Vibrational and Electronic Circular Dichroism of Dimethyl Mesobilirubins-XIIIα

The vibrational circular dichroism (VCD) spectra of (α<i>R</i>,α′<i>R</i>)-, (α<i>S</i>,α′<i>S</i>)-, (β<i>R</i>,β′<i>R</i>)-, and (β<i>S</i>,β′<i>S</i>)-dimethylmesobilirubin-XIIIα have been recorded in the range of 1800–900 cm^–1 in CDCl₃ solution and in mixed DMSO-<i>d</i>₆/CDCl₃ solutions. Ab initio density functional theory (DFT) calculations predict IR vibrational absorption (VA) and VCD spectra in excellent to good correspondence with observed data. The same calculations confirmed the ridge-tile conformation that has been known for a long time. Assignment of vibrational normal modes (NMs) sheds light on the relative importance of local moieties and groups in determining conformational properties of the molecules, as well as their interaction with solvent molecules. Time-dependent DFT (TDDFT) calculations were also performed to provide an understanding of electronic circular dichroism (ECD) spectra and confirm the well-known interpretation based on the exciton model

Importance of C*–H Based Modes and Large Amplitude Motion Effects in Vibrational Circular Dichroism Spectra: The Case of the Chiral Adduct of Dimethyl Fumarate and Anthracene

The role played by the C*–H based modes (C* being the chiral carbon atom) and the large amplitude motions in the vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra is investigated. The example of an adduct of dimethyl fumarate and anthracene, i.e., dimethyl-(+)-(11<i>R</i>,12<i>R</i>)-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylate, and two deuterated isotopomers thereof specially synthesized for this goal, are considered. By comparing the experimental and DFT calculated spectra of the undeuterated and deuterated species, we demonstrate that the C*–H bending, rocking, and stretching modes in the VA and VCD spectra are clearly identified in well defined spectroscopic features. Further, significant information about the conformer distribution is gathered by analyzing the VA and VCD data of both the fingerprint and the C–H stretching regions, with particular attention paid to the band shape data. Effects related to the large amplitude motions of the two methoxy moieties have been simulated by performing linear transit (LT) calculations, which consists of varying systematically the relative positions of the two methoxy moieties and calculating VCD spectra for the partially optimized structures obtained in this way. The LT method allows one to improve the quality of calculated spectra, as compared to experimental results, especially in regard to relative intensities and bandwidths

Conformational Studies of Phe-Rich Foldamers by VCD Spectroscopy and ab Initio Calculations

Employing VCD spectroscopy, we demonstrate that the structural behavior of the oligomers Boc-(l-Phe-l-Oxd)_<i>n</i>-OBn is similar from <i>n</i> = 2 to <i>n</i> = 6; ab initio calculations for the <i>n</i> = 1 case provide physical insight into the conformational properties. Further information is gained by IR, ¹H NMR, and ECD spectroscopies. ECD spectra suggest the presence of different conformations between n = 1 on one side and longer chain foldamers on the other side. VCD and absorption IR spectra in methanol solutions can be interpreted as indicative of a PPII structure. In the case of Boc-l-Phe-l-Oxd-OBn, VCD spectra in CCl₄ and detailed DFT computational analysis allow one to demonstrate that the most populated conformers exhibit backbone dihedral angles similar to those of a PPII geometry. This is a remarkable outcome, as we had previously demonstrated that the Boc-(l-Ala-d-Oxd)<i>_n</i>-OBn series folds in a β-band ribbon spiral that is a subtype of the 3₁₀ helix

pH Dependent Chiroptical Properties of (1<i>R</i>,2<i>R</i>)- and (1<i>S</i>,2<i>S</i>)-<i>trans</i>-Cyclohexane Diesters and Diamides from VCD, ECD, and CPL Spectroscopy

Diesters of (1<i>R</i>,2<i>R</i>)- and (1<i>S</i>,2<i>S</i>)-cyclohexanediols and diamides of (1<i>R</i>,2<i>R</i>)- and (1<i>S</i>,2<i>S</i>)-diaminocyclohexane with <i>p</i>-hydroxycinnamic acid have been known for some time to exhibit intense bisignate electronic circular dichroism (ECD) spectra in CH₃OH. It was also known that added NaOH causes a bathochromic shift of ∼50 nm in CH₃OH, and an even higher one in DMSO. We have measured vibrational circular dichroism (VCD) spectra both for neutral compounds and in the presence of NaOH and other bases. The VCD and IR spectra in the mid-IR region for CD₃OD and DMSO-<i>d</i>₆ solution exhibit high sensitivity to the charged state for the diesters. They possess two strong bisignate features in the presence of bases in the mid-IR, which are interpreted in terms of vibrational exciton couplets, while this phenomenon is less evident in diamides. VCD allied to density functional theory (DFT) calculations allows one to shed some light on the spectral differences of diesters and diamides by studying their conformational properties. Optical rotatory dispersion (ORD) curves confirm the ECD data. Circularly polarized luminescence (CPL) data have been also acquired, which are rather intense in basified solution: the CPL spectra are monosignate and are as intense in the diester and in the diamide case

Importance and Difficulties in the Use of Chiroptical Methods to Assign the Absolute Configuration of Natural Products: The Case of Phytotoxic Pyrones and Furanones Produced by <i>Diplodia corticola</i>

