Natural and abundant plant triterpenoids are attractive starting materials for the synthesis of conformationally rigid and chiral building blocks for functional soft materials. Here, we report the rational design of three oleanolic acid–triazole–spermine conjugates, containing either one or two spermine units in the target molecules, using the Cu­(I)-catalyzed Huisgen 1,3-dipolar cycloaddition reaction. The resulting amphiphile-like molecules 2 and 3, bearing just one spermine unit in the respective molecules, self-assemble into highly entangled fibrous networks leading to gelation at a concentration as low as 0.5% in alcoholic solvents. Using step-strain rheological measurements, we show rapid self-recovery (up to 96% of the initial storage modulus) and sol ⇔ gel transition under several cycles. Interestingly, rheological flow curves reveal the thixotropic behavior of the gels. To the best of our knowledge, this kind of behavior was not shown in the literature before, neither for a triterpenoid nor for its derivatives. Conjugate 4, having a bolaamphiphile-like structure, was found to be a nongelator. Our results indicate that the position and number of spermine units alter the gelation properties, gel strength, and their self-assembly behavior. Preliminary cytotoxicity studies of the target compounds 2–4 in four human cancer cell lines suggest that the position and number of spermine units affect the biological activity. Our results also encourage exploring other triterpenoids and their derivatives as sustainable, renewable, and biologically active building blocks for multifunctional soft organic nanomaterials.

Supramolecular chirality of amyloid fibrils, protein aggregates related to many neurodegenerative diseases, is a remarkable property associated with fibril structure and polymorphism. Since its discovery almost 10 years ago there is still little understanding of this phenomenon, including the cause of the highly enhanced vibrational circular dichroism (VCD) intensity arising from fibril supramolecular chirality. In this study, VCD spectra, enhanced by filament supramolecular chirality, are presented for lysozyme and insulin fibrils above and below pH 2 and after deuterium exchange, above and below pD 2. Supramolecular chirality (observed by VCD) and fibril morphology (documented by atomic force microscopy) are not affected by protein deuteriation. In D O the fibril VCD sign pattern changes to fewer bands, with implications for the amide I/II origin of enhanced VCD intensity. Separation of amide I and II signals will facilitate calculations of enhanced VCD spectra of amyloid fibrils and enable a better understanding of the origin of the VCD sign pattern.

We report the first vibrational circular dichroism (VCD) measurement of spatial heterogeneity in a sample using infrared (IR) microsampling. Vibrational circular dichroism spectra are typically measured using a standard IR cell with an IR beam diameter of 10 mm or greater making it impossible to investigate the spatial heterogeneity of a solid film sample. We have constructed a VCD sampling assembly with either 3 mm or 1 mm spatial resolution. An XY-translation stage was used to measure spectra at different spatial locations producing IR and VCD maps of the sample. In addition, a rotating sample stage was employed using a dual photoelastic modulator (PEM) setup to suppress artifacts due to linear birefringence in solid-phase or film samples. Infrared and VCD mapping of an insulin fibril film has been carried out at both 3 and 1 mm spatial resolution, and lysozyme films were mapped at 1 mm resolution. The IR spectra of different spots vary in intensity due primarily to sample thickness. The changes in the VCD intensity across the map largely correlate to corresponding changes in the IR map. Closer inspection of the insulin map revealed changes in the relative intensities of the VCD spectra not present in the parent IR spectra, which indicated differences in the degree of supramolecular chirality of the fibrils in the various spatial regions. For lysozyme films, in addition to different degrees of supramolecular chirality, reversal of the net fibril chirality was observed. The large signal-to-noise ratio observed at 1 mm resolution implies the feasibility of further increasing the spatial resolution by one or two orders of magnitude for protein fibril film samples.

The asymmetric synthesis of optically pure and conformationally locked oxabenzo[5]­helicenes bearing pyridin-2-yl or isoquinolin-3-yl substituents and their transformation into the corresponding cycloiridated organometallics are described. These helically chiral Cp*IrIII(X)­C,N-complexes (X = Cl, I) also contain a configurationally unstable pseudotetrahedral iridium center. This center undergoes epimerization at room temperature, and its relative stereochemistry, especially in the solid state, depends on the nature of the coordinated ligands. Cycloiridated helicenes were used in the asymmetric transfer hydrogenation of prochiral aromatic imines with formic acid/triethylamine to reach up to 96:4 er. It is assumed that the chirality transfer is controlled by the auxiliary helix rather than the IrIII stereogenic center of the chiral iridacycles.

