Wyniki wyszukiwania - Biblioteka Nauki

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Strukturalne zagadki kompleksowanych gramicydyn w ich kryształach

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Główka M. L. , Olczak A. , Bojarska J. , Szczesio M.

Wiadomości Chemiczne

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2007

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tom [Z] 61, 3-4

167-187

EN

Gramicidin D (gD) is a naturally occurring mixture of six linear pentadecapeptides produced by Bacillus brevis species in soil, which exhibit significant antibacterial activity against all Gram+ bacteria. The peptides are modified at both ends and the six forms differ at 1st (Val or Ile) and 11th (Trp, Phe or Tyr) positions. The most populated gramicidin A (VgA) has a sequence CH(O)-Val_1-Gly-Ala-D-Leu-Ala-D-Val-Val-D-Val-Trp-D-Leu-Trp_11-D-Leu-Trp-D-Leu-Trp-NHCH_2CH_2OH. Gramicidin was isolated in 1939 and its sequence was determined five years later. First models of its secondary structure based on helical conformation were proposed in 1971-1974 explaining channel-like properties of the peptide. Despite over 60 years of studies on gramicidin, there are still numerous unans-wered questions concerning its structure and functioning. Even fundamental issues of dimers type and handedness of helices in the gramicidin channel formed by the peptide in the bacterial cell membranes are not univocally accepted. After refining several crystal structures of gramicidin D complexes with alkali metal salts, we have at hands not only more facts concerning double-stranded antiparallel gramicidin dimers, but also new puzzled observations due to very high reso-lution data collected. These, among others, include visible asymmetry of cations' sites and occupations in the gramicidin channels, as well as specific arrangement of heterodimers in the crystals. The paper describes these interesting features of the complexed gramicidin structure and proposes some new explanations.

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Asymmetric p(O)-pi(Triazine) Conjugation in the Formally Symmetric Bis(4,6-dimethoxy-1,3,5-triazinyl) Ether in Its Crystal Structure

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Główka M. L. , Błaszczyk M. , Olczak A. , Kamińska J. E. , Kamiński Z. J.

Polish Journal of Chemistry

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2009

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tom Vol. 83, nr 12

2163-2172

EN

Reported is the crystal structure of a formally symmetric bis(s-triazinyl) ether obtained as a by-product in the acylation reaction of less reactive nucleophiles with 2-acyloxy-4,6-dimethoxy-1,3,5-tri azines. The title ether shows conformational asymmetry evi - denced by different orientations of the four methoxy groups and the two aromatic rings in relation to the central etheric plane. Differentiation of the two O–C(triazine) bond lengths of the bis(triazinyl) ether can be correlated with the angles between s-triazine rings with the central etheric plane, indicating different degree of conjugation of the two pi-systems with alone pair of the central oxygen atom. It seems that primary reason for the observed asymmetry is crystal packing, causing different orientations of methoxy groups, which in turn induced bond lengths alternation in the s-triazine rings affecting their conjugations with central O atom through AGIBA effect. The experimental obser - vations are in agreement with ab in itio calculation.

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Structural Basis for Superiority of N-Methyl-N-(4,6-dimethoxy-1,3,5-triazin-2-yl)morpholinium Tetrafluoroborate as Modular Coupling Reagent. Crystal Structures of N-Methyl-N-(4,6-dimethoxy-1,3,5-tri azin-2-yl)morpholinium and -piperidinium Tetrafluorobor

88%

Olczak A. , Błaszczyk M. , Główka M. L. , Kolesińska B. , Kamiński Z. J.

Polish Journal of Chemistry

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2008

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tom Vol. 82, nr 7

1413-1413

EN

Structural characteristics of tetrafluoroborates of N-methylated N-(4,6- dialkoxy- 1,3,5- triazin-2-yl)-morpholine and -piperidine, as determined by X-ray diffraction study, not only explain their good performance as modular coupling reagents in the peptide synthesis but also their higher stability than appropriate chlorides, which degrade easily through demethylation. Not expected axial orientation of 4,6-dialkoxy-1,3,5-triazinyl substituent at the quaternary N atom of the aliphatic ring observed in the crystal state has been confirmed in a solution by NMR technique. Quarternary triazinyl ammonium salts have been found as excellent modular coupling reagents, particularly in peptide syntheses. Crystal structure determination of N-methyltriazinylmorpholinium and -piperidinium tetrafluoroborates as well as their demethylated products provides a structural reason for the observed reaction. Namely, formation of very stable guanidine system around C(2) atom in 2-morpholine- and 2-piperidine-s-tri azines is controlled thermodynamically, while the difference in stability of chlorides and tetrafluoroborates (being superior) of quarternary N-methyl-N-triazinylmorpholinium (or piperidinium) results from conservative conformation of their cations, which hinders an a cess of large and less nucleophilic tetrafluoroborate an ion than Cl– to the methyl carbon atom.

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Crystal structure of 2-(1,4,5,6-tetrahydropyrimidin-2-yl)phthalazinone-1 hydroiodide

75%

Główka M. L. , Książek W. , Staszewska A. , Kozłowska K. , Ołubek Z. , Krężel I.

Polish Journal of Chemistry

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1998

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tom Vol. 72, nr 8

2009-2013

EN

X-ray crystal study of the 2-(1,4,5,6-tetrahydropyrimidin-2-yl)phthalazinone-1 hydroiodide showed different hydrogen bonding network and packing than those observed in the crystal of two other phthalazinones-1, comprising either smaller (4,5-dihydroimidazolyl) or larger (4,5,6,7-tetrahydrodiazepinyl) ring substituent in position 2 and having very similar crystal structures.