Conformational Preferences and Cis−Trans Isomerization of l-Lactic Acid Residue

Synthesis of Depsipeptides via Isocyanide-Based Consecutive Bargellini–Passerini Multicomponent Reactions

An efficient and straightforward approach has been established for the preparation of a new class of depsipeptide structures via isocyanide-based consecutive Bargellini–Passerini multicomponent reactions. 3-Carboxamido-isobutyric acids bearing an amide bond were obtained via Bargellini multicomponent reaction from isocyanides, acetone, and chloroform in the presence of sodium hydroxide. Next, via a Passerini multicomponent-reaction strategy, a new class of depsipeptides was synthesized using the Bargellini reaction products, isocyanides, and aldehydes. The depsipeptides thus prepared have more flexible structures than their pseudopeptidic analogues.

Neutral Peptides in the Gas Phase: Conformation and Aggregation Issues

Combined IR and UV laser spectroscopic techniques in molecular beams merged with theoretical approaches have proven to be an ideal tool to elucidate intrinsic structural properties on a molecular level. It offers the possibility to analyze structural changes, in a controlled molecular environment, when successively adding aggregation partners. By this, it further makes these techniques a valuable starting point for a bottom-up approach in understanding the forces shaping larger molecular systems. This bottom-up approach was successfully applied to neutral amino acids starting around the 1990s. Ever since, experimental and theoretical methods developed further, and investigations could be extended to larger peptide systems. Against this background, the review gives an introduction to secondary structures and experimental methods as well as a summary on theoretical approaches. Vibrational frequencies being characteristic probes of molecular structure and interactions are especially addressed. Archetypal biologically relevant secondary structures investigated by molecular beam spectroscopy are described, and the influences of specific peptide residues on conformational preferences as well as the competition between secondary structures are discussed. Important influences like microsolvation or aggregation behavior are presented. Beyond the linear α-peptides, the main results of structural analysis on cyclic systems as well as on β- and γ-peptides are summarized. Overall, this contribution addresses current aspects of molecular beam spectroscopy on peptides and related species and provides molecular level insights into manifold issues of chemical and biochemical relevance.

Strategies to Develop Inhibitors of Motif-Mediated Protein-Protein Interactions as Drug Leads

Protein-protein interactions are fundamental for virtually all functions of the cell. A large fraction of these interactions involve short peptide motifs, and there has been increased interest in targeting them using peptide-based therapeutics. Peptides benefit from being specific, relatively safe, and easy to produce. They are also easy to modify using chemical synthesis and molecular biology techniques. However, significant challenges remain regarding the use of peptides as therapeutic agents. Identification of peptide motifs is difficult, and peptides typically display low cell permeability and sensitivity to enzymatic degradation. In this review, we outline the principal high-throughput methodologies for motif discovery and describe current methods for overcoming pharmacokinetic and bioavailability limitations.

Structural investigations on a linear isolated depsipeptide: the importance of dispersion interactions

The first molecular beam investigations of an isolated linear depsipeptide are presented. By applying IR/UV spectroscopic methods and DFT calculations three structural arrangements are identified with the most stable structure being only stable by including dispersion interactions.

Helical α/β-depsipeptides with alternating residue types: conformational change from the 11-helix to the 14/15-helix

Short α/β-depsipeptides of which the third residue from the N-terminus is an (S)-lactic acid residue predominantly adopt 14/15-helical conformations analogous to the α-helix.

Effects of peptide backbone amide-to-ester bond substitution on the cleavage frequency in electron capture dissociation and collision-activated dissociation

Probing the mechanism of electron capture dissociation on variously modified model peptide polycations has resulted in discovering many ways to prevent or reduce N-Cα bond fragmentation. Here we report on a rare finding of how to increase the backbone bond dissociation rate. In a number of model peptides, amide-to-ester backbone bond substitution increased the frequency of O-Cα bond cleavage (an analogue of N-Cα bonds in normal peptides) by several times, at the expense of reduced frequency of cleavages of the neighboring N-Cα bonds. In contrast, the ester linkage was only marginally broken in collisional dissociation. These results further highlight the complementarity of the reaction mechanisms in electron capture dissociation (ECD) and collision-activated dissociation (CAD). It is proposed that the effects of amide-to-ester bond substitution on fragmentation are mainly due to the differences in product ion stability (ECD, CAD) as well as proton affinity (CAD). This proposal is substantiated by calculations using density functional theory. The implications of these results in relation to the current understanding of the mechanisms of electron capture dissociation and electron transfer dissociation are discussed.

Conformational properties of the residues connected by ester and methylated amide bonds: theoretical and solid state conformational studies

Peptides produced by bacteria and fungi often contain an ester bond in the main chain. Some of them have both an ester and methylated amide bond at the same residue. A broad spectrum of biological activities makes these depsipeptides potential drug precursors. To investigate the conformational properties of such modified residues, a systematic theoretical analysis was performed on N-acetyl-L-alanine N'-methylamide (Ac-Ala-NHMe) and the analogues with the ester bond on the C-terminus (Ac-Ala-OMe), N-terminus (Ac-[psi](COO)-Ala-NHMe) as well as the analogues methylated on the N-terminus (Ac-(Me)Ala-OMe) and C-terminus (Ac-[psi](COO)-Ala-NMe(2)). The phi, psi potential energy surfaces and the conformers localised were calculated at the B3LYP/6-311++G(d,p) level of theory both in vacuo and with inclusion of the solvent (chloroform, water) effect (SCRF method). The solid state conformations of the studied residues drawn from The Cambridge Structural Database have been also analysed. The residues with a C-terminal ester bond prefer the conformations beta, C5, and alpha(R), whereas those with N-terminal ester bond prefer the conformations beta, alpha(R), and the unique conformation alpha' (phi, psi = -146 degrees , -12 degrees ). The residues with N-terminal methylated amide and a C-terminal ester bond prefer the conformations beta, beta2, and interestingly, the conformation alpha(L). The residues with a C-terminal methylated amide and an N-terminal ester bond adopt primarily the conformation beta. The description of the selective structural modifications, such as those above, is a step towards understanding the structure-activity relationship of the depsipeptides, limited by the structural complexity of these compounds.