Enantioselective organocatalytic aminomethylation of aldehydes: a role for ionic interactions and efficient access to beta2-amino acids

Organocatalytic Mannich addition of aldehydes to a formaldehyde-derived iminium species catalyzed by proline-derived chiral pyrrolidines provides beta-amino aldehydes with >/=90% ee. Mechanistic analysis of the proline-catalyzed reactions suggests that non-hydrogen-bonded ionic interactions at the Mannich reaction transition state can influence stereochemical outcome. The beta-amino aldehydes from our process bear a substituent adjacent to the carbonyl and can be efficiently converted to protected beta2-amino acids, which are important building blocks for beta-peptide foldamers that display useful biological activities.     

Helical secondary structures in 2:1 and 1:2 α/γ-peptide foldamers

Oligomers containing both α- and γ-amino acid residues in a 1:1 alternating pattern have recently been shown by several research groups to adopt helical secondary structures. We have begun to explore the folding behavior of oligomers with different α-residue/γ-residue backbone patterns. Previously we reported that the γ-amino acids bearing a cyclohexyl constraint at the C_β–C_γ bond and a variable side chain at C_α strongly promote a helical conformation containing 12-atom CO(i)?H–N(i+3) hydrogen bonds for 1:1 α:γ backbones. Here we report synthesis and crystallographic analysis of 2:1 and 1:2 α/γ-peptides that adopt CO(i)?H–N(i+3) hydrogen-bonded helical conformations.     

The CO coadsorption and reactions of sulfur,hydrogen and oxygen on clean and sulfided Mo(100) and on MoS2(0001) crystal faces

The chemisorption and reactivity of O₂ and H₂ with the sulfided Mo(100) surface and the basal (0001) plane of MoS₂ have been studied by means of Thermal Desorption Spectroscopy (TDS), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). These studies have been carried out at both low (10^?8–10^?5Torr) and high (1 atm) pressures of O₂ and H₂. Sulfur desorbs from Mo(100) both as an atom and as a diatomic molecule. Sulfur adsorbed on Mo(100) blocks sites of hydrogen adsorption without noticeably changing the hydrogen desorption energies. TDS of ¹⁸O coadsorbed with sulfur on the Mo(100) surface produced the desorption of SO at 1150 K, and of S, S₂ and O, but not SO₂. A pressure of 1 × 10^?7 Torr of O₂ was sufficient to remove sulfur from Mo(100) at temperatures over 1100 K. The basal plane of MoS₂ was unreactive in the presence of 1 atm of O₂ at temperatures of 520 K. Sputtering of the MoS₂ produced a marked uptake of oxygen and the removal of sulfur under the same conditions.     

(alpha/beta+alpha)-peptide antagonists of BH3 domain/Bcl-x(L) recognition: toward general strategies for foldamer-based inhibition of protein-protein interactions

The development of molecules that bind to specific protein surface sites and inhibit protein-protein interactions is a fundamental challenge in molecular recognition. New strategies for approaching this challenge could have important long-term ramifications in biology and medicine. We are exploring the concept that unnatural oligomers with well-defined conformations ("foldamers") can mimic protein secondary structural elements and thereby block specific protein-protein interactions. Here, we describe the identification and analysis of helical peptide-based foldamers that bind to a specific cleft on the anti-apoptotic protein Bcl-xL by mimicking an alpha-helical BH3 domain. Initial studies, employing a fluorescence polarization (FP) competition assay, revealed that among several alpha/beta- and beta-peptide foldamer backbones only alpha/beta-peptides intended to adopt 14/15-helical secondary structure display significant binding to Bcl-xL. The most tightly binding Bcl-xL ligands are chimeric oligomers in which an N-terminal alpha/beta-peptide segment is fused to a C-terminal alpha-peptide segment ((alpha/beta + alpha)-peptides)). Sequence-affinity relationships were probed via standard and nonstandard techniques (alanine scanning and hydrophile scanning, respectively), and the results allowed us to construct a computational model of the ligand/Bcl-xL complex. Analytical ultracentrifugation with a high-affinity (alpha/beta + alpha)-peptide established 1:1 ligand:Bcl-xL stoichiometry under FP assay conditions. Binding selectivity studies with the most potent (alpha/beta + alpha)-peptide, conducted via surface plasmon resonance measurements, revealed that this ligand binds tightly to Bcl-w as well as to Bcl-xL, while binding to Bcl-2 is somewhat weaker. No binding could be detected with Mcl-1. We show that our most potent (alpha/beta + alpha)-peptide can induce cytochrome C release from mitochondria, an early step in apoptosis, in cell lysates, and that this activity is dependent upon inhibition of protein-protein interactions involving Bcl-xL.     

Practical synthesis of enantiomerically pure beta2-amino acids via proline-catalyzed diastereoselective aminomethylation of aldehydes

Proline-catalyzed diastereoselective aminomethylation of aldehydes using a chiral iminium ion, generated from a readily prepared precursor, provides alpha-substituted-beta-amino aldehydes with 85:15 to 90:10 dr. The alpha-substituted-beta-amino aldehydes can be reduced to beta-substituted-gamma-amino alcohols, the major diastereomer of which can be isolated via crystallization or column chromatography. The amino alcohols are efficiently transformed to protected beta2-amino acids, which are valuable building blocks for beta-peptides, natural products, and other interesting molecules. Because conditions for the aminomethylation and subsequent reactions are mild, beta2-amino acid derivatives with protected functional groups in the side chain, such as beta2-homoglutamic acid, beta2-homotyrosine, and beta2-homolysine, can be prepared in this way. The synthetic route is short, and purifications are simple; therefore, this method enables the preparation of protected beta2-amino acids in useful quantities.     

