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Spectral studies of 1-(2-hydroxy-5-chloro-phenyl)-3,5-dioxo-1 H -imidazo-[3,4- b ] isoindol (ADCL) in various solvents reveal the occurrence of excited state intramolecular proton transfer (ESIPT) process. Excitation at the shorter wavelengths preferentially leads to dual emission from an equilibrium mixture of excited state normal (N*, lactim form) and tautomeric (T*, lactam form) form due to the operation of proton transfer process. The proton transfer dynamics is found to be very slow in various organic solvents and it occurs in the nano-second time scale. This may be due to high energy barrier along the proton transfer coordinate through a strained four member hydrogen bonded network compared to the usual six member hydrogen bonded ring. [ABSTRACT FROM AUTHOR]

Highlights: [•] Photophysical study in heterocycles of isoindole fused imidazole system (ADII). [•] Lactim–lactam tautomerism reaction across four member hydrogen bonded network. [•] Effect of intermolecular hydrogen bonding interaction on ESIPT process. [•] Computational study on conformational stability and PEC for ESIPT process. [Copyright &y& Elsevier]

Abstract: We report conformational analysis of an end-protected tripeptide, BOC–Phe–Aib–Leu–OMe in CCl4 solution and solid crystal at room temperature by infrared spectroscopy and quantum chemistry calculation. The measured spectra in those physical conditions appear different, which has been attributed to different secondary structures of the peptide in the two phases. Our analysis reveals that the peptide has only one secondary structure in the crystal, but in the solution there are two different secondary structures. While the spectrum in solution shows excellent correspondence with the superposition of predicted spectra of the two most favored gas-phase secondary structures, the spectrum of crystal shows best correspondence with the predicted spectrum of a relatively higher energy (∼6kcal/mol) secondary structure. However, the latter in crystal can gain maximum stability by intermolecular hydrogen bonding. The secondary structure of the peptide in crystal that we have deduced agrees nicely with the one suggested by X-ray crystallographic study. [Copyright &y& Elsevier]

Single crystal X-ray diffraction studies show that the three designed tripeptides Boc-Leu-Aib-m-NA-NO2 (I), Boc-Phe-Aib-m-NA-NO2 (II) and Boc-Pro-Aib-m-ABA-OMe (III) (Aib, α-aminoisobutyric acid; m-NA, m-nitroaniline; m-ABA, m-aminobenzoic acid; Boc, t-butyloxycarbonyl) containing aromatic rings in the backbones adopt β-turn structures that are self-assembled through intermolecular hydrogen bonds and van der Waals interactions to create layers of β-sheets. Solvent-dependent NMR titration and CD studies show that the β-turn structures of the peptides also exist in the solution phase. The field emission scanning electron microscopic and transmission electron microscopic images of the peptides in the solid state reveal fibrillar structures of flat morphology that are formed through β-sheet mediated self-assembly of the preorganised β-turn building blocks. [ABSTRACT FROM AUTHOR]

Single crystal X-ray diffraction studies show that the β-turn structure of tetrapeptide I, Boc-Gly-Phe-Aib-Leu-OMe (Aib: α-amino isobutyric acid) self-assembles to a supramolecular helix through intermolecular hydrogen bonding along the crystallographic a axis. By contrast the β-turn structure of an isomeric tetrapeptide II, Boc-Gly-Leu-Aib-Phe-OMe self-assembles to a supramolecular β-sheet-like structure via a two-dimensional (a, b axis) intermolecular hydrogen bonding network and π-π interactions. FT-IR studies of the peptides revealed that both of them form intermolecularly hydrogen bonded supramolecular structures in the solid state. Field emission scanning electron micrographs (FE-SEM) of the dried fibrous materials of the peptides show different morphologies, non-twisted filaments in case of peptide I and non-twisted filaments and ribbon-like structures in case of peptide II. [ABSTRACT FROM AUTHOR]

