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Hans Geiger

Bio: Hans Geiger is an academic researcher from University of Hohenheim. The author has contributed to research in topics: High-performance liquid chromatography & Kaempferol. The author has an hindex of 15, co-authored 31 publications receiving 1600 citations.

Papers
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Journal ArticleDOI
TL;DR: In this article, 13 C NMR spectra for a variety of flavonoid glycosidcs are presented and analyzed, and evidence is presented which demonstrates that 13 C nMR spectroscopy is a valuable technique for distinguishing the sites of methylation, glycosylation and acylation in glycoiides.

858 citations

Journal ArticleDOI
TL;DR: The 13C NMR spectra of a range of amentoflavone and hinokiflavone biflavonoids are reported, most for the first time as mentioned in this paper.

150 citations

Journal ArticleDOI
TL;DR: In this article, the ability of a reversed phase high-performance liquid chromatographic system to separate flavonoids [LiChrosorb RP-18 and a combination of an isocratic and a gradient (5% aqueous formic acid and methanol) technique] was studied.

116 citations

Journal ArticleDOI
TL;DR: In this paper, a reversed-phase system for the separation of simple phenolics and related substances is described, which consists of a LiChrosorb RP-18 (10 μm) Knauer column and a combination of isocratic and linear gradient elution (solvent a, water-formic acid (95:5); solvent B, methanol; temperature, 35°C).

77 citations

Journal ArticleDOI
01 Aug 1984
TL;DR: The absorption and fluorescence spectra of all isomeric hydroxyflavones and methoxyflavone, except for the photolabile 3-methoxyflava, have been measured in organic solvents and in aqueous solutions of various acidity as mentioned in this paper.
Abstract: The absorption and fluorescence spectra of all isomeric hydroxyflavones and methoxyflavones, except for the photolabile 3-methoxyflavone, have been measured in organic solvents and in aqueous solutions of various acidity. With the exception of 5-hydroxyflavone, all are found to be fluorescent in methanol and water solution. On the other hand, there is virtually no fluorescence observed in cyclohexane solution, a fact that is interpreted in terms of high intersystem crossing rates and the El-Sayed selection rules. – Except for the 3-hydroxy and 7-hydroxy isomers, the anions of hydroxyflavones are non-fluorescent, whereas the protonated forms exhibit strong fluorescence emission. Methoxyflavones have higher fluorescence quantum yields than the corresponding hydroxyflavones and have fluorescence decay times ranging from 1.3 to 6.9 ns in methanol at room temperature. – The ground state pKa values of the hydroxyflavones range from 7.8 to 9.8, except for the 5-hydroxy isomer (11.6). The pKa's governing the protonation step range from – 1.22 to – 1.55, again with the exception of the 5-hydroxy isomer (– 3.1). The dissociation constants of the first excited singlet state were calculated with the help of the Forster-Weller equation. The results predict a reversal of the most basic and most acidic sites of the hydroxyflavones. In aqueous solutions, this should result in the formation of excited state tautomers which, however, could be detected only for the 3-hydroxy and 7-hydroxy isomers. Apparently, the lifetimes of the other isomers are too short to allow the establishment of excited state equilibria.

70 citations


Cited by
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Book ChapterDOI
TL;DR: The radical chemistry of flavonoids not only is of interest from a kinetic or mechanistic point of view but also offers considerable insight into structural relationships of highly evolved plant components.
Abstract: Publisher Summary This chapter discusses the radical-scavenging efficiencies of flavonoids as antioxidants To test whether a certain substance acts as radical scavenger, the clearest evidence comes from the determination of reaction rate constants with a set of different specifically produced radicals In the tests discussed in the chapter, hydroxyl, azide, superoxide, linoleic acid peroxyl, tert-butoxyl, and sulfite radicals are used The spectral observation of radical reactions with flavonoids is assisted by the strong absorption characteristics of both the parent compounds and their respective aroxyl radicals Owing to the low solubility of flavonoids, conditions for pseudo-first order reactions with radical species can rarely be achieved for flavonoid aglycones Therefore, kinetic modeling calculations are employed The radical chemistry of flavonoids not only is of interest from a kinetic or mechanistic point of view but also offers considerable insight into structural relationships of highly evolved plant components First, the consistently high rate constants for attack by different types of radicals demonstrate the effective radicals scavenging capabilities of most flavonoids Second, owing to extensive electron delocalization as a prerequisite for radical stabilization, multiple mesomeric structures exist for aroxyl radical species of flavonoids

1,651 citations

Journal ArticleDOI
29 Aug 1986-Science
TL;DR: The induction of nodABC expression by alfalfa exudates demonstrates host-symbiont signaling at an early stage in nodule development.
Abstract: The symbiotic interaction of Rhizobium meliloti and alfalfa results in the formation of nitrogen-fixing root nodules. Rhizobium meliloti nodABC genes are required for the early host responses of cortical cell divisions and root hair curling. The induction of nodABC expression by alfalfa exudates demonstrates host-symbiont signaling at an early stage in nodule development. The inducer molecule for nodABC expression was isolated from plant exudate by constructing a nodABC-lacZ fusion to monitor the inducing activity. From ultraviolet-visible absorption spectra, proton nuclear magnetic resonance, and mass spectrometry, the inducer was determined to be 3',4', 5,7-tetrahydroxyflavone (luteolin). Luteolin is a normal secondary plant metabolite found throughout the plant kingdom that may serve to control nodABC expression during nodule development. This regulatory role for a flavone contrasts with the function of some flavonoids as defense compounds.

948 citations

Journal ArticleDOI
TL;DR: This review provides an updated and extensive overview of methods and their applications in natural product matrices and samples of biological origin and critically appraises recent developments and trends.
Abstract: The impetus for developing analytical methods for phenolic compounds in natural products has proved to be multifaceted. Hundreds of publications on the analysis of this category of compounds have appeared over the past two decades. Traditional and more advanced techniques have come to prominence for sample preparation, separation, detection, and identification. This review provides an updated and extensive overview of methods and their applications in natural product matrices and samples of biological origin. In addition, it critically appraises recent developments and trends, and provides selected representative bibliographic examples.

872 citations

Journal ArticleDOI
TL;DR: In this article, 13 C NMR spectra for a variety of flavonoid glycosidcs are presented and analyzed, and evidence is presented which demonstrates that 13 C nMR spectroscopy is a valuable technique for distinguishing the sites of methylation, glycosylation and acylation in glycoiides.

858 citations

Journal ArticleDOI
01 Jun 1997
TL;DR: This work focuses on the recent developments in pollen biology that help to understand how the male gamete survives and accomplishes its successful delivery to the ovule of the sperm to effect sexual reproduction.
Abstract: Many aspects of Angiosperm pollen germination and tube growth are discussed including mechanisms of dehydration and rehydration, in vitro germination, pollen coat compounds, the dynamic involvement of cytoskeletal elements (actin, microtubules), calcium ion fluxes, extracellular matrix elements (stylar arabinogalactan proteins), and control mechanisms of gene expression in dehydrating and germinating pollen. We focus on the recent developments in pollen biology that help us understand how the male gamete survives and accomplishes its successful delivery to the ovule of the sperm to effect sexual reproduction.

778 citations