1,720,970 research outputs found
L-Stercobilin-HCl and d-Urobilin-HCl. Analysis of Their Chiroptical and Conformational Properties by VCD, ECD, and CPL Experiments and MD and DFT Calculations
No full textVibrational circular dichroism (CD) and IR spectra of dichloromethane solutions of l-stercobilin and d-urobilin hydrochlorides have been recorded in the mid-IR region. The spectra are best interpreted by combining molecular dynamics calculations and density functional theory (DFT) calculations within the quantum mechanics/molecular mechanics ONIOM-type framework, and the combined predicted results are better and more informative than the more standard analysis provided by DFT calculations. The same approach also sheds light on the Cotton effect sign inversion of room temperature versus low-temperature electronic CD spectra of the same compounds in methanol–glycerol solution. Finally, circularly polarized luminescence spectra for l-stercobilin in chloroform solution provide information on the excited-state geometry of this molecule
Bilirubin and its congeners: conformational analysis and chirality from metadynamics and related computational methods
No full textThe conformational properties of natural bilirubin, bisbutyricmesobilirubin-XIIIα, and their chiral derivatives (βS,β′S)-
dimethylmesobilirubin-XIIIα and 8,12-(γR,γ′R)- dimethylbisbutyricmesobilirubin-XIIIα were investigated by metadynamics,
an improved molecular dynamics computational methodology useful for conformational analysis. Applied to linear
tetrapyrroles, which may be treated as two blade molecular propellers, our results are represented by conformational energy
hypersurface maps built as functions of the two central dihedral torsion angles (φ1 and φ2) following rotations of two dipyrrinone
units about the connecting CH2,
and are compared successfully with maps previously obtained from molecular
mechanics. With an emphasis on the conformationally more flexible 8,12-(γR,γ′R)-dimethylbisbutyricmesobilirubin-XIIIα,
a conformationally mobile optically active bilirubinoid with butyric acid replacing propionic, metadynamics reveals global
and nearby minima. Its electronic circular dichroism and UV–Vis spectra predicted by DFT calculations correlate well with
the experimental spectra and are explained on the basis of equilibrium between the predicted conformational energy minima
PH Dependent Chiroptical Properties of (1R,2R)- and (1S,2S)-trans-Cyclohexane Diesters and Diamides from VCD, ECD, and CPL Spectroscopy
No full textDiesters of (1R,2R)- and (1S,2S)-cyclohexanediols and diamides of (1R,2R)- and (1S,2S)-diaminocyclohexane with p-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-d6 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
On the aggregation of bilirubinoids in solution as evidenced by VCD and ECD spectroscopy and DFT calculations
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Insights into the Structures of Bilirubin and Biliverdin from Vibrational and Electronic Circular Dichroism: History and Perspectives
Full text linkIn this work we review research activities on a few of the most relevant structural aspects of bilirubin (BR) and biliverdin (BV). Special attention is paid to the exocyclic C=C bonds being in mostly Z rather than E configurations, and to the overall conformation being essentially different for BR and BV due to the presence or absence of the double C=C bond at C-10. In both cases, racemic mixtures of each compound of either M or P configuration are present in achiral solutions; however, imbalance between the two configurations may be easily achieved. In particular, results based on chiroptical spectroscopies, both electronic and vibrational circular dichroism (ECD and VCD) methods, are presented for chirally derivatized BR and BV molecules. Finally, we review deracemization experiments monitored with ECD data from our lab for BR in the presence of serum albumin and anesthetic compounds
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