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ABSTRACT: The Raman Optical Activity (ROA) spectra of both enantiomers of 2-Br-hexahelicene in chloroform solution have been measured in the range 1700-300 cm-1. Density Functional Theory (DFT) calculations accurately reproduce the observed features. The most intense ROA features are also the most intense Raman features, in the region 1350-1400 cm-1, and correspond to the so-called D modes, which play a major role in coronene and other PAH's (polycyclic aromatic hydrocarbons). Together with a detailed analysis of the normal mode structure, the polarizability tensors for the intense Raman features are investigated and related to the principal characteristics of helicene systems, namely chirality and π-conjugation. Through electron-phonon coupling analysis we propose a mechanism that justifies the intense ROA signals.
The Journal of Physical Chemistry B 01/2013; · 3.70 Impact Factor
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ABSTRACT: Strongly fluorescent dipyrrinones can be prepared by bridging the pyrrole and lactam nitrogens with a carbonyl group, from
reaction with N,N′-carbonyldiimidazole in the presence of a strong, non-nucleophilic base. The yellow, N,N′-carbonyl-bridged dipyrrinones typically have fluorescent quantum yields (φF) approaching 1.0. Thus, in chloroform, N,N′-bridged 9H-dipyrrinones with β-alkyl substituents: 2,3-diethyl-7,8-dimethyl has φF = 0.90 (λem = 465 nm) and 2,3-dimethyl-7,8-dimethoxy has φF = 0.84 (λem = 482 nm). In contrast, 2,3-dimethoxy-7,8-dimethyl and 2,3,7,8-tetramethoxy show red-shifted λem but with strongly reduced φF: φF = 0.10 (λem = 511 nm) and 0.08 (λem = 511 nm), respectively. Methoxy substituents on the lactam, but not the pyrrole ring act to quench the fluorescence and
shift the emission and excitation wavelengths bathochromically. The first X-ray crystal structure of an N,N′-carbonyl-bridged dipyrrinone was obtained from 7,8-dimethoxy-2,3-dimethyl-10H-dipyrrin-1-one.
Monatshefte fuer Chemie/Chemical Monthly 05/2012; 139(11):1377-1385. · 1.53 Impact Factor
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ABSTRACT: AbstractSix 10-nor-bilirubin analogs have been synthesized and investigated. Lacking the C(10) CH2 group, these linear tetrapyrroles have a bipyrrole core rather than a dipyrrylmethane core and thus a different shape. Whereas
the propionic acid groups of bilirubin are well engaged in intramolecular hydrogen bonding to the dipyrrinones, molecular
modeling studies of the 10-nor-rubins predict that propionic acid chains are too short to engage the CO2H hydrogen fully in intramolecular hydrogen bonding with the dipyrrinones. Butyric acid chains, however, can and do lead to
a stabilized conformation with a bipyrrole dihedral angle of approximately 115°. Spectroscopic studies verify the predictions
and vapor pressure osmometry indicates that the 10-nor-rubins with butyric acids are monomeric in CHCl3.
Graphical abstract
Monatshefte fuer Chemie/Chemical Monthly 04/2012; 140(1):97-110. · 1.53 Impact Factor
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ABSTRACT: Replacing the typical lactam β-alkyl substituents of xanthobilirubinic acid and kryptopyrromethenone, two bilirubin analogs
long used as model compounds in studies of its photochemistry and metabolism, leads to increased amphiphilicity. Synthesized
by base-catalyzed condensation of 3,4-dimethoxypyrrolin-2-one with the appropriate pyrrole α-aldehyde, the 2,3-dimethoxyl
analogs of xanthobilirubinic acid and kryptopyrromethenone are yellow-colored dipyrrinones that form intermolecular hydrogen-bonded
dimers in the solid, as determined by X-ray crystallography, and in CHCl3, as revealed by 1H NMR and vapor pressure osmometry. These two new dipyrrinones are approximately ten times more soluble in water than their
parent dipyrrinones.
