The Exciton Origin of the Visible Circular Dichroism Spectrum of Bacteriorhodopsin

The visible CD spectrum of bacteriorhodpsin (bR) in purple membrane has a negative CD band at ∼600 nm and a positive band at ∼530 nm and has been variously interpreted as resulting from exciton coupling within the bR trimer, heterogeneity in protein conformation, or the presence of two distinct low-energy electronic transitions in bR. We have performed time-dependent density functional theory (TDDFT) calculations on the protonated Schiff base of retinal (retPSB) in bR to predict the intrinsic CD. The resulting spectroscopic parameters have been used to predict the long-wavelength CD spectrum of retPSB trimers. TDDFT, exciton theory, and classical polarizability (DeVoe) predict a strong negative couplet centered near 570 nm, with a magnitude in good agreement with experiment. Coupling of the retPSB chromophore with aromatic and peptide chromophores has been considered by means of perturbation theory and is responsible for the net positive CD of the 570 nm band. The visible CD spectrum of bR is dominated by exciton interactions

Natural α‑Amino Acid-Functionalized Poly(phenyleneethynylene)s (PPEs): Synthesis and Chiroptical Characterization of Aggregate States

The synthesis of several novel poly­(phenyleneethynylene)­s (PPEs) functionalized with different natural α-amino acids methyl esters has been achieved through Cassar–Heck–Sonogashira reaction. Five different derivatives have been prepared varying the nature of the amino acid (Gly, Leu, <i>N</i>-methyl Leu, Phe, and Val), and their aggregation behavior has been investigated by means of UV–vis absorption, circular dichroism (CD), and fluorescence spectroscopies in different conditions of aggregation. ECD measurements provided unique information about the structural organization of the aggregates dispersed in solution and as thin films. The effects of the nature of the amino acidic moiety, the consequences of chirality, and the role played by intermolecular hydrogen bonds have been elucidated

An Exciton-Coupled Circular Dichroism Protocol for the Determination of Identity, Chirality, and Enantiomeric Excess of Chiral Secondary Alcohols

Chiral mono-ols are among the most sought after targets in asymmetric synthesis, and therefore, their chemical characterization and associated enantiomeric excess (<i>ee</i>) values are commonly reported. A simple optical method for determining alcohol identity and <i>ee</i> could be widely used. Toward this end, an in situ-generated multicomponent assembly that creates diastereomeric tris­(pyridine) metal complexes incorporating chiral secondary alcohols was explored using exciton-coupled circular dichroism (ECCD). Qualitative models were proposed to predict the preferential diastereomer and its twist, and computational studies provided a rationalization of the CD spectra. Different ECCD spectra found for diastereomers formed in the self-assembled tris­(pyridine) complexes were used to determine the absolute configurations of chiral mono-ols. Linear discriminant analysis was successfully employed to classify the alcohol analytes, thereby allowing identification of the alcohols. Conformational effects imparted by heteroatoms were also explored, further expanding the substrate scope. Finally, <i>ee</i> calibration curves allowed the determination of the <i>ee</i> of unknown samples of three chiral secondary alcohols with an average error of 3%. The assay described here is unique because no preparation of structurally elaborated chiral hosts is needed

An Exciton-Coupled Circular Dichroism Protocol for the Determination of Identity, Chirality, and Enantiomeric Excess of Chiral Secondary Alcohols

Chiral mono-ols are among the most sought after targets in asymmetric synthesis, and therefore, their chemical characterization and associated enantiomeric excess (<i>ee</i>) values are commonly reported. A simple optical method for determining alcohol identity and <i>ee</i> could be widely used. Toward this end, an in situ-generated multicomponent assembly that creates diastereomeric tris­(pyridine) metal complexes incorporating chiral secondary alcohols was explored using exciton-coupled circular dichroism (ECCD). Qualitative models were proposed to predict the preferential diastereomer and its twist, and computational studies provided a rationalization of the CD spectra. Different ECCD spectra found for diastereomers formed in the self-assembled tris­(pyridine) complexes were used to determine the absolute configurations of chiral mono-ols. Linear discriminant analysis was successfully employed to classify the alcohol analytes, thereby allowing identification of the alcohols. Conformational effects imparted by heteroatoms were also explored, further expanding the substrate scope. Finally, <i>ee</i> calibration curves allowed the determination of the <i>ee</i> of unknown samples of three chiral secondary alcohols with an average error of 3%. The assay described here is unique because no preparation of structurally elaborated chiral hosts is needed

