Unusual N(amide)⋯C–X noncovalent carbon bonding interactions stabilize the trans–cis (t–c) amide bond rotamers of N-methyl-N,N′-diacylhydrazines over the expected trans–trans (t–t) rotamers in solution.

An unusual Namide···H–Namide hydrogen bond (HB) was previously proposed to stabilize the azapeptide β-turns. Herein we provide experimental evidence for the Namide···H–Namide HB and show that this HB endows a stabilization of 1–3 kcal·mol–1 and enforces the trans–cis–trans (t–c–t) and cis–cis–trans (c–c–t) amide bond conformations in azapeptides and N-methyl-azapeptides, respectively. Our results indicate that these Namide···H–Namide HBs can have stabilizing contributions even in short azapeptides that cannot fold to form β-turns.

We report the solid-phase synthesis of N,N′-di­(acylamino)-2,5-diketopiperazine, an acylhydrazide-based conformationally rigid 2,5-DKP scaffold having exocyclic N–N bonds. We also show that different combinations of acylhydrazides, carbazates, semicarbazides, amino acids, and primary amines can be used to synthesize a highly diverse collection of hybrid DKP molecules via the solid-phase submonomer synthesis route. Finally, we show incorporation of a methyl substituent in one of the carbon atoms of the DKP ring to generate chiral daa- and hybrid-DKPs without compromising the synthetic efficiency.

New fluorescent molecular rotors (FMRs) were developed by modification of the Thioflavin T (ThT) structure via introduction of methyl and methoxy groups. Effects of the substituents on fluorescence properties and twisted intramolecular charge transfer (TICT) rate in the excited state of the molecules were studied using steady-state fluorescence and time-resolved absorption spectroscopy. Quantum chemical calculations of the molecules in the ground and excited states were carried out to aid interpretation of the experimental results. Only cationic forms of ThT derivatives have FMR properties and exhibit viscosity-dependent fluorescence. The TICT rate was found to be affected by the size of the molecular fragments, which experience mutual rotation, dihedral angle [Formula: see text] between the fragments in the ground state as well as their donor/acceptor properties.

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