Rationalizing the Extent of the "Sergeants-and-Soldiers" Effect in Supramolecular Helical Catalysts: Effect of Copper Coordination.

Aggregation of supramolecular helices, for example through interdigitation of their alkyl side chains or through more directional supramolecular interactions, leads to hierarchical architectures with original structural and chiroptical properties. However, when a chiral monomer (the "sergeant") is introduced as a minor component in these assemblies composed of a majority of achiral monomers (the "soldiers"), it is not clear how the aggregation changes the ability of the sergeant to induce a preferential helicity to the polymer main chain (the so-called "sergeants-and-soldiers" effect). This study reports a detailed investigation of the influence of [Cu(OAc)2 ⋅H2 O] coordination on the structure and chiroptical properties of helical hydrogen-bonded co-assemblies composed of a catalytically-active benzene-1,3,5-tricarboxamide (BTA) monomer, acting as the "soldier", and an enantiopure BTA monomer derived from cyclohexylalanine, playing the role of the "sergeant". The copper actually significantly influences the extent of the "sergeants-and-soldiers" effect since it acts as a crosslink that induces some chiral defects in the supramolecular helices. These crosslinks appear to be preserved during the catalytic hydrosilylation of 4-nitroacetophenone. The aggregation of helices through the formation of copper crosslinks is reversible since homochiral single helices are exclusively formed in the case of sergeant-rich assemblies. The fact that both main chain and side chain aggregation affects the chiroptical properties of supramolecular helices must be considered in the design of elaborated chiral materials.

Molecule-specific vibration-based chiral differentiation of Raman spectra using cysteine modified gold nanoparticles: the cases of tyrosine and phenylalanine.

The chirality of amino acids plays a key role in many biochemical processes, with the development of spectroscopic analysis methods for the chiral differentiation of amino acids being significant. Normal Raman spectroscopy is blind to chirality; however, chiral discrimination of tyrosine (Tyr) (or phenylalanine, Phe) enantiomers using Raman spectra can be achieved assisted by the construction of a simple chiral selector (i.e., cysteine (Cys)-modified Au nanoparticles (NPs)). Due to the synergetic effect between Cys and the Au NPs, the characteristic Raman scattering intensities of the Tyr (or Phe) enantiomer with the same chirality of Cys are enantioselectively boosted by over four-fold compared with those of the counter enantiomer of Tyr (or Phe). The large differences in the Raman signals allow for the determination of enantiomeric excess. Interestingly, such enantiomeric discrimination is not revealed by the common chiral analysis method of circular dichroism spectroscopy. Consequently, it is anticipated that Raman spectroscopy based on molecular vibrations will find broad applications in chirality-related detection with high sensitivity and species specificity.

Chirality Detection by Raman Spectroscopy: The Case of Enantioselective Interactions between Amino Acids and Polymer-Modified Chiral Silica.

In chirality research area, it is of interest to reveal the chiral feature of inorganic nanomaterials and their enantioselective interactions with biomolecules. Although common Raman spectroscopy is not regarded as a direct chirality analysis tool, it is in fact effective and sensitive to study the enantioselectivity phenomena, which is demonstrated by the enantio-discrimination of amino acid enantiomers using the polydopamine-modified intrinsically chiral SiO2 nanofibers in this work. The Raman scattering intensities of an enantiomer of cysteine are more than twice as high as those of the other enantiomer with opposite handedness. Similar results were also found in the cases of cystine, phenylalanine, and tryptophan enantiomers. In turn, these organic molecules could be used as chirality indicators for SiO2, which was clarified by the unique Raman spectra-derived mirror-image relationships. Thus, an indirect chirality detection method for inorganic nanomaterials was developed.

Polydopamine/Silver Substrates Stemmed from Chiral Silica for SERS Differentiation of Amino Acid Enantiomers.

Polydopamine (PDA) and silver (Ag) nanoparticles were first generated on chiral silica nanofibers and then detached from silica to form PDA/Ag composites. The as-obtained PDA/Ag showed surface-enhanced Raman scattering (SERS) activity but very weak circular dichroism optical activity. Interestingly, the PDA/Ag substrates could make a pair of tyrosine (or phenylalanine) enantiomers show different Raman scattering signal intensities, where the differences could reach 3 times. In contrast, PDA/Ag prepared by using racemic or achiral silica did not exhibit such discrimination performance. Therefore, this research offered a novel SERS-based enantiomeric differentiation method with the assistance of plasmonic metal-containing substrates stemmed from intrinsically chiral inorganic silica.

Chiral Plasmonic Nanoparticle Assisted Raman Enantioselective Recognition.

Au nanoparticles (NPs) labeled with the handedness tag of "d-" or "l-", which were detached from inorganic chiral silica, showed both intrinsic chirality and surface enhanced Raman scattering (SERS) activity. In the presence of these chiral Au substrates, it was found that the enantiomer of cystine with the same handedness tag of Au NPs would show stronger Raman scattering signal intensities than those of the enantiomer with the opposite tag, where the differences could be over three times. Consequently, this work afforded a novel enantioselective recognition method on ordinary Raman spectroscopy by using chiral plasmonic metallic nanomaterials.