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     

The chemistry of wine polyphenolic C-glycosidic ellagitannins targeting human topoisomerase II

Polyphenolic nonahydroxyterphenoyl-containing C-glycosidic oak ellagitannins are found in wine as a result of the aging of this beverage in oak-made barrels. Once in the slightly acidic wine (pH approximately 3-4), some of these complex natural products such as (-)-vescalagin (1), but not its C-1 epimer (-)-castalagin (2), can capture grape-derived nucleophilic entities such as ethanol, the flavanols catechin (10a) and epicatechin (10b), the anthocyanin oenin (13b), and the thiolic glutathione (16) to furnish condensation products with retention of configuration at the C-1 locus. A computer-aided rationale of this high diastereoselectivity is given. These condensation products can contribute to the modulation of organoleptic properties of the wine, as evidenced by the 23 nm bathochromic shift color absorbance observed with the novel oenin-based anthocyano-ellagitannin (15b). Hydrolysis of 1 under solvolytic conditions furnished another novel compound that we refer to as vescalene (21), in addition to the known (-)-vescalin (18). Of pharmacological importance is the fact that most of these found-in-wine water-soluble ellagitannin derivatives are much more potent than etoposide (VP-16) at inhibiting top2-mediated DNA decatenation in vitro (top2=topoisomerase II)). The known (-)-vescalin (18) and the novel vescalene (21) fully inhibited top2 at 10 microM concentration!     

Convenient Synthesis of Alkenyl‐, Alkynyl‐, and Allenyl‐Substituted Imidazo[1,2‐a]pyridines via Palladium‐Catalyzed Cross‐Coupling Reactions

A systematic study on the Stille and Sonogashira cross‐coupling of iodinated imidazo[1,2‐a]pyridines was performed, permitting the preparation of various vinyl‐, ethynyl‐, and allenyl‐substituted derivatives. These methods are particularly valuable, given their experimental simplicity and high degree of flexibility with regard to functional groups that can be introduced in positions 3, 6, or 8 of the imidazo[1,2‐a]pyridine core. Effects concerning different substitution positions and the nature of the 2‐substituent under various reaction conditions are reported in detail for the above types of unsaturated groups introduced.     

Half-projected and projected Hartree-Fock calculations for singlet ground states. II. Lithium hydride

The half-projected Hartree–Fock function (HPHF ) for singlet states is defined as a linear combination of two Slater determinants which contains only spin eigenstates with even spin quantum numbers. The possible uses of such an approach for determining molecular properties are investigated computing the potential energy curve, binding energy, force constant, and dipole moment variation corresponding to the lithium hydride ground state. Full projected and restricted Hartree–Fock calculations (PHF and RHF ) are performed simultaneously for comparison purposes. It is found that the HPHF model yields very satisfactory results, very close to those of the PHF scheme. Both models predict properly the molecular behavior as a function of nuclear separation, whereas the RHF one fails. A discussion is given in terms of configuration equivalents. It is concluded that the HPHF scheme seems to be useful for determining molecular properties specially in the case of large systems in which the more sophisticated methods are unmanageable.     

Synthese de polyphosphonates et polyphosphonamides a partir de monomeres diphosphoniques polycondensation en solution

Synthesis of linear polyphosphonates and polyphosphonamides is achieved by polycondensation in solution of aromatic diols and diamines with alkyldiphosphonyl tetrachlorides, in the presence of a hydrochloric acid trapping agent (triethylamine). The main limiting factors for chain growth are the insolubility of the polymers and the occurrence of side reactions.     

Oriented binding of the His6-tagged carboxyl-tail of the L-type Ca2+ channel alpha1-subunit to a new NTA-functionalized self-assembled monolayer

Oriented stable binding of functional proteins on surfaces is of fundamental interest for receptor/ligand studies in atomic force microscopy (AFM) and surface plasmon resonance (SPR) experiments. Here we have chosen the His6-tagged carboxyl-tail (C-tail) of the alpha1c-subunit of the L-type Ca2+ channel and calmodulin (CaM) as its cognitive partner as a model system to develop a new functional surface. Covalently attached self-assembled monolayers on ultraflat gold containing NTA-thiols to which the His6-tagged C-tail was bound and thiols with triethylene-glycol groups as matrix-thiols represented the system of choice. The topography of this surface was characterized using AFM; its ability to bind C-tail proteins oriented and stable was confirmed by SPR measurements and by complementary force spectroscopy experiments with a CaM4-construct covalently attached to the tip. The developed anchoring strategy can now be used to study receptor/ligand interactions in general applying force spectroscopy and SPR on His6-tagged proteins oriented immobilized onto this new NTA-functionalized self-assembled monolayer.