Lessons from Nature

Terrestrial and marine animals, plants, insects, and microorganisms produce a large variety of compounds for the organism's development, daily survival, self-defence, symbiosis, sexual attraction, etc. Certain of these compounds are discussed here, with emphasis not only on their isolation and structure determination, but also on the question as to why they are active. This requires a multidisciplinary approach, for a better understanding of life's processes and nature's mysteries

Increased duplex stabilization in porphyrin-LNA zipper arrays with structure dependent exciton coupling

Porphyrins were attached to LNA uridine building blocks via rigid 5-acetylene or more flexible propargyl-amide linkers and incorporated into DNA strands. The systems show a greatly increased thermodynamic stability when using as little as three porphyrins in a zipper arrangement. Thermodynamic analysis reveals clustering of the strands into more ordered duplexes with both greater negative ??S and ??H values, and less ordered duplexes with small positive ??S differences, depending on the combination of linkers used. The exciton coupling between the porphyrins is dependent on the flanking DNA sequence in the single stranded form, and on the nature of the linker between the nucleobase and the porphyrin in the double stranded form; it is, however, also strongly influenced by intermolecular interactions. This system is suitable for the formation of stable helical chromophore arrays with sequence and structure dependent exciton coupling

Configurational Assignment of Brassinosteroid Sidechain by Exciton Coupled Circular Diehroic Spectroscopy

Abstract: A microscale method was developed to determine the sidechain configuration of brassinosteroids, a class of potent plant growth promoters. Microscale naphthoylation followed by circular dichroie (CD) spectroscopy measurement in acetonitrile unambiguously differentiate between the two possible vieinal syn-diol configurations, (22R,23R) and (22S,23S). © 1997 Elsevier Science Ltd. Brassinosteroids are a new group of phytohormones with high growth stimulating and anti-stress activities, and which exist in only minuscule amounts. 1-3 After the isolation and characterization of brassinolide in 19794 about 40 members have been discovered in a broad spectrum of monocots and dicots as well as in gymnosperms, thus suggesting their ubiquitous occurrence in plants. 5 Structurally most brassinosteroids are AIB-trans fused steroids with 20t,3ot-diol functionalities, where ring B is 6a-oxa-6-oxo-, 6-oxo-or 6-deoxo. A characteristic feature for all native members found so far is the (22R,23R) vicinal diol in the side chain. Hydroxylation of suitable A22-unsaturated sterol precursors with OsO4 represents a key step in the synthesis of brassinosteroids, which are necessary for structural characterization of new brassinosteroids and for practical applications. Although usage of chiral ligands leads to a higher stereoselectivity of the desired native (22R,23R)-diol, 6 the (22S,23S)-isomer with lower bioactivity 7 is also formed in these reactions. 8 However, since there exists no general method for configurational assignments, a versatile and rapid analytical method based on exciton coupled circular dichroism (ECCD) of their pemaphthoates is reported here. ECCD method is a microscale procedure for determining the absolute configurations and conformations of compounds containing two or more chromophores in solution. 9,10 Hydroxyl and amino groups are converted into para-substituted benzoates, naphthoates, and other chromophores. Provided the chromophores are close-by in space, the electric transition moments will couple and give rise to a bisignate CD. A positive absolute twist between the electron transition moments of the coupled chromophores, i.e., positive chirality, will give rise to a split CD curve with positive first and negative second Cotton effects at longer and shorter wavelength, respectively. The amplitude (A-value) of the split CD, defined as the difference between AEl(first Cotton effect, longer wavelength) and Ae2(second Cotton effect, shorter wavelength), is inversely proportional to the distance between the two chromophores. 9 Importantly, in case of three or more identical 11,12 or different chromophores,13,14 the exciton split CD curve can be approximated by pair-wise addition of the interacting 1196

Exciton coupled circular dichroic studies of self-assembled brevetoxin-porphyrin conjugates in lipid bilayers and polar solvents

Background: Brevetoxins, involved in the ‘red tide' as well as shellfish poisoning, are known to bind to cell membranes and membrane proteins. Brevetoxin B (BTX-B) interacts specifically with neuronal sodium channels. We recently found that BTX also induces selective ion movements across lipid bilayers through transmembrane BTX self-assemblies.Results: We examined the self-assembly of several BTX derivatives in the presence and absence of cations and lipid bilayers using the powerful porphyrin chromophores as circular dichroism labels. BTX derivatives self-assemble into tubes, which can bind to metals both when soluble and when inserted into the bilayer to form transmembrane pores. Depending on the tendency of the BTX derivative to self-aggregate (the critical “micelle” concentration, cmc), it may aggregate in solution before membrane insertion, or may insert itself into the membrane as a monomer before assembling the pore.Conclusions: The active BTX-B complex in lipid bilayers is a cyclic, transmembrane self-assembly consisting of antiparallel aligned BTX molecules that can mediate selective ion movement through membranes. The differences in pore formation mechanisms between BTX derivatives may be reflected in differences in pore formation by natural BTX variants, perhaps explaining their varying levels of toxicity

Intramolecular Porphyrin π,π-Stacking: Absolute Configurational Assignment of Acyclic Compounds with Single Chiral Centers by Exciton Coupled Circular Dichroism

We report a new concept based on exciton coupled circular dichroism (CD) for assigning absolute configurations to a single chiral center *CXYSL, where X is -OH or -NH2, Y is an acyclic chain with terminal OH or -NH2, and S (small) and L (large) represent sterically distinct groups. It consists of a one step attachment of porphyrins to X and Y followed by CD measurement. The key event is intramolecular porphyrin pi,pi-stacking, which converts the flexible, acyclic substrate into a rigid stacked conformation characterized by bisignate exciton split CD curves. Since the stacked conformer is sterically controlled by the two groups, S and L, the sign of the exciton split CD is directly governed by the spatial arrangement of these groups. This approach is applicable to various acyclic compounds with different C/C distances (1,3 approximately 1,15) between the functional groups, but not to 1,2-C/C

Special issue Proceedings of 16th international conference on chiroptical spectroscopy, Rennes 2017

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