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2 results on '"Ultraviolet/methods"'

1. Aromaticity as a Guiding Concept for Spectroscopic Features and Nonlinear Optical Properties of Porphyrinoids

Author

Mercedes Alonso, Frank De Proft, Paul Geerlings, Benoît Champagne, Tatiana Woller, Faculty of Sciences and Bioengineering Sciences, and Chemistry

Subjects
Porphyrins, Absorption spectroscopy, Hyperpolarizability, Pharmaceutical Science, nonlinear optical properties, Porphyrinoids, 010402 general chemistry, 01 natural sciences, Article, Analytical Chemistry, lcsh:QD241-441, Structure-Activity Relationship, lcsh:Organic chemistry, Isomerism, Polarizability, Ultraviolet/methods, Drug Discovery, Molecular symmetry, Physical and Theoretical Chemistry, Absorption (electromagnetic radiation), Spectrophotometry, Ultraviolet/methods, 010405 organic chemistry, Aromaticity, Organic Chemistry, Absorption spectra, Porphyrins/chemistry, Planarity testing, 0104 chemical sciences, Nonlinear Dynamics, Chemical physics, Chemistry (miscellaneous), Spectrophotometry, Molecular Medicine, Anisotropy, Spectrophotometry, Ultraviolet, aromaticy, Antiaromaticity
Abstract

With their versatile molecular topology and aromaticity, porphyrinoid systems combine remarkable chemistry with interesting photophysical properties and nonlinear optical properties. Hence, the field of application of porphyrinoids is very broad ranging from near-infrared dyes to opto-electronic materials. From previous experimental studies, aromaticity emerges as an important concept in determining the photophysical properties and two-photon absorption cross sections of porphyrinoids. Despite a considerable number of studies on porphyrinoids, few investigate the relationship between aromaticity, UV/vis absorption spectra and nonlinear properties. To assess such structure-property relationships, we performed a computational study focusing on a series of H&uuml, ckel porphyrinoids to: (i) assess their (anti)aromatic character, (ii) determine the fingerprints of aromaticity on the UV/vis spectra, (iii) evaluate the role of aromaticity on the NLO properties. Using an extensive set of aromaticity descriptors based on energetic, magnetic, structural, reactivity and electronic criteria, the aromaticity of [4n+2] &pi, electron porphyrinoids was evidenced as was the antiaromaticity for [4n] &pi, electron systems. In agreement with previous studies, the absorption spectra of aromatic systems display more intense B and Q bands in comparison to their antiaromatic homologues. The nature of these absorption bands was analyzed in detail in terms of polarization, intensity, splitting and composition. Finally, quantities such as the average polarizability and its anisotropy were found to be larger in aromatic systems, whereas first and second hyperpolarizability are influenced by the interplay between aromaticity, planarity and molecular symmetry. To conclude, aromaticity dictates the photophysical properties in porphyrinoids, whereas it is not the only factor determining the magnitude of NLO properties.

Published
2018

2. Perturbation of DNA hairpins containing the EcoRI recognition site by hairpin loops of varying size and composition : physical (NMR and UV) and enzymatic (EcoRI) studies

Author

Germann, Markus W., Kalisch, Bernd W., Lundberg, Peter, Vogel, Hans J., van de Sande, Johan H., Germann, Markus W., Kalisch, Bernd W., Lundberg, Peter, Vogel, Hans J., and van de Sande, Johan H.

Abstract

We have investigated loop-induced structural perturbation of the stem structure in hairpins d(GAATTCXnGAATTC) (X = A, T and n = 3, 4, 5 and 6) that contain an EcoRI restriction site in close proximity to the hairpin loop. Oligonucleotides containing either a T3 or a A3 loop were not hydrolyzed by the restriction enzyme and also showed only weak binding to EcoRI in the absence of the cofactor Mg2+. In contrast, hairpins with larger loops are hydrolyzed by the enzyme at the scission site next to the loop although the substrate with a A4 loop is significantly more resistant than the oligonucleotide containing a T4 loop. The hairpin structures with 3 loop residues were found to be thermally most stable while larger hairpin loops resulted in structures with lower melting temperatures. The T-loop hairpins are thermally more stable than the hairpins containing the same number of A residues in the loop. As judged from proton NMR spectroscopy and the thermodynamic data, the base pair closest to the hairpin loop did form in all cases studied. The hairpin loops did, however, affect the conformation of the stem structure of the hairpins. From 31P and 1H NMR spectroscopy we conclude that the perturbation of the stem structure is stronger for smaller hairpin loops and that the extent of the perturbation is limited to 2-3 base pairs for hairpins with T3 or A4 loops. Our results demonstrate that hairpin loops modulate the conformation of the stem residues close to the loop and that this in turn reduces the substrate activity for DNA sequence specific proteins., 0305-1048 (Print) Journal Article Research Support, Non-U.S. Gov't

Published
1990
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