12 research outputs found
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<sup>–1</sup> in CDCl<sub>3</sub> solution and in mixed DMSO-<i>d</i><sub>6</sub>/CDCl<sub>3</sub> 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)<sub><i>n</i></sub>-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, <sup>1</sup>H 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 CCl<sub>4</sub> 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><sub>n</sub></i>-OBn series folds in a β-band
ribbon spiral that is a subtype of the 3<sub>10</sub> 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 CH<sub>3</sub>OH. It was also
known that added NaOH causes a bathochromic shift of ∼50 nm
in CH<sub>3</sub>OH, 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 CD<sub>3</sub>OD and DMSO-<i>d</i><sub>6</sub> 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<sup>–1</sup> cm<sup>–1</sup> 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<sup>–1</sup> cm<sup>–1</sup> 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