trimeric head-to-tail connected aminopyrazole, of nine similar aminopyrazole ligands were identified as efficient aggregation retardants whose minimum energy conformations showed a perfect complementarity to a beta-sheet. The concentration dependence of the inhibitory effect of a trimeric aminopyrazole derivative allowed an estimation of the dissociation constant in the range of 10(-5) m. Finally, electrospray ionization mass spectrometry (ESI-MS) was used to determine the aggregation kinetics of Abeta-(1-40) in the absence and in the presence of the ligands. From the comparable decrease in Abeta monomer concentration, we conclude that these beta-sheet ligands do not prevent the initial oligomerization of monomeric Abeta but rather block further aggregation of spontaneously formed small oligomers. Together with the results from density gradient centrifugation and fluorescence correlation spectroscopy it is now possible to restrict the approximate size of soluble Abeta aggregates formed in the presence of both inhibitors from 3- to 15-mers.

Molecular clips form isostructural dimeric aggregates from benzene to water

We report the synthesis and characterization of eight C-shaped methylene-bridged glycoluril dimers (1-8) bearing hydrogen-bonding amide groups on their aromatic rings. Compounds 1-6 undergo tight dimerization in CDCl3 solution (Ks > 9 x 10(5) M(-1)); binary mixtures of 1-7 form mixtures of homodimers and heterodimers in moderately selective dimerization processes (0.23 < or = Keq < or = 768; 0.253 < or = chiAB < 0.933). The high affinity formation of 1.1-6.6 is due to the commensurate nature of the geometrical constraints imposed by the pi-pi interactions and only two hydrogen bonds. The differential response of the strengths of the pi-pi interactions and H-bonds of 2.2 to changes in solvent polarity--from C6D6 to D2O--results in the formation of a solvent-independent isostructural aggregate that exhibits high affinity dimerization across the full range of solvents.

Synthesis and Single-Molecule Studies of a Well-Defined Biomimetic Modular Multidomain Polymer Using a Peptidomimetic β-Sheet Module

In the pursuit of advanced biomaterials with combined strength, toughness, and elasticity, a new class of well-defined modular polymers has been synthesized, and their nanomechanical properties have been studied using atomic force microscopy. These polymers are based on a peptidomimetic beta-sheet-based double-closed loop (DCL) module, which was designed to overcome the limitation of the modular polymers we reported previously (J. Am. Chem. Soc. 2004, 126, 2059). Single-molecule force-extension experiments revealed the sequential unfolding of these modules as the polymer is stretched, resulting in more regular sawtooth-patterned curves similar to those seen in titin and other biopolymers. The single-molecule data agreed well with computer modeling, which suggested that hydrogen bonding and pi-stacking are both involved in the formation of small DCL clusters along the polymer chain.

Solvophobically-Driven Oligo(ethylene glycol) Helical Foldamers. Synthesis, Characterization, and Complexation with Ethane-1,2-diaminium

Oligo(ethylene glycols) 1a-h, which are incorporated with one to eight 2,3-naphthylene units, respectively, have been synthesized and characterized. The conformational changes of the new oligomers have been investigated in chloroform-acetonitrile binary solvents by the UV-vis, (1)H NMR, and fluorescent spectroscopy. It has been revealed that the naphthalene units in hexamer 1f, heptamer 1g, and octamer 1h are driven by solvophobic interaction to stack in polar solvents. As a result, compact helical conformations are formed that give rise to a cavity similar to that of 18-crown-6. Shorter oligomers 1b-e exhibit weaker folding tendency. (1)H NMR studies reveal that 1f-h are able to complex ammonium or ethane-1,2-diaminium 19, but not secondary ammonium compounds. The association constants of complexes 1f.19, 1g.19, and 1h.19 in acetonitrile are determined to be 3.5(+/-0.4) x 10(3), 1.0(+/-0.12) x 10(4), and 2.5(+/-0.4) x 10(4) M(-1), respectively, with the (1)H NMR titration method. For comparison, hexamer 22, which incorporates six 1,5-naphthylene units, is also prepared. The UV-vis and fluorescent investigations show that 22 is also able to fold in polar solvents, but no helical structure can be produced due to mismatch of the stacking naphthalene units and consequently there is no obvious complexation between 22 with ethane-1,2-diaminium ion. The structures of the longest foldamer 1h and its complex with 19 have been studied with molecular mechanics calculations. This work represents a new approach to building folding conformations from flexible linear molecules.

Sequences in dendrons and dendrimers

Sequential incorporation of a variety of functional groups forms the basis for specificity in biomacromolecules. Introduction of such diversity and sequencing ability in artificial macromolecules is fundamentally interesting. In this paper, three different synthetic approaches have been used to build dendrons and dendrimers in which all the monomer units are different from each other. The synthetic strategies described in this paper involve the use of (i) an ABB(p) monomer, (ii) an ABB' monomer, and (iii) an ABB(m) monomer. The complementarity and the versatility of these synthetic approaches should render them useful for a variety of applications.

General synthesis of unsymmetrical norbornane scaffolds as inducers for hydrogen bond interactions in peptides

Starting from readily accessible endo-cis-(2S,3R)-norbornene dicarboxylic acid benzyl monoester, a general and efficient synthetic approach toward unsymmetrical two-stranded peptidic structures was developed. In these structures the peptide strands are oriented in a parallel geometry. Their synthesis is easily applicable to a variety of amino acids and peptides. Specifically, a norbornane template as molecular scaffold induces hydrogen bonding between the adjacent peptide strands. The specific hydrogen bonding patterns between these strands were revealed by detailed NMR analysis including TOCSY/NOE experiments.