The half projected Hartree-Fock function for determining singlet excited states. Application to cyclobutanone and 3-cyclopenten-1-one

The Half-Projected-Hartree-Fock procedure (HPHF) for determining singlet ground states is briefly described and extended to the direct determination of singlet excited states. The procedure is applied, using a [7s,3p/2s,1p] basis set, to determine the optimal geometry of two relatively large molecules, to which large CI calculations cannot be easily applied. These two molecules are cyclobutanone and 3-cyclopenten-1-one in their lowest singlet (n → π) excited state. Both molecules are found to exhibit in their excited state a pyramidal structure with the carbonyl oxygen atom pointing outward from the molecular plane. RHF calculations for the singlet ground state were also performed for comparison. The theoretical geometrical parameters compare well with the experimental data.     

Role of the Na+ ion on phenol derivatives/hydroxypropyl-beta-cyclodextrin complex formation on porous graphitic carbon phase

The reversed-phase liquid chromatography retention of phenol derivatives was investigated over a concentration range of sodium chloride (0-10(-2) M) and hydroxypropyl-beta-cyclodextrin (HP-beta-CD) (0-35x10(-3) M) using a porous graphitic carbon (PGC) stationary phase and a methanol/water mixture (50:50 (v/v)) as the mobile phase. A theoretical treatment was developed to investigate the effect of the sodium chloride and hydroxypropyl-beta-cyclodextrin on the equilibrium between the solutes with the PGC surface and the aqueous medium, respectively. The thermodynamic parameter variations were calculated using van't Hoff plots. It was expected that the sodium ion acted on the solute-PGC association process by modifying the surface tension of both the bulk solvent and the PGC surface. The phenol derivative/HP-beta-cyclodextrin complexation was shown to be entropically controlled for all the solutes except for the one which contained the -NO2 group in its structure, i.e. the nitro phenol derivative. A comparison of the compensation temperature of the solute-PGC association process when sodium chloride and HP-beta-CD concentration changed in the mobile phase led to the conclusion that these two modifiers acted via a variation in the hydrophobic effect.     

Synthesis of polysubstituted-2-naphthols

Substituted 3-(triisopropylsilyl)-2-naphthols are prepared by an aluminum chloride catalyzed condensation of phenyl acetyl chloride derivatives and triisopropylsilyl acetylene. The yields obtained are in the range of 41% to 67%. The reaction is regiospecific and gives only the 3-(trialkylsilyl)-2-naphthol isomer.     

Dendrimer-like PEO glycopolymers exhibit anti-inflammatory properties

A new class of high molecular weight polysulfated PEO dendrimer-like glycopolymer has been synthesized by a combination of arm-first and core-first methodologies followed by trichloroacetimidate glycosidation as a facile bioconjugation strategy. An L-selectin antagonist was identified that exhibits 103-fold greater activity than other multivalent sLex glycopolymers and 20-50 times greater potency than other linear heparinoids. A significant reduction in inflammatory cell recruitment was observed in vivo.     

Die Struktur des Bisindolalkaloides Amatain ( = Grandifolin,Subsessilin)

The structure of the bisindole alkaloid amataine (= grandifoline, subsessiline) Amataine ( 1 ) was isolated from the roots of Hedranthera barteri (HOOK . F .) PICHON (Apocynaceae). In comparison to the alkaloid vobtusine ( 5 ), which was isolated from the same plant, 1 contains two hydrogen atoms less. By reduction of 1 with H₂/PtO₂ or with NaBH₄ dihydromataine ( = isovobtusine, 13 ) was formed which is isomeric to 5 . Acid catalysed water addition to 1 afforded hydratoamataine ( 15 ), which was reduced to 5 with NaBH₄. Thermolysis of 15 gave 1 . The difference between the two isomeric bases 5 and 13 has to be attributed to a different configuration at the spirocenter C(7). In contrast to 5 amataine ( 1 ) contains an ether bridge instead of a hydroxy group; this bridge is located between C (2′) and C (8).     

A FLUORESCENCE STUDY OF TRYPTOPHAN-HISTIDINE INTERACTIONS IN THE PEPTIDE ANANTIN AND IN SOLUTION

Abstract—Anantin is a heptadecapeptide in which the C-terminal peptide chain pierces the covalently cyclized peptide ring formed by an amide link between the α-NH₂ end group and the β-carboxyl group of Asp(8). It contains a tryptophan and a histidine at positions 5 and 12 , respectively. Des-Phe(17)-anantin lacks the C-terminal phenylalanine. Fluorescence emission intensity as a function of pH follows the ionization of a single residue. The pK_a amounts to 7.23 ± 0.03 for anantin and is attributed to His(12). At pH 9 the quantum yield is 0.12 ± 0.01 for anantin, whereas at pH 4.5 the quantum yield decreases more than two-fold (0.05 2 0.01). Practically identical parameters are observed for des-Phe(17)-anantin. This pH dependency reveals intramolecular quenching of the excited indole ring of Trp(5) by the imidazole of His(12), which results in a marked decrease of the tryptophan fluorescence at low pH. In a multifrequency phase fluorometric study the fluorescence lifetimes for both peptides at pH 4.5 and pH 9 are determined. At both, pH fluorescence decay is well described by a sum of two exponentials. For anantin at pH 4.5 the lifetimes are 0.72 ± 0.07 ns and 1.67 ± 0.07 ns. At pH 9 the lifetimes are 1.11 ±0.12 ns and 2.55 ± 0.03 ns. In methanol we find two lifetimes for anantin: 0.68 ± 0.01 ns and 2.57 ± 0.01 ns. The lifetimes are found to be slightly dependent upon emission wavelength. For des-Phe(17)-anantin practically the same values are observed. The quenching of tryptophan fluorescence by histidine is further studied in solution using N-acetyl-tryptophanamide in the presence of increasing concentrations of imidazole in the protonated (pH 4.5) and unprotonated (pH 9) state and in methanol. At both pH values and in methanol, a linear increase in both the inverse of the steady-state fluorescence F_o/F and the inverse of the lifetime 1/τ with increasing imidazole concentration indicates that a collisional mechanism is at the root of the observed quenching. The quenching efficiency values, γ, are calculated and amount to about 0.32 at pH 4. 5 , 0.02 at pH 9 and 0.002 in methanol, showing that protonated imidazole is a better quencher than the unprotonated form, and that the nature of the solvent is involved even in the quenching by unprotonated imidazole. Tryptophan-histidine interactions in solution and in the peptide are compared.     

Speciation of activators in electroluminescent thin films: an EXAFS study of cerium and terbium doped strontium sulfide

Extended X-ray absorption fine structure (EXAFS) was used to determine the local structure of the luminescent centers in Tb³⁺ and Ce³⁺ -doped strontium sulfide thin films deposited by Atomic Layer Epitaxy (ALE). The rare earths were observed to enter mainly the substitutional sites but at the same time a part of the atoms form RES clusters (RE = rare earth). The presence of both substitutional sites and RES clusters has been observed for the first time in this study for rare earths in II–VI compounds. Received: 9 February 1998 / Revised: 22 May 1998 / Accepted: 29 May 1998