Monatshefte fuer Chemie/Chemical Monthly 04/2012; 138(7):687-697. · 1.53 Impact Factor
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ABSTRACT: A set of four regioisomeric dipyrrinone propionic acids has been synthesized and their hepatic metabolism examined in rats:
xanthobilirubinic acid and pseudo-xanthobilirubinic acid each with a propionic acid on a pyrrole ring; exo-ψ-xanthobilirubinic acid and endo-ψ-xanthobilirubinic acid, each with a propionic acid transposed to a lactam ring. After intravenous injection all four isomers
were excreted to some degree in unchanged form in bile in normal rats. Xanthobilirubinic acid, the structurally closest dipyrrinone
to bilirubin, and exo-ψ-xanthobilirubinic acid were excreted almost entirely in unchanged form. However, a small fraction of xanthobilirubinic
acid acyl glucuronide was also detected. More extensive acyl glucuronidation was observed for pseudo-xanthobilirubinic acid,
and endo-ψ-xanthobilirubinic acid underwent slow metabolism to unidentified more polar products that did not seem to be glucuronides.
Monatshefte fuer Chemie/Chemical Monthly 04/2012; 137(11):1463-1476. · 1.53 Impact Factor
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ABSTRACT: Abstract
13CO2H-Labeled [6]-semirubin, 9-(6-[13C]-5-carboxypentyl)-2,3,7,8-tetramethyl-(10H)-dipyrrin-2-one, and its 91-oxo analog were synthesized from 2,3,7,8-tetramethyl-(10H)-dipyrrin-2-one. 13C NMR T
1 measurements of the 13CO2H carbon were run in CDCl3, dioxane, (CH3)2SO and aqueous solvents and compared with those from the corresponding methyl esters. The data as well as those from nuclear
Overhauser effect (NOE) studies indicate intramolecular hydrogen bonding between the CO2H and dipyrrinone groups. Studies of the acids in aqueous solvent predict (1) intramolecular hydrogen bonding over a wide
range of pH and (2) a pK
a
of ~4.4.
Graphical Abstract
Monatshefte fuer Chemie/Chemical Monthly 04/2012; 140(10):1229-1236. · 1.53 Impact Factor
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ABSTRACT: AbstractNovel bilirubin analogs with two nonionic, water-solubilizing polyethyleneglycol (PEG) β-substituents of varying chain length
on the lactam ring of each dipyrrinone were synthesized. In contrast to bilirubin, which is insoluble in CH3OH and in H2O at pH 7 but somewhat soluble in CHCl3 (~1mM), the PEGylated rubins are soluble in all three solvents, with H2O solubility increasing with increasing number of ethyleneglycol units in the PEG chain(s). Vapor pressure osmometry indicates
that, like bilirubin, they are monomeric in CHCl3 and in CH3OH. Nuclear magnetic resonance (NMR) studies indicate that their most stable structure in these solvents and in H2O has the 4Z,15Z configuration that is bent into a ridge-tile shape with the pigment’s dipyrrinones engaged in intramolecular hydrogen bonding
to the propionic acid carboxyl groups. Aqueous pK
a values for the intramolecularly hydrogen-bonded carboxyl groups of these compounds, determined by vacuum-assisted multiplexed
capillary electrophoresis in H2O–CH3OH mixtures followed by Yesuda-Shedlovsky extrapolations to pure H2O, were found to be 4.9, as previously determined by NMR titrations for mesobilirubin-XIIIα.
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Monatshefte fuer Chemie/Chemical Monthly 04/2012; 141(1):101-109. · 1.53 Impact Factor
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ABSTRACT: AbstractOrange-colored 7,8-dimethyl-2,3,12,13-tetramethoxy-(15H,17H)-tripyrrin-1,14-dione and its 2,3,7,8,12,13-hexaethyl analog were synthesized and converted to fluorescent derivatives by
bridging a lactam nitrogen to the adjacent pyrrole nitrogen in a reaction with carbonyldiimidazole. The tripyrrindiones were
also converted to the corresponding mono lactim methyl ethers by reaction with neat trimethylphosphite, and a corresponding
difluoroboryl (BODIPY) derivative was made from the lactim by reaction with boron trifluoride etherate. X-ray structures of
the tetramethoxytripyrrindione, the N,N′-carbonyl-bridged hexa-ethyltripyrrindione, and the lactim methyl ether of the hexa-ethyltripyrrindione were determined.
The N,N′-bridged pigments exhibited very large Stokes’ shifts with fluorescence quantum yields ranging from 1.0 to 0.02.
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Monatshefte fuer Chemie/Chemical Monthly 04/2012; 140(10):1171-1181. · 1.53 Impact Factor
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ABSTRACT: AbstractThe title compound, labeled with 13C in the ethyl groups was synthesized from K13CN and low-molecular-weight components. The synthetic relay compound was 31(32)[13C]-xanthobilirubinic acid methyl ester in a synthetic route that leads to a label in the ethyl β-substituent of a dipyrrinone
model for bilirubin. This labeled dipyrrinone was oxidatively coupled to the dimethyl ester of mesobiliverdin-XIIIα, thereby
providing a route to a 13C-labeled mesobiliverdin and mesobilirubin, with one carbon of each ethyl being 98% 13C-enriched.
