Stability of doubly and triply H-bonded complexes governed by acidity-basicity relationships

We used our recently proposed acidity-basicity interplay (ABI) model (Chem. Sci., 2018, 9, 4402) and the Jorgensen secondary interactions hypothesis (JSIH) to rationalise the experimentally observed trends in the formation constants of doubly and triply H-bonded systems with -NHO[double bond, length as m-dash]C- and -NHN- interactions. Unlike the JSIH, the ABI interpretation can explain the trends in the complexation of amide/imide homo- and heterodimers as well as ADA-DAD clusters. We found that the strongest H-bonds play a very important role, a condition which offers an alternative to the well established JSIH to modulate the stability of these relevant systems.

Self-Organisation in the Assembly of Gels from Mixtures of Different Dendritic Peptide Building Blocks

This paper investigates dendritic peptides capable of assembling into nanostructured gels, and explores the effect on self-assembly of mixing different molecular building blocks. Thermal measurements, small angle X-ray scattering (SAXS) and circular dichroism (CD) spectroscopy are used to probe these materials on macroscopic, nanoscopic and molecular length scales. The results from these investigations demonstrate that in this case, systems with different "size" and "chirality" factors can self-organise, whilst systems with different "shape" factors cannot. The "size" and "chirality" factors are directly connected with the molecular information programmed into the dendritic peptides, whilst the shape factor depends on the group linking these peptides together--this is consistent with molecular recognition hydrogen bond pathways between the peptidic building blocks controlling the ability of these systems to self-recognise. These results demonstrate that mixtures of relatively complex peptides, with only subtle differences on the molecular scale, can self-organise into nanoscale structures, an important step in the spontaneous assembly of ordered systems from complex mixtures.

Asymmetry Induction by Cooperative Intermolecular Hydrogen Bonds in Surface-Anchored Layers of Achiral Molecules

The mesoscale induction of two-dimensional supramolecular chirality (formation of 2D organic domains with a single handedness) was achieved by self-assembly of 1,2,4-benzenetricarboxylic (trimellitic) acid on a Cu(100) surface at elevated temperatures. The combination of spectroscopic [X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS)], real-space-probe [scanning tunneling microscopy (STM)], and computational [density functional theory (DFT)] methods allows a comprehensive characterization of the obtained organic adlayers, where details of molecular adsorption geometry, intermolecular coupling, and surface chemical bonding are elucidated. The trimellitic acid species, comprising three functional carboxylic groups, form distinct stable mirror-symmetric hydrogen-bonded domains. The chiral ordering is associated with conformational restriction in the domains: molecules anchor to the substrate with an ortho carboxylate group, providing two para carboxylic acid moieties for collective lateral interweaving through H bonding, which induces a specific tilt of the molecular plane. The ease of molecular symmetry switching in domain formation makes homochiral-signature propagation solely limited by the terrace width. The molecular layer modifies the morphology of the underlying copper substrate and induces mum-sized strictly homochiral terraces.

Synthesis, Multiphase Characterization, and Helicity Control in Chiral DACH-Linked Oligothiophenes

A new class of chiral oligothiophenes is described. Mono-, bi-, ter-, and quarterthiophenes have been linked to enantiopure trans-1,2-cyclohexanediamine (DACH) via diamino or diimino moieties. The stereochemistry of DACH, the type of linker, and oligothiophene size determine the conformational flexibility of these molecules and consequently their molecular and supramolecular helicity in solution and in the solid state. The case of diaminobis(bithiophene), which inverts helicity and shows chiral amplification in the transition from solution to film, is described in detail. Based on the combined use of circular dichroism in solution and in the solid state, single-crystal/thin-film X-ray diffraction, and polarized optical microscopy, a working mechanism has been proposed to explain this unexpected behavior.

Homochiral Conglomerates and Racemic Crystals in Two Dimensions: Tartaric Acid on Cu(110)

Two-dimensional lattice structures formed by racemic tartaric acid on a single crystalline Cu(110) surface have been studied and compared with the enantiopure lattices. At low coverage, the doubly deprotonated bitartrate species is separated into two-dimensional conglomerates showing opposite enantiomorphism. At higher coverage, however, a singly deprotonated monotartrate species forms a heterochiral, racemic crystal lattice. While the enantioseparated bitartrate system undergoes decomposition at the same temperature as the enantiopure system, the racemic monotartrate lattice has a lower thermal stability than the enantiopure lattice of identical periodicity and surface density. At monolayer saturation coverage, the pure enantiomers form a denser lattice than the racemate. This is in contrast to the three-dimensional tartaric acid crystals, where the racemate crystallizes in a lattice of higher density, which is also more thermally stable than the enantiopure tartaric acid crystals.

Simple and complex lattices of N-alkyl fatty acid amides on a highly oriented pyrolytic graphite surface

STM investigations of three N-alkyl fatty acid amide molecules have been carried out to get information of their molecular arrangement on a highly oriented pyrolytic graphite surface. With variable positions of amide along the alkyl chain, complex lattices with different lattice constants were observed. Besides the lattices with a repeat unit matching one or two molecular lengths, a lattice with a repeat unit corresponding to three molecular lengths was found. In addition, the portion of different lattices depends on the length of the shorter alkyl chain. DFT-D calculations point to interactions of antiparallel oriented dipoles due to the amide group, which are distance dependent and thus larger for shorter N-alkyl chains.

Octyl-Decorated Fréchet-Type Dendrons: A General Motif for Visualisation of Static and Dynamic Behaviour Using Scanning Tunnelling Microscopy?

A detailed STM study of monolayers of 3,5-bis[(3,5-bisoctyloxyphenyl)methyloxy]benzaldehyde and 3,5-bis[(3,5-bisoctyloxyphenyl)methyloxy]benzyl alcohol adsorbed on graphite is presented. Very highly resolved scanning tunnelling microscopy images are observed at room temperature in air allowing the analysis of the conformation of the adsorbed molecules. These long-chain alkyl-decorated Fréchet-type dendrons are a powerful assembly motif and initially form a pattern based on trimeric units, assembled into hexagonal host structures with a pseudo-unit cell of seven molecules, one of which remains highly mobile. Over time, the supramolecular ordering changes from a trimeric into a dimeric pattern. The chirality arising from the adsorption onto a surface of the dendrons is discussed.

Chirality of living systems: a helping hand from crystals and oligopeptides

Left-right asymmetry is ubiquitous in nature. Recent studies reveal changes in the energy and growth rate of crystal surfaces to which D or L amino acids bind, with the binding itself being dictated by stereochemical matching. Likewise, oligomerization of amino acids appears to be a chiroselective process that enables the propagation of sequences with defined handedness.[[For a definition of chiroselective self-assembly, see: M. Bolli, R. Micura, A. Eschenmoser, Chem. Biol. 1997, 4, 309-320.]] These results, along with related findings on symmetry breaking and further amplification of asymmetry at a supramolecular level, constitute new insights into the origin of homochirality in living species.