Electronic Circular Dichroism in Exciton-Coupled Dimers: Vibronic Spectra from a General All-Coordinates Quantum-Dynamical Approach

We present a computational approach of general applicability to simulate the vibronic line shapes of absorption and electronic circular dichroism (ECD) spectra in rigid exciton-coupled dimers based on a time-dependent expression of the spectra and quantum dynamical calculations. We adopt a diabatic model of interacting states localized on the monomers whose electronic potential energy surfaces are described within harmonic approximation, including the effect of displacements, frequency changes, and normal-mode mixings. Spectra that fully account for the effect of all nuclear degrees of freedom of the system are obtained through a hierarchical representation of the Hamiltonian in blocks, defined so that few blocks accurately describe the short-time dynamics of the system. With this approach, on the ground of time-dependent density functional theory calculations, we simulate the absorption and ECD spectra of a covalent compound representing a “dimer” of anthracene, in the spectral region of the ¹L_a monomer transition, obtaining results in good agreement with the experiment

Vibronic Coupling Dominates the Electronic Circular Dichroism of the Benzene Chromophore ¹L_b band

The alkylbenzene derivatives (<i>R</i>)-PhCH­(CH₃)^tBu (<b>1</b>) and (<i>R</i>)-PhCH­(CH₃)ⁱPr (<b>2</b>) were taken as paradigms of chiral benzene compounds and their vibronic electronic circular dichroism (ECD) spectrum in the ¹L_b band region analyzed in detail. The ¹L_b ECD band of chiral benzene compounds is often used to assign absolute configurations on the basis of sector rules. However, ¹L_b ECD bands of several benzene derivatives are associated with a forbidden character and show marked vibrational progressions strongly modulating their shape. This is also true for compounds <b>1</b> and <b>2</b>, the latter also showing a peculiar thermochromism. The low-temperature ECD spectrum of <b>2</b> displays in fact an alternation of positive and negative ECD maxima. Vibronic ECD calculations performed within a TDDFT scheme allowed a full rationalization of the observed ECD spectra of <b>1</b> and <b>2</b>. Especially in the case of <b>2</b>, the ECD spectrum in the ¹L_b band region results from a complex balance of Franck–Condon and Herzberg–Teller effects, as well as of conformational factors. Therefore, straightforward sector rules cannot be safely used to assign the absolute configuration of even these simple derivatives

Chiroptical Properties of Glucose-Substituted Poly(<i>p</i>‑phenylene-ethynylene)s in Solution and Aggregate State

The aggregation behavior of two d-glucose-substituted phenyleneethynylenes, an alternate copolymer (<b>AP</b>) and a homooligomer (<b>HO</b>), has been investigated by means of UV–vis absorption, circular dichroism (CD) and fluorescence spectroscopy. CD reveals superior capability to detect the early stages of aggregation and to provide information about aggregate geometries. The multiband CD spectrum of the <b>AP</b> and of analogous chiral PPEs is rationalized on the basis of the exciton coupling between vibronic transitions localized on proximate portions of the chromophoric chains

Phytotoxic activity against <i>Bromus tectorum</i> for secondary metabolites of a seed-pathogenic <i>Fusarium</i> strain belonging to the <i>F. tricinctum</i> species complex