Graphical Abstract
Monatshefte fuer Chemie/Chemical Monthly 04/2012; 140(1):111-119. · 1.53 Impact Factor
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ABSTRACT: AbstractXanthobilirubinic acid, a model dipyrrinone for one-half of the bilirubin molecule in photochemical and metabolism studies,
is more polar than bilirubin and insoluble in water and in chloroform. Replacing the β-alkyl substituents on the lactam ring
of xanthobilirubinic acid with methyl-capped ethylene glycol, diethylene glycol, and triethylene glycol (PEG) groups steadily
increased the water solubility of the pigment so that the last is completely soluble in both water and chloroform. Synthesized
by base-catalyzed condensation of the corresponding methyl-capped 3,4-diPEG-pyrrolin-2-one with 3,5-dimethyl-4(2-ethoxycarbonylethyl)-2-formyl-1H-pyrrole, these new PEGylated analogs of xanthobilirubinic acid are yellow-colored dipyrrinones that form intermolecular hydrogen-bonded
dimers in chloroform solution but are monomeric in methanol and water, as revealed by 1H NMR spectroscopy and vapor pressure osmometry. Methyl xanthobilirubinate has served as a synthetic precursor to bilirubinoids;
its amphiphilic PEGylated analogs suggest a route to water-soluble bilirubinoids and biliverdinoids.
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Monatshefte fuer Chemie/Chemical Monthly 04/2012; 140(2):161-170. · 1.53 Impact Factor
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ABSTRACT: The vibrational circular dichroism (VCD) spectra of (αR,α'R)-, (αS,α'S)-, (βR,β'R)-, and (βS,β'S)-dimethylmesobilirubin-XIIIα have been recorded in the range of 1800-900 cm(-1) in CDCl(3) solution and in mixed DMSO-d(6)/CDCl(3) 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.
The Journal of Physical Chemistry B 04/2012; · 3.70 Impact Factor
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ABSTRACT: The vibrational circular dichroism (VCD) and IR absorption spectra of the (-)-enantiomer of 2-Br-hexahelicene have been measured and interpreted by use of density functional theory (DFT) calculations. From time dependent DFT calculations we also interpret the electronic circular dichroism (ECD) spectra of 2-Br-hexahelicene. We compare the calculated IR, VCD and ECD spectra to the corresponding calculated data of hexahelicene and 2-aza-hexahelicene; for the last compound we also recorded the ECD spectra. Comparison with current literature allows an insight to be gained on the meaning and usefulness of some VCD features.
Physical Chemistry Chemical Physics 10/2009; 11(40):9039-43. · 3.57 Impact Factor
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ABSTRACT: The absorption spectra and vibrational circular dichroism (VCD) spectra in the mid-IR range 1600-950 cm(-1) of 10 camphor-related compounds have been recorded and compared to DFT calculated spectra at the B3PW91/TZ2P level and have been examined together with the corresponding data of the parent molecules. The rigidity of the bridged structure common to all compounds investigated permits (a) identification of three spectroscopic regions in the mid-IR range that can be "used" separately by the interested stereochemist for structural diagnosis and assignment of some major characteristics of the VCD spectra in these regions to what we call "skeletal chiral sense" and (b) recognition of possible conformers for flexible substituent groups, when present. VCD spectra of the 10 molecules have been recorded and analyzed also in the CH-stretching region, 3100-2800 cm(-1). Here, we have been able to identify and characterize features of vibrational excitons by comparison of data within the 10-molecule class. To find a theoretical justification of result (a), we have examined the potential energy distribution of the normal modes in the mid-IR range, the partitioning of the calculated rotational strengths in terms of contributions from all couples of internal coordinates, the angle formed by the two vectors, the electric dipole transition moment and the magnetic dipole transition moment, and finally the overlap of normal modes of different molecules. A discussion is provided as to the usability of the introduced algorithms.
The Journal of Physical Chemistry A 09/2009; 113(42):11390-405. · 2.95 Impact Factor
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ABSTRACT: The 10-thia analog of mesobilirubin-XIIIα, with the central C(10) CH2 group replaced by sulfur (1a) was synthesized by condensation of neoxanthobilirubic acid (3) with sulfur dichloride. Thia-rubin 1a is a brilliant yellow solid, forming bright yellow solutions with uv-visible absorption maximum from 425–440 nm.