α-Pyrones and furanones are metabolites produced by <i>Diplodia corticola</i>, a pathogen of cork oak. Previously, the absolute configuration (AC) of diplopyrone was defined by chiroptical methods and Mosher’s method. Using X-ray and chiroptical methods, the AC of sapinofuranone C was assigned, while that of the (4<i>S,</i>5<i>S</i>)-enantiomer of sapinofuranone B was established by enantioselective total synthesis. Diplofuranone A and diplobifuranylones A–C ACs are still unassigned. Here electronic and vibrational circular dichroism (ECD and VCD) and optical rotatory dispersion (ORD) spectra are reported and compared with density functional theory computations. The AC of the (4<i>S</i>,5<i>S</i>)-enantiomer of sapinofuranone B and sapinofuranone C is checked for completeness. The AC of diplobifuranylones A–C is assigned as (2<i>S</i>,2′<i>S</i>,5′<i>S</i>,6′<i>S</i>), (2<i>S</i>,2′<i>R</i>,5′<i>S</i>,6′<i>R</i>), and (2<i>S</i>,2′<i>S</i>,5′<i>R</i>,6′<i>R</i>), respectively, with the Mosher’s method applied to define the absolute configuration of the carbinol stereogenic carbon. The AC assignment of sapinofuranones is problematic: while diplofuranone A is (4<i>S</i>,9<i>R</i>), sapinofuranones B and C are (4<i>S</i>,5<i>S</i>) according to ORD and VCD, but not to ECD. To eliminate these ambiguities, ECD and VCD spectra of a di-<i>p</i>-bromobenzoate derivative of sapinofuranone C are measured and calculated. For phytotoxicity studies, it is relevant that all six compounds share the <i>S</i> configuration for the stereogenic carbon atom of the lactone moiety

Mode Robustness in Raman Optical Activity

By reformulating Raman and ROA invariants we provide ground for the definition of robust modes in ROA spectroscopy. Introduction of two parameters defining robustness helps characterization and assignment of ROA bands. Application and use of robustness parameters to [<i>n</i>]­helicenes and oxirane/thiirane derivatives are presented

Chiral Peropyrene: Synthesis, Structure, and Properties

Herein we describe the synthesis, structure, and properties of chiral peropyrenes. Using <i>p</i>-terphenyl-2,2″,6,6″-tetrayne derivatives as precursors, chiral peropyrenes were formed after a 4-fold alkyne cyclization reaction promoted by triflic acid. Due to the repulsion of the two aryl substituents within the same bay region, the chiral peropyrene adopts a twisted backbone with an end-to-end twist angle of 28° that was unambiguously confirmed by X-ray crystallographic analysis. The chiral peropyrene products absorb and emit in the green region of the UV–visible spectrum. Circular dichroism spectroscopy shows strong Cotton effects (Δε = ±100 M^–1 cm^–1 at 300 nm). The Raman data shows the expected D-band along with a split G-band that is due to longitudinal and transversal G modes. This data corresponds well with the simulated Raman spectra of chiral peropyrenes. The chiral peropyrene products also display circularly polarized luminescence. The cyclization reaction mechanism and the enantiomeric composition of the peropyrene products are explained using DFT calculations. The inversion barrier for racemization was determined experimentally to be 29 kcal/mol and is supported by quantum mechanical calculations

Looking at Human Cytosolic Sialidase NEU2 Structural Features with an Interdisciplinary Approach

Circular dichroism (CD) spectra at variable temperatures have been recorded for human cytosolic sialidase NEU2 in buffered water solutions and in the presence of divalent cations. The results show the prevalence of β-strands together with a considerable amount of α-helical structure, while in the solid state, from both previous X-ray diffraction analysis and our CD data on film samples, the content of β-strands is higher. In solution, a significant change in CD spectra occurs with an increase in temperature, related to a decrease in α-helix content and a slight increase in β-strand content. In the same range of temperatures, the enzymatic activity decreases. Although the overall structure of the protein appears to be particularly stable, molecular dynamics simulations performed at various temperatures evidence local conformational changes possibly relevant for explaining the relative lability of enzymatic activity

Chiral Peropyrene: Synthesis, Structure, and Properties

Herein we describe the synthesis, structure, and properties of chiral peropyrenes. Using <i>p</i>-terphenyl-2,2″,6,6″-tetrayne derivatives as precursors, chiral peropyrenes were formed after a 4-fold alkyne cyclization reaction promoted by triflic acid. Due to the repulsion of the two aryl substituents within the same bay region, the chiral peropyrene adopts a twisted backbone with an end-to-end twist angle of 28° that was unambiguously confirmed by X-ray crystallographic analysis. The chiral peropyrene products absorb and emit in the green region of the UV–visible spectrum. Circular dichroism spectroscopy shows strong Cotton effects (Δε = ±100 M^–1 cm^–1 at 300 nm). The Raman data shows the expected D-band along with a split G-band that is due to longitudinal and transversal G modes. This data corresponds well with the simulated Raman spectra of chiral peropyrenes. The chiral peropyrene products also display circularly polarized luminescence. The cyclization reaction mechanism and the enantiomeric composition of the peropyrene products are explained using DFT calculations. The inversion barrier for racemization was determined experimentally to be 29 kcal/mol and is supported by quantum mechanical calculations

Pyrene-Containing <i>ortho</i>-Oligo(phenylene)ethynylene Foldamer as a Ratiometric Probe Based on Circularly Polarized Luminescence

In this manuscript, we report the first synthesis of an organic monomolecular emitter, which behaves as a circularly polarized luminescence (CPL)-based ratiometric probe. The enantiopure helical <i>ortho</i>-oligo­(phenylene)­ethynylene (<i>o</i>-OPE) core has been prepared by a new and efficient macrocyclization reaction. The combination of such <i>o</i>-OPE helical skeleton and a pyrene couple leads to two different CPL emission features in a single structure whose ratio linearly responds to silver­(I) concentration