Variable-temperature x-ray structural investigation of [Fe[HC(3,5-Me2pz)3]2](BF4)2 (pz= pyrazolyl ring): observation of a thermally induced spin state change from all high spin to an equal high spin-low spin mixture, concomitant with the onset of nonmerohedral twinning

The complex [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (pz = pyrazolyl ring) undergoes a phase transition that occurs concomitantly with a thermally induced spin conversion between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states. Above 204 K the compound is completely HS with the structure in the C2/c space group with Z = 4. A crystal structure determination of this phase was performed at 220 K yielding the cell constants a = 20.338(2) A, b = 10.332(1) A, c = 19.644(2) A, beta = 111.097(2) degrees, and V = 3851.5(6) A(3). There is one unique iron(II) site at this temperature. Below 206 K the compound converts to a 50:50 mixture of HS and LS. The radical change in the coordination sphere for half of the iron(II) sites, most notably a shortening of the Fe-N bond distances by ca. 0.2 A, that accompanies this magnetic transition causes a phase transition. The crystal system changes from C-centered monoclinic to primitive triclinic with Z = 2 with two half-molecules on independent inversion centers. A crystal structure determination was performed at 173 K in space group P1 with a = 10.287(2) A, b = 11.355(3) A, c = 18.949(4) A, alpha = 90.852(4) degrees, beta = 105.245(4) degrees, gamma = 116.304(4) degrees, and V = 1892.3(8) A(3). All specimens investigated below the phase transition temperature were determined to be nonmerohedral twins. Temperature cycling between these two forms does not appear to degrade crystal quality. Previous magnetic susceptibility measurements indicate a second, irreversible increase in the magnetic moment the first time the crystals are cooled below 85 K. A crystal structure determination at 220 K of a specimen precooled to 78 K was not significantly different from those not cooled below 220 K.     

Novel two-band ratiometric fluorescence probes with different location and orientation in phospholipid membranes

3-hydroxyflavone (3-HF) derivatives are very attractive fluorescence sensors due to their ability to respond to small changes in their microenvironment via a dramatic alteration of the relative intensities of their two well-separated emission bands. We developed fluorescence probes with locations at different depths and orientations of 3-HF moiety in the phospholipid bilayer, which determine their fluorescence behavior. While the spectral shifts of the probes correlate with their binding site polarity, the intensity ratio is a complex parameter that is also sensitive to the local hydration. We demonstrate that even the deeply located probes sense this hydration effect, which can be modulated by the charge of the lipid heads and is anisotropic with respect to the bilayer plane. Thus the two-band ratiometric fluorescence probes can provide multiparametric information on the properties of lipid membranes at different depths.     

Fluorescent Probes Based on Charge and Proton Transfer for Probing Biomolecular Environment

Fluorescent probes for sensing fundamental properties of biomolecular environment, such as polarity and hydration, help to study assembly of lipids into biomembranes, sensing interactions of biomolecules and imaging physiological state of the cells. Here, we summarize major efforts in the development of probes based on two photophysical mechanisms: (i) an excited-state intramolecular charge transfer (ICT), which is represented by fluorescent solvatochromic dyes that shift their emission band maximum as a function of environment polarity and hydration; (ii) excited-state intramolecular proton transfer (ESIPT), with particular focus on 5-membered cyclic systems, represented by 3-hydroxyflavones, because they exhibit dual emission sensitive to the environment. For both ICT and ESIPT dyes, the design of the probes and their biological applications are summarized. Thus, dyes bearing amphiphilic anchors target lipid membranes and report their lipid organization, while targeting ligands direct them to specific organelles for sensing their local environment. The labels, amino acid and nucleic acid analogues inserted into biomolecules enable monitoring their interactions with membranes, proteins and nucleic acids. While ICT probes are relatively simple and robust environment-sensitive probes, ESIPT probes feature high information content due their dual emission. They constitute a powerful toolbox for addressing multitude of biological questions.     

