The Ant-Pro Reverse-Turn Motif. Structural Features and Conformational Characteristics

Protein Backbone Alteration in Non-Hairpin β-Turns: Impacts on Tertiary Folded Structure and Folded Stability

The importance of β-turns to protein folding has motivated extensive efforts to stabilize the motif with non-canonical backbone connectivity. Prior work has focused almost exclusively on turns between strands in a β-sheet (i. e., hairpins). Turns in other structural contexts are also common in nature and have distinct conformational preferences; however, design principles for their mimicry remain poorly understood. Here, we report strategies that stabilize non-hairpin β-turns through systematic evaluation of the impacts of backbone alteration on the high-resolution folded structure and folded stability of a helix-loop-helix prototype protein. Several well-established hairpin turn mimetics are shown detrimental to folded stability and/or hydrophobic core packing, while less-explored modification schemes that reinforce alternate turn types lead to improved stability and more faithful structural mimicry. Collectively, these results have implications in control over protein folding through chemical modification as well as the design of protein mimetics.

Chiral anthranilic pyrrolidine as custom-made amine catalyst for enantioselective Michael reaction of nitroalkenes with carbonyl compounds

A chiral anthranilic pyrrolidine catalyst as a custom-made amine-catalyst was developed for the enantio- and diastereo selective Michael reaction of nitroalkenes with carbonyl compounds. In particular, a peptide-like catalyst in which an α-amino acid is attached to the anthranilic acid skeleton induced the high stereoselectivity of the reaction with aldehydes. Studies of the reaction mechanism indicated that the catalyst exhibits a divergent stereocontrol in the reaction, namely, steric control by a 2-substituted group on the catalyst and hydrogen-bonding control by a carboxylic acid group on the catalyst.

AIE-MOF materials for biological applications

Metal-Organic Frameworks (MOFs), coming under the realm of coordination chemistry, are unparalleled and the most studied among the group of porous materials. Structurally, these are well-defined three-dimensional crystalline products that can be tuned for various potential applications with a range of physico-chemical properties. More recently, aggregation-induced emission (AIE) and AIE of MOF material has attracted tremendous attention due to promising applications in biology. However, a chapter summarizing the work in AIE-MOFs materials has never been reported till date. A comprehensive review on the AIE and MOFs separately is beyond the reach of this chapter. Hence, we have summarized overview of recent developments in the syntheses and biological applications such as cell imaging, heparin detection, and drug delivery. In the end, conclusion, prospects and challenges in the arena of AIE-MOF materials are also highlighted.

Bis(2-methyl-pyridinium) tetra-chlorido-cuprate(II): synthesis, structure and Hirshfeld surface analysis

The title compound, (C6H8N)2[CuCl4], crystallizes in the monoclinic space group I2/c. The coordination around the copper atom is a distorted tetra-hedron. The 2-methyl-pyridinium ion (C6H8N+) inter-acts with the tetra-chloro-cuprate anion through N-H⋯Cl and C-H(phen-yl)⋯Cl contacts, forming a hydrogen-bonded layer-like structure. The supra-molecular structure is further stabilized by C-H(meth-yl)⋯Cl inter-actions between the layers.

Supramolecular architectures sustained by delocalised C–I⋯π(arene) interactions in molecular crystals and the propensity of their formation

A survey of delocalised C–I⋯π(chelate ring) interactions is presented.

Crystal structure and Hirshfeld surface analysis of di-chlorido-(methanol-κO)bis-(2-methyl-pyridine-κN)copper(II)

In the title complex, [CuCl2(C6H7N)2(CH3OH)], the copper atom is five-coordinated by two nitro-gen atoms of 2-methyl-pyridine ligands, two chloro ligands and an oxygen atom of the methanol mol-ecule, being in a tetra-gonal-pyramidal environment with N and Cl atoms forming the basal plane. In the crystal, complex mol-ecules related by the twofold rotation axis are joined into dimeric units by pairs of O-H⋯Cl hydrogen bonds. These dimeric units are assembled through C-H⋯Cl inter-actions into layers parallel to (001).

Conformational studies of Ant–Pro motif-incorporated cyclic peptides: gramicidin S and avellanin

Conformational studies suggest that an Ant^DPro motif-incorporated synthetic gramicidin S analog retains β-sheet conformation, while its truncated analog avellanin disturbs the β-sheet conformation.

Direct N-alkylation of unprotected amino acids with alcohols

N-alkyl amino acids find widespread application as highly valuable, renewable building blocks. However, traditional synthesis methodologies to obtain these suffer from serious limitations, providing a major challenge to develop sustainable alternatives. We report the first powerful catalytic strategy for the direct N-alkylation of unprotected α-amino acids with alcohols. This method is highly selective, produces water as the only side product leading to a simple purification procedure, and a variety of α-amino acids are mono- or di-N-alkylated, in most cases with excellent retention of optical purity. The hydrophobicity of the products is tunable, and even simple peptides are selectively alkylated. An iron-catalyzed route to mono-N-alkyl amino acids using renewable fatty alcohols is also described that represents an ideal green transformation for obtaining fully bio-based surfactants.

The role of N-terminal proline in stabilizing the Ant–Pro zipper motif

This paper deals with the role of N-terminal proline in stabilizing the Ant–Pro zipper structure by the co-operative contribution of competing forces viz. hydrogen bonding, aromatic stacking and backbone chirality.

Synthetic turn mimetics and hairpin nucleators: Quo Vadimus?

Structural mimicry of peptides has witnessed perceptible progress in the last three decades. Reverse turn and β-hairpin units are the smallest secondary structural motifs that are some of the most scrutinized functional cores of peptides and proteins. The practice of mimicking, without altering the function of the bioactive core, ranges from conformational locking of the basic skeleton to total replacement of structural architecture using synthetic analogues. Development of heterogeneous backbones--using unnatural residues in place of natural ones--has broadened further opportunities for efficient structural rigidification. This feature article endeavours to trail the path of progress achieved hitherto and envisage the possibilities that lie ahead in the development of synthetic turn mimetics and hairpin nucleators.

Formation of a pseudo-β-hairpin motif utilizing the Ant–Pro reverse turn: consequences of stereochemical reordering

Herein, we report a special case of pseudo-β-hairpin formation by tetrapeptide sequences featuring a two-residue Ant–Pro dipeptide motif (Ant = anthranilic acid and Pro = proline) at the loop region.