Aggregation modes of the spin mono-labeled tylopeptin B and heptaibin peptaibiotics in frozen solutions of weak polarity as studied by PELDOR spectroscopy

The X band PELDOR spectroscopy was used to investigate the magnetic dipole-dipole interactions in glassy solutions of nitroxide mono-labeled tylopeptin B and heptaibin peptaibiotics at 77 K. Specifically, a study was performed of the tylopeptin B peptides labeled at either position 3, 8, or 13, denoted as T3, T8, and T13, respectively. The heptaibin analogs labeled at either position 2 or 14, denoted as H2 and H14, respectively, were also investigated. It was shown that in frozen glassy peptide solutions in methanol, the spin labels are randomly distributed over the solvent volume. This result points to the absence of specific dipolar interactions between the peptides under these conditions. However, peptide aggregation was detected in weakly polar methanol/toluene environments. To study the properties of the resulting aggregates, we examined the depth of modulation for the PELDOR traces as a function of the concentration of the peptides in solution and the distances between the spin labels in the aggregates. Based on the concentration dependencies, the number of peptide molecules in the aggregates was estimated. We find that this value ranges from 2 to 3, depending on the position of the spin label in the peptide sequence. The combined analysis of the distance spectra and the number of peptide molecules in the aggregates allows us to suggest that dimer formation is the prevailing mode of self-association. In the case of spin-labeled tylopeptin B, the molecules in the dimer are head-to-head oriented. In addition, the distance spectra of the aggregates show that the C-termini of the molecules in the tylopeptin B dimer are more mobile than the Ntermini. This phenomenon leads to an increase in the spread of the distances between the nitroxides as the label position is approaching the peptide C-terminus. For heptaibin, we show that two forms of dimerization (head-to-head and head-to-tail) occur. Finally, in addition to dimers, aggregates containing 3 or 4 peptide molecules, which give broad lines in the distance spectra, are seen in solution.     

Reactivity of Aliphatic and Phenolic Hydroxyl Groups in Kraft Lignin towards 4,4′ MDI

Several efforts have been dedicated to the development of lignin-based polyurethanes (PU) in recent years. The low and heterogeneous reactivity of lignin hydroxyl groups towards diisocyanates, arising from their highly complex chemical structure, limits the application of this biopolymer in PU synthesis. Besides the well-known differences in the reactivity of aliphatic and aromatic hydroxyl groups, experimental work in which the reactivity of both types of hydroxyl, especially the aromatic ones present in syringyl (S-unit), guaiacyl (G-unit), and p-hydroxyphenyl (H-unit) building units are considered and compared, is still lacking in the literature. In this work, the hydroxyl reactivity of two kraft lignin grades towards 4,4′-diphenylmethane diisocyanate (MDI) was investigated. ³¹P NMR allowed the monitoring of the reactivity of each hydroxyl group in the lignin structure. FTIR spectra revealed the evolution of peaks related to hydroxyl consumption and urethane formation. These results might support new PU developments, including the use of unmodified lignin and the synthesis of MDI-functionalized biopolymers or prepolymers.     

The nature of the [20.0] 1Sigma+ electronic state of RhB: a multiconfigurational study

Multiconfigurational second order perturbation theory, with extended atomic basis sets and inclusion of scalar relativistic effects, was employed to investigate the low-lying (1)Sigma(+) electronic states of RhB. The [20.0] (1)Sigma(+) state is represented by a single configuration, mid R:[ellipsis (horizontal)]10sigma(2)11sigma(1)5pi(4)2delta(4)12sigma(1), derived from a single excitation (11sigma-->12sigma) from the ground state, which defines its electronic nature. A new excited state, coined as [9.0] (1)Delta (R(0)=1.786A, DeltaG(12)=792 cm(-1)), located 9221 cm(-1) above the X(1)Sigma(+) state, and described by the |...10sigma(2)11sigma(2)5pi(4)2delta(3)12sigma(1)> electronic configuration, was also identified.     