Abstract: The tripeptides Boc-Gly-Aib-m-ABA-OMe (I), Boc-βAla-Aib-m-ABA-OMe (II) and Boc-γAbu-Aib-m-ABA-OMe (III) (Aib: α-aminoisobutyric acid, βAla: β-alanine, γAbu: γ-aminobutyric acid, m-ABA: meta-aminobenzoic acid) with homologated amino acids at the N-terminus, the rigid γ-amino acid m-ABA at the C-terminus and the helicogenic Aib at the central position have been chosen to create unusual turns. Single crystal X-ray diffraction studies, solvent dependent NMR titrations and 2D NMR analysis reveal that peptides II and III adopt unusual turns of 11- and 12-membered rings stabilized by modified 4→1 type intramolecular hydrogen bonds. Solution phase studies indicate that peptide I exists in the β-turn conformation stabilized by 10-membered intramolecular hydrogen bonding. [Copyright &y& Elsevier]

Abstract: Single crystal X-ray diffraction studies reveal that the incorporation of meta-amino benzoic acid in the middle of a helix forming hexapeptide sequence such as in peptide I Boc-Ile(1)-Aib(2)-Val(3)-m-ABA(4)-Ile(5)-Aib(6)-Leu(7)-OMe (Aib: α-amino isobutyric acid; m-ABA: meta-amino benzoic acid) breaks the helix propagation to produce a turn-linker-turn (T-L-T) foldamer in the solid state. In the crystalline state two conformational isomers of peptide I self-assemble in antiparallel fashion through intermolecular hydrogen bonds and aromatic π–π interactions to form a molecular duplex. The duplexes are further interconnected through intermolecular hydrogen bonds to form a layer of peptides. The layers are stacked one on top of the other through van der Waals interactions to form hydrophilic channels filled with solvent methanol. [Copyright &y& Elsevier]

Three tripeptides Boc-Phe-Aib-Val-OMe (1), Boc-Leu-Aib-p-NA-NO2 (2) and Boc-Pro-Aib-m-NA-NO2 (3) (Aib: α-aminoisobutyric acid; p- and m-NA: para- and meta-nitroaniline) have been designed by incorporating aromatic rings to study the self-assembly and fibril formation. Single crystal X-ray diffraction studies show that all the peptides adopt turn-like structures that are self-assembled through intermolecular hydrogen bonds and van der Waals interactions to create layers of β-sheets. Solvent dependent NMR titration and CD studies show that the turn structures of the peptides also exist in the solution phase. The field emission scanning electron microscopic (FE-SEM) images of the peptides in the solid state reveal fibrillar structures of flat morphology that are formed through β-sheet mediated self-assembly of the preorganized turn building blocks. [ABSTRACT FROM AUTHOR]

Abstract: Single crystal X-ray diffraction study reveals that the water soluble tetrapeptide H2N–Ile–Aib–Leu–m-ABA–CO2H, containing non-coded Aib (α-amino isobutyric acid) and m-ABA (meta-amino benzoic acid), crystallizes with two smallest possible diastereomeric β-hairpin molecules in the asymmetric unit. Although in both of the molecules the chiralities at Ile(1) and Leu(3) are S, a conformational reversal in the back bone chain is observed to produce the β-hairpins with β-turn conformations of type II and II′. Interestingly Aib which is known to adopt helical conformation, adopts unusual semi-extended conformation with : −49.5(5)°, ψ: 135.2(5)° in type II and : 50.6(6)°, ψ: −137.0(4)° in type II′ for occupying the i +1 position of the β-turns. The two hairpin molecules are further interlocked through intermolecular hydrogen bonds and electrostatic interactions between – and −+NH3 groups to form dimeric supramolecular β-hairpin aggregate in the crystal state. The CD measurement and 2D NMR study of the peptide in aqueous medium support the existence of β-hairpin structure in water. [Copyright &y& Elsevier]

Abstract: Single crystal X-ray diffraction studies and solvent dependent NMR titration reveal that the designed peptides I and II, Boc-Xx(1)-Aib(2)-Yy(3)-NH(CH2)2NH-Yy(3)-Aib(2)-Xx(1)-Boc, where Xx and Yy are Ile and Leu in peptide I and Leu and Val in peptide II, respectively, fold into a turn-linker-turn (T-L-T) conformation both in the solid state and in solution. In the crystalline state the T-L-T foldamers of peptide I and II self-assemble to form a three-dimensional framework of channels. The insides of the channels are hydrophilic and found to contain solvent CHCl3 hydrogen bonded to exposed Clocated at the turn regions. [Copyright &y& Elsevier]