Aromatic Contacts Research Articles

Structural and electrochemical properties of a novel N,N′-dialkyl-substituted quinone diimine

trans,trans-3,3,7,7,10,10,14,14-Octamethyl-1,2,3,4,4a,7,7a,8,9,10,11,11a,14,14a-tetradecahydroquino[2,3-b]acridine 5, formed by a Lewis acid-catalyzed biscyclization of N-arylbisimine 3 and subsequent autoxidation, represents the first stable example of the unexplored class of N,N′-dialkyl-substituted quinone diimines. The X-ray crystal structure of 5 exhibits the following features: space group P21/c; a ribbon-like arrangement of two parallel strands along the b-axis. The smallest intermolecular distance between two quinoid systems is 4.846 Å and therefore any close aromatic contacts in this material can be precluded. A chemically reversible single electron reduction at E0 = –1.87 V has been observed by cyclic voltammetry.

Structure and rotational dynamics of fluorescently labeled insulin in aqueous solution and at the amphiphile-water interface of reversed micelles.

Insulin is a proteohormone with amphipathic three-dimensional structure and the ligand of a receptor, which itself spans the plasma membrane of glucose-metabolizing cells. In this study, the possible impact of amphiphiles on structural and dynamic properties of the hormone was investigated in reversed micelles mimicking the amphipathic nature of biological membranes. To make insulin