Nonlinear Hanle effect in the active discharge of an argon laser

 We present experimental evidence of the NonLinear Hanle Effect (NLHE) on the 488 nm transition in an active discharge of an argon laser. Experimental data are used to estimate the homogeneous linewidth of the upper level of the transition. The role of the NLHE in the laser power enhancement is also discussed. Received: 22 February 1996 / Revised version: 17 June 1996     

Ground and lowest-lying electronic states of CoN. A multiconfigurational study

The lowest-lying X1Sigma+, a3Phi, b3II, c5Delta, A1Phi, and B1II electronic states of CoN have been investigated at the ab initio MRCI and MS-CASPT2 levels, with extended atomic basis sets and inclusion of scalar relativistic effects. Among the singlet states, the A1Phi and B1II states have been described for the first time. Potential energy curves, excitation energies, spectroscopic constants, and bonding character for all states are reported. Comparison with other early transition-metal nitrides (ScN, TiN, VN, and CrN), isoelectronic (NiC) and isovalent (RhN and IrN) species has been made, besides analyzing the B1II <=> X1+ electronic transition in terms of Franck-Condon factors, Einstein coefficients, and radiative lifetimes. At both levels of theory, the following energetic order has been obtained: X1Sigma+, a3Phi, b3II, c5Delta, A1Phi, and B1II, with good agreement with experimental results. In contrast, previous DFT and MRCI calculations predicted the ground state to be the 5Delta state.     

Opioid peptides: synthesis and biological properties of [(N gamma-glucosyl,N gamma-methoxy)-alpha, gamma-diamino-(S)-butanoyl]4-deltorphin-1-neoglycopeptide and related analogues

The [D-Ala2]deltorphin 1 sequence in which the aspartic acid residue is replaced by the N gamma-OCH3-alpha, gamma-diamino (S) butanoyl residue was synthesized using the Fmoc-chemistry-based solid phase procedure. The resulting deltorphin analogue was chemoselectively glucosylated by reaction with unprotected D-glucose (Glc). The Asn4-, (2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-beta-D-galactopyranosyl)-Asn4- and the (2-acetamido-2-deoxy-D-galactopyranosyl)-Asn4-deltorphin I were also prepared for comparison. The affinity of the new compounds for the delta-opioid receptor was expressed by the inhibition constant (Ki) of the binding of the delta-receptor selective ligand [3H]naltrindole (NTI) to rat brain membrane preparations. The in vitro biological activity of the synthetic peptides was compared with that of the mu-opioid receptor agonist dermorphin in guinea pig ileum (GPI) preparations and with that of the delta-opioid receptor agonist deltorphin I in mouse vas deferens (MVD) preparations. The substitution of Asp4 with Asn failed to affect drastically the Ki and IC50 values for delta-sites, suggesting that an electrostatic interaction does not play an essential role in the binding to delta-opioid sites. The steric hindrance of the side chain of the residue in position 4 affects binding to delta-sites. The increase of the Ki value is smaller when the sugar-peptide linkage involves the gamma-nitrogen of the Dab residue in comparison with the Asn amide side chain.     

PHOTOSENSITIZATION OF MITOCHONDRIA BY LIPOSOME-BOUND PORPHYRINS

We have compared the photodynamic activities of hematoporphyrin (HP) and protoporphyrin (PP) on isolated rat liver mitochondria by measuring the decline of the respiratory control ratio (RCR) after irradiation at 365 nm. Before addition to the respiratory mcdium, the dyes were dissolved in phosphate-buffered saline (PBS) or incorporated into unilamellar liposomes of dipalmitoyl-phosphatidylcholine (DPPC), sometimes enriched with cholesterol (Chol) or cardiolipin (Card), which are naturally present in mitochondrial membranes. Chol and especially Card strongly increase the porphyrin uptake by mitochondria. In all experimental conditions, PP is taken up by mitochondria to a higher extent than HP. Nevertheless, under conditions giving the same amount of mitochondriabound dye, HP is a morc efficient photosensitizer than PP. As the efficiency of singlet oxygen production has been shown to be equivalent for the two porphyrins in monomeric state, the resulting photobiological effects are explained in terms of different localization of HP and PP in the mitochondrial membrancs. In particular, HP preferentially localizes in the protein-rich polar domains of the inner mitochondrial membrane, whereas PP dissolvcs in the lipid regions of the mcmbrancs.     

The effect of tricarbonylchromium on the thermodynamic stability of stable carbocations

The pK_R+'s for the ionization equilibria between a number of p,p′-substituted diphenylmethanols mono- and bis-complexed with the Cr(CO)₃ group and the corresponding carbocations in aqueous sulfuric acid were determined by spectrophotometric methods. The same pK_R+'s were measured for two ferrocenyltricarbonylchromiumarylmethanols in aqueous trifluoroacetic acid. The introduction of the Cr(CO)₃ group can make the complexed cation either more or less stable than the corresponding free ion, depending upon the structure. The effect of the complexation is explained in terms of electronic and conformational effects.