Halictine-1 (Hal-1)-a linear antibacterial dodecapeptide isolated from the venom of the eusocial bee Halictus sexcinctus-has been subjected to a detailed spectroscopic study including circular dichroism, fluorescence, and vibrational spectroscopy. We investigated Hal-1 ability to adopt an amphipathic α-helical structure upon interaction with model lipid-based bacterial membranes (phosphatidylcholine/phosphatidylglycerol-based large unilamellar vesicles and sodium dodecylsulfate micelles) and helix inducing components (trifluoroethanol). It was found that Hal-1 responds sensitively to the composition of the membrane model and to the peptide/lipid ratio. The amphipathic nature of the helical Hal-1 seems to favour flat charged surfaces of the model lipid particles over the nondirectional interaction with trifluoroethanol. Increasing fraction of polyproline II type conformation was detected at low peptide/lipid ratios.

Triterpenoids are among the largest groups of functional plant secondary metabolites but with intrinsically low water solubility. Because of their rigid backbone, multiple chiral centers, and functional groups, they are suitable for synthesizing water-soluble and conformationally rigid triterpenoid amphiphiles with unique self-assembly behavior. In this context, we present the aqueous self-assembly, structural transition, and antimicrobial properties of nanoscale oleanolic acid–spermine conjugates (2–4). The conjugates contain either one or two spermine moieties connected through a 1,4-disubstituted 1,2,3-triazole linker. We use cryogenic transmission electron microscopy (cryo-TEM) imaging to show that conjugates 2 and 3 self-assemble in water initially into kinetically favored metastable micellar nanoparticles (d ≈ 6–10 nm). The nanoparticles further reorganize to form thermodynamically stable helical nanofibers. Notably, cryo-TEM imaging also suggests the formation of spherulite-like structures. Time-dependent infrared (IR) spectroscopy reveals the role of hydration and dehydration in the structural transition of initial micelle-like structures into thermodynamically stable nanofibers. Electronic and vibrational circular dichroism (ECD and VCD, respectively) spectroscopy in the solution state suggests the formation of chiral superstructures with a left-handed helical twist. The conjugates display antibacterial properties with high selectivity against Gram-positive bacterial strains. The results help us understand fibrillar network formation in supramolecular gels, and demonstrate that the position and number of spermine groups influence the self-assembly behavior of the conjugates in aqueous media and their biological properties.

Using home‐built Raman optical activity (ROA) spectrometer and a relative intensity correction for different instrument responses, we report ROA spectra beyond the traditionally utilized spectral region of 200–2500 cm−1. With three different interchangeable gratings covering partially overlapping spectral regions, we can study ROA in the whole region of fundamental molecular vibrations (150–4000 cm−1). Complete panoramic spectra are assembled from subparts collected with different gratings after a relative intensity correction based on the National Institute of Science and Technology standards known from the analogous application in Raman and fluorescence spectroscopy. Using this setup, we report the still little known ROA from C–H and C–D stretching region of the testing substances α‐pinene and a tricyclic spirodilactam. The intensity‐corrected experimental data were compared with calculated ROA and Raman spectra of these substances both with and without anharmonic corrections. A comparison revealed that above 1200 cm−1, the anharmonic correction provides a clear improvement of the agreement. While the calculation of Raman spectra achieves already good accuracy, the analogous ROA calculations still need further development. Copyright © 2014 John Wiley & Sons, Ltd.

We investigate amide nonplanarity in vibrational optical activity (VOA) spectra of tricyclic spirodilactams 5,8-diazatricyclo[6,3,0,0(1,5)]undecan-4,9-dione (I) and its 6,6',7,7'-tetradeuterio derivative (II). These rigid molecules constrain amide groups to nonplanar geometries with twisted pyramidal arrangements of bonds to amide nitrogen atoms. We have collected a full range vibrational circular dichroism (VCD) and Raman optical activity (ROA) spectra including signals of C-H and C-D stretching vibrations. We report normal-mode analysis and a comparison of calculated to experimental VCD and ROA. The data provide band-to-band assignment and offer a possibility to evaluate roles of constrained nonplanar tertiary amide groups and rigid chiral skeletons. Nonplanarity shows as single-signed VCD and ROA amide I signals, prevailing the couplets expected to arise from the amide-amide interaction. Amide-amide coupling dominates amide II (mainly C'-N stretching, modified in tertiary amides by the absence of a N-H bond) transitions (strong couplet in VCD, no significant ROA) probably due to the close proximity of amide nitrogen atoms. At lower wavenumbers, ROA spectra exhibit another likely manifestation of amide nonplanarity, showing signals of amide V (δ(oop)(N-C) at ~570 cm(-1)) and amide VI (δ(oop)(C'═O) at ~700 cm(-1) and ~650 cm(-1)) vibrations.