1,721,241 research outputs found
Intercrystalline interactions and other lattice effects in solid-state electronic circular dichroism of organic crystals
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Recent applications of ECD spectra calculations of natural and synthetic porducts - from TDDFT back to coupled oscillators
No full textIntermolecular Interactions and Solid-State Electronic Circular Dichroism in Organic Crystals
No full textSolid-state Circular Dichroism and Hydrogen Bonding, Part 2: The Case of Hypothemycin Re-investigated
No full textThe impact of crystalline forces on the solid-state circular dichroism (CD) spectrum of hypothemycin (), a biologically active molecule extracted from natural sources, has been analyzed by means of time-dependent density functional theory CD calculations. Input structures were extracted from the X-ray geometry of and consisted in the isolated molecule, its cluster with five water molecules, and 20 different dimers (plus water molecules) representative of all the closest neighbors found in the crystal. The effects of solid-state intermolecular hydrogen bonds and through-space exciton couplings in determining the solid-state CD spectrum of hypothemycin were evaluated and compared. Chirality 24:718724, 2012. (c) 2012 Wiley Periodicals, Inc
For a correct application of the CD exciton chirality method: The case of laucysteinamide A
Full text linkThe circular dichroism (CD) exciton chirality method (ECM) is a very popular approach for assigning the absolute configuration (AC) of natural products, thanks to its immediacy and ease of application. The sign of an exciton couplet (two electronic CD bands with opposite sign and similar intensity) can be directly correlated with the molecular stereochemistry, including the AC. However, a correct application of the ECM necessitates several prerequisites: knowledge of the molecular conformation; knowledge of transition moment direction; and preeminence of the exciton coupling mechanism with respect to other sources of CD signals. In recent years, by using quantum-chemical CD calculations, we have demonstrated that some previous applications of ECM were wrong or based on incorrect assumptions. In a recent publication of this journal (Mar. Drugs, 2017, 15(4), 121), the ECM was employed to assign the AC of a marine metabolite, laucysteinamide A. This is a further case of incorrect application of the method, where none of the aforementioned prerequisites is fully met. Using this example, we will discuss the criteria required for a correct application of the ECM
The Exciton Origin of the Visible Circular Dichroism Spectrum of Bacteriorhodopsin
No full textThe 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
Comment on “Breakdown of Exciton Splitting through Electron Donor–Acceptor Interaction: A Caveat for the Application of Exciton Chirality Method in Macromolecules”
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The Role of Heme Chirality in the Circular Dichroism of Heme Proteins
No full textThe rotational strength (R) of the Soret transition in sperm-whale myoglobin (SW Mb), the hemoglobin from Chirononms thummi thummi (C17 Hb), and human hemoglobin (hHb) has been calculated using 20 high-resolution ( R-aro > R-pep. For CTT Hb and hHB, the orders were, respectively, R-int > R-pep > R-aro and R-int > R-aro approximate to R-pep. Human Hb alpha chains showed the same trend as CTT Hb. Only in the hHb beta chains did R-aro predominate, with the order R-aro > R-iot > R-pep. The total predicted R-tot for SW Mb, CTT Hb, and hHb averaged +0.77 +/- 0.10 (0.56-0.80), -0.37 +/- 0.12 (-0.5), and +0.31 +/- 0.17 DBM (0.23-0.50), respectively. (Values in parentheses are experimental values.) Thus, contrary to the currently accepted view, coupling with aromatic side-chain or peptide transitions is not the dominant factor in the Soret circular dichroism (CD) of these proteins. The Soret CD is dominated by intrinsic CD of the heme chromophore, of which vinyl torsion is the major determinant. This result suggests an explanation for the large effect of heme isomerism on the Soret CD of Mb and Hb. Rotation about the alpha-gamma axis may be associated with large changes in vinyl torsion and thus substantially alter the intrinsic CD, even reversing its sign
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