Journal of Heterocyclic Chemistry 04/2009; 40(6):1113 - 1116. · 1.22 Impact Factor
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ABSTRACT: Replacing the typical beta-alkyl substituents of [6]-semirubin and [6]-oxosemirubin, two intramolecularly hydrogen-bonded bilirubin analogs, with methoxy groups produces amphiphilic dipyrrinones. Synthesized from the respective 9H-dipyrrinones prepared by base-catalyzed condensation of 3,4-dimethoxypyrrolin-2-one with the appropriate pyrrole alpha-aldehyde, the 2,3-dimethoxy and 2,3,7,8-tetramethoxy analogs of [6]-semirubin are yellow-colored dipyrrinones that form intramolecularly hydrogen-bonded monomers in CDCl(3), as deduced from (1)H-NMR NH chemical shifts. They are monomeric in CHCl(3), as determined by vapor pressure osmometry. In contrast, in the solid, X-ray crystallography reveals supramolecular ribbons of intermolecularly hydrogen-bonded (dipyrrinone to dipyrrinone and acid to acid) 2,3,7,8-tetramethoxy-[6]-semirubin. The latter is approximately 20 times more soluble in water than the parent [6]-semirubin with four beta-methyl groups.
Tetrahedron 03/2009; 65(12):2399-2407. · 3.03 Impact Factor
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ABSTRACT: An optically active analog 1 of etiobilirubin-IV with a single fluorine on each of the C(8) and C(12) alkyl groups has been synthesized in order to examine its potential for hydrogen bonding with fluorine. Circular dichroism spectroscopy reveals an unusually strong influence of 2,2,2-trifluoroethanol solvent on diastereo-selection of the M-helical conformation of (81S,121S)-1.
Journal of Heterocyclic Chemistry 03/2009; 36(4):969 - 977. · 1.22 Impact Factor
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ABSTRACT: Thiolactam analogs of kryptopyrromethenone and xanthobilirubic acid methyl ester were prepared from the parent pyrromethenone using Lawesson's reagent and are shown to form dimeric association complexes through intermolecular hydrogen bonding.
Journal of Heterocyclic Chemistry 03/2009; 28(7):1753 - 1756. · 1.22 Impact Factor
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ABSTRACT: Unsymmetric bile pigment analogs of mesobilirubin-XIII and mesobilirubin-IV were synthesized following oxidative coupling of dipyrrinones: methyl xanthobilirubinate with kryptopyrromethenone; methyl ψ-xanthobilirubinate with kryptopyrromethenone.
Journal of Heterocyclic Chemistry 03/2009; 32(1):113 - 121. · 1.22 Impact Factor
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ABSTRACT: 2,3-Benzannelated dipyrrinone analogs (1 and 2) of xanthobilirubic acid (3) are prepared by base-catalyzed condensation of isoindolinone (5) and indolin-2-one (6) respectively, with methyl 3-(2-formyl-3,5-dimethyl-1H-pyrrol-4-yl)propanoate (4). Nuclear Overhauser effect H-nmr studies indicate that both 1 and 2 adopt preferentially a syn-Z configuration. The former forms a hydrogen-bonded homodimer in nonpolar solvents; the latter is intramolecularly hydrogen bonded.
Journal of Heterocyclic Chemistry 03/2009; 40(1):181 - 185. · 1.22 Impact Factor
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ABSTRACT: Bilirubin and its analogs are carboxylic acids that engage in intramolecular hydrogen bonding and are thus thought to be monomelic in solution, although the evidence for the molecularity in solution is indirect. Contrastingly, the dimethyl esters favor intermolecular hydrogen bonding and are thought to be dimeric, yet they, like the bilirubin (acids), exhibit essentially no concentration dependence of their NH nmr chemical shifts upon dilution from 10−2 to 10−5 (or even 10−6) M in chloroform-d. Vapor phase osmometry (vpo) studies of chloroform solutions of eight bilirubins and their dimethyl esters clearly indicate that the former are monomelic, while the latter are dimeric — except when a β-methyl group (but not an *-methyl) is present in each methyl propionate chain. Bilirubin mono-esters might be monomelic or dimeric in solution. Using vpo to study some seven mono-esters or mono acids, we found that the pigments were monomelic in chloroform.
Journal of Heterocyclic Chemistry 03/2009; 37(4):863 - 870. · 1.22 Impact Factor