<p>The winter annual grass <i>Bromus tectorum</i> (cheatgrass) has become highly invasive in semiarid ecosystems of western North America. In these areas, a natural phenomenon, complete cheatgrass stand failure (‘die-off’), is apparently caused by a complex interaction among soilborne fungal pathogens. Several <i>Fusarium</i> strains belonging to the <i>Fusarium tricinctum</i> species complex were isolated from these soils and found to be pathogenic on <i>B. tectorum</i> seeds. One of these strains was produced in cheatgrass seed culture to evaluate its ability to produce phytotoxins. Six metabolites were isolated and identified by spectroscopic methods (essentially 1D and 2D NMR and ESIMS) as acuminatopyrone (<b>1</b>), blumenol A (<b>2</b>), chlamydosporol (<b>3</b>), isochlamydosporol (<b>4</b>), ergosterol (<b>5</b>) and 4-hydroxybenzaldehyde (<b>6</b>). Upon testing against <i>B.</i> <i>tectorum</i> in a seedling bioassay, (<b>6</b>) the coleoptile and radicle length of cheatgrass seedlings were significantly reduced. Compounds <b>1</b> and <b>2</b> showed moderate activity, while <b>3</b>–<b>5</b> were not significantly different from the control.</p

Chloromonilinic Acids C and D, Phytotoxic Tetrasubstituted 3‑Chromanonacrylic Acids Isolated from <i>Cochliobolus australiensis</i> with Potential Herbicidal Activity against Buffelgrass (<i>Cenchrus ciliaris</i>)

The fungal pathogen <i>Cochliobolus australiensis</i> isolated from infected leaves of the invasive weed buffelgrass (<i>Pennisetum ciliare</i>) was grown in vitro to evaluate its ability to produce phytotoxic metabolites that could potentially be used as natural herbicides against this weed. Two new tetrasubstituted 3-chromanonacrylic acids, named chloromonilinic acids C (<b>1</b>) and D (<b>2</b>), were isolated from the liquid cultures of <i>C. australiensis</i>, together with the known chloromonilinic acid B. Chloromonilinic acids C and D were characterized by spectroscopic and chemical methods as (<i>E</i>)-3-chloro-3-[(5-hydroxy-3-(1-hydroxy-2-methoxy-2-oxoethyl)-7-methyl-4-oxo-4<i>H</i>-chromen-2-yl)]­acrylic acid and (<i>Z</i>)-3-chloro-3-[(5-hydroxy-3-(2-methoxy-2-oxoethyl)-7-methyl-4-oxo-4<i>H</i>-chromen-2-yl)]­acrylic acid, respectively. The stereochemistry of chloromonilinic acids C and D was determined using a combination of spectroscopic and computational methods, including electronic circular dichroism. The fungus produced these compounds in two different liquid media together with cochliotoxin, radicinin, radicinol, and their 3-epimers. The radicinin-related compounds were also produced when the fungus was grown in wheat seed solid culture, but chloromonilinic acids were not found in the solid culture organic extract. All three chloromonilinic acids were toxic to buffelgrass in a seedling elongation bioassay, with significantly delayed germination and dramatically reduced radicle growth, especially at a concentration of 5 × 10^–3 M

In Situ Assembly of Octahedral Fe(II) Complexes for the Enantiomeric Excess Determination of Chiral Amines Using Circular Dichroism Spectroscopy

A method for discriminating between α-chiral primary amine enantiomers is reported. The method utilizes circular dichroism (CD) spectroscopy and a sensing ensemble composed of 2-formyl-3-hydroxypyridine (<b>4</b>) and Fe­(II)­(TfO)₂. Aldehyde <b>4</b> reacts rapidly with chiral amines to form chiral imines, which complex Fe­(II) to form a series of diastereomeric octahedral complexes that are CD-active in both the UV and visible regions of the spectrum. NMR studies showed that for enantiomerically pure imine complexes, the Δ-<i>fac</i> isomer is preferred. A statistical analysis of the distribution of stereoisomers accurately modeled the calibration curves for enantiomeric excess (ee). CD signals appearing in the UV region were bisignate, and the nulls of the CD signals were coincident with maxima in the UV spectrum, consistent with exciton coupling. Time-dependent density functional theory and semiempirical calculations confirmed that the CD signals in the UV region arise from coupling of the π–π* transitions in the imine chromophores and that they can be used to describe the signs and magnitudes of the curves accurately. The CD signals in the visible region arise from metal-to-ligand charge-transfer bands, and these signals can be used to determine the ee values of chiral amines with an average absolute error of ±5%. Overall, the strategy presented herein represents a facile in situ assembly process that uses commercially available simple reagents to create large optical signals indicative of ee values