Fluorescence Probing of Cell Membranes with Azacrown Substituted Ketocyanine Dyes

The three dyes with similar fluorescence properties but different lipophility—azacrown- and di-methylamino-substituted ketocyanines—are proposed as probes for the studies of biomembrane structure and dynamics. Their attractive feature is an extremely strong solvatofluorochromism, covering the range from 470 to 650 nm. Two photophysical mechanisms are responsible for these features, the general polarity effect associated with substantial increase of the probe dipole moment on electronic excitation and the excited-state stabilization due to hydrogen bonding to the central carbonyl groups. On the binding of these probes with erythrocyte membranes, three components in fluorescence spectra are resolved. They are attributed to probe molecules bound in two discrete types of binding sites inside the membrane: hydrophilic polar sites, in which carbonyl groups of the probes molecules are hydrogenbonded with hydrogen donor surrounding, and hydrophobic non-polar sites. The third component present in the emission spectra was attributed to the unbound probe in the near-membrane aqueous phase. Our results suggest that such a complex response of the probes is sensitive to the dynamics of hydration of the membrane interior, and this feature can be easily studied in simple ratiometric measurements. Among three studied compounds containing two crown-, two dimethylamino-, or both crown- and diethylamino- substituents, the latter proves to be most prospective in biomembrane research. This probe was tested in monitoring the phase transition of human erythrocyte membrane.     

Effective synthesis of 7-hydroxyisoflavone O-glucosides

An effective procedure for condensing acetobromoglucose with 2,4-dihydroxy-phenyl benzyl ketones has been developed. The 2-hydroxy-4-(-D-tetra-O-acetylglucopyranosyloxy)phenyl benzyl ketones synthesized with yields of 47–69% have been converted under the action of ethyl orthoformate in pyridine solution into 7-hydroxyisoflavone O-glucosides and these have also been obtained by the glycosylation of 7-hydroxyisoflavone. The advantages and disadvantages of the alternative pathways of the synthesis of 7-hydroxyisoflavone O-glycosides are discussed.T. G. Shevchenko Kiev State University. Khar'kov State Pharmaceutical Institute. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 511–519, July–August, 1988.     

Synthesis of α-aryloxy-2,4-dihydroxyacetophenone 4-O-β-D-glucopyranosides and their conversion into 3-aryloxy-7-glucosyloxychromones

The hydrolysis of the glycoside quisqualoside B, isolated from the herbAstragalus quisqualis Bunge, has given the new cycloartane triterpenoid quisquagenin, which has the structure of 20(R),24(S)-epoxycycloartane-3,16,25-triol.All-Union Scientific-Research Institute of Medicinal Plants, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 527–532, July–August, 1987.     

Synthetic and modified isoflavonoids. XIII. Synthesis of analogues of fujikinin

Analogues of a natural isoflavone glycoside — fujikinin — have been synthesized. Their structures have been confirmed by their PMR spectra.Taras Shevenko Kiev University: KIEN, Karakalpak Division, Academy of Sciences of the Republic of Uzbekistan, Nukus. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 347–351, May–June, 1994.     

Structurally Rigid 2,6-distyrylpyridines—A New Class of Fluorescent Dyes. 1. Synthesis, Steric Constitution and Spectral Properties

A group of structurally rigid analogues of 2,6-distyrylpyridine was synthesised. Molecular geometry of the synthesised dyes in solutions was studied by ¹ H-NMR, electronic absorption and fluorescence spectrometry. The spectral data testify all the compounds exist in E-configuration of their styryl residues. The most planar molecular conformation is typical for the compounds with five-membered side aromatic moieties. In the case of pyridines with six-membered aromatic residues steric hindrance results in turning the above mentioned cyclic groups out of the plane of the central pyridine moiety. The violation of planarity in this case is not significant and saves the possibility of -electronic conjugation in the molecules. The synthesised compounds are characterized by high fluorescence quantum yields in solutions. The electronic absorption spectra of titled pyridines demonstrate low sensitivity to the nature of the substituents introduced into the side aromatic rings. In contrast to this, the fluorescence bands considerably change their position under the influence of electron donor substituents. The fluorescence spectra display substantial positive solvatofluorochromism only in the cases of the dialkylamino-derivatives, especially on going from aprotic solvents to proton donor ones. Generally, the synthesised structurally rigid distyrylpyridine derivatives have prospects for their application as multi-purposes fluorescent probes.