2D Ising Model for Adsorption-induced Enantiopurification of Racemates

Mechanisms for the spontaneous transformation of achiral chemical systems into states of enantiomeric purity have important ramifications in modern pharmacology and potential relevance to the origins of homochirality in life on Earth. Such mechanisms for enantiopurification are needed for production of chiral pharmaceuticals and other bioactive compounds. Previously proposed chemical mechanisms leading from achiral systems to near homochirality are initiated by a symmetry-breaking step resulting in a minor excess of one enantiomer via statistical fluctuations in enantiomer concentrations. Subsequent irreversible processes then amplify the majority enantiomer concentration while simultaneously suppressing minority enantiomer production. Herein, equilibrium adsorption of amino acid enantiomer mixtures onto chiral and achiral surfaces reveals amplification of surface enantiomeric excess relative to the gas phase; i. e. enantiopurification of chiral adsorbates by adsorption. This adsorption-induced amplification of enantiomeric excess is shown to be well-describe by the 2D Ising model. More importantly, the 2D-Ising model predicts formation of homochiral monolayers from adsorption of racemic mixtures or prochiral molecules on achiral surfaces; i. e. enantiopurification with no apparent chiral driving force.     

Structure-activity studies of 14-helical antimicrobial beta-peptides: probing the relationship between conformational stability and antimicrobial potency

Antimicrobial alpha-helical alpha-peptides are part of the host-defense mechanism of multicellular organisms and could find therapeutic use against bacteria that are resistant to conventional antibiotics. Recent work from Hamuro et al. has shown that oligomers of beta-amino acids ("beta-peptides") that can adopt an amphiphilic helix defined by 14-membered ring hydrogen bonds ("14-helix") are active against Escherichia coli [Hamuro, Y.; Schneider, J. P.; DeGrado, W. F. J. Am. Chem. Soc. 1999, 121, 12200-12201]. We have created two series of cationic 9- and 10-residue amphiphilic beta-peptides to probe the effect of 14-helix stability on antimicrobial and hemolytic activity. 14-Helix stability within these series is modulated by varying the proportions of rigid trans-2-aminocyclohexanecarboxylic acid (ACHC) residues and flexible acyclic residues. We have previously shown that a high proportion of ACHC residues in short beta-peptides encourages 14-helical structure in aqueous solution [Appella, D. H.; Barchi, J. J.; Durell, S. R.; Gellman, S. H. J. Am. Chem. Soc. 1999, 121, 2309-2310]. Circular dichroism of the beta-peptides described here reveals a broad range of 14-helix population in aqueous buffer, but this variation in helical propensity does not lead to significant changes in antibiotic activity against a set of four bacteria. Several of the 9-mers display antibiotic activity comparable to that of a synthetic magainin derivative. Among these 9-mers, hemolytic activity increases slightly with increasing 14-helical propensity, but all of the 9-mers are less hemolytic than the magainin derivative. Previous studies with conventional peptides (alpha-amino acid residues) have provided conflicting evidence on the relationship between helical propensity and antimicrobial activity. This uncertainty has arisen because alpha-helix stability can be varied to only a limited extent among linear alpha-peptides without modifying parameters important for antimicrobial activity (e.g., net charge or hydrophobicity); a much greater range of helical stability is accessible with beta-peptides. For example, it is very rare for a linear alpha-peptide to display significant alpha-helix formation in aqueous solution and manifest antibacterial activity, while the linear beta-peptides described here range from fully unfolded to very highly folded in aqueous solution. This study shows that beta-peptides can be unique tools for analyzing relationships between conformational stability and biological activity.     

Hydrophobic Variations of N‐Oxide Amphiphiles for Membrane Protein Manipulation: Importance of Non‐hydrocarbon Groups in the Hydrophobic Portion

Amphipathic agents called detergents serve as membrane‐mimetic systems to maintain the native structures of membrane proteins during their manipulation. However, membrane proteins solubilized in conventional detergents tend to undergo denaturation and aggregation, necessitating the development of novel amphipathic agents with enhanced properties. Here we describe several new amphiphiles that contain an N‐oxide group as the hydrophilic portion. The new amphiphiles have been evaluated for the ability to solubilize and stabilize a fragile multi‐subunit assembly from biological membranes. We found that cholate‐based agents were promising in supporting retention of the native protein quaternary structure, while deoxycholate‐based amphiphiles were highly efficient in extracting/solubilizing the intact superassembly from the native membrane. Monitoring superassembly solubilization and stabilization as a function of variation in amphiphile structure led us to propose that a non‐hydrocarbon moiety such as an amide, ether, or a hydroxy group present in the lipophilic regions can manifest distinctive effects in the context of membrane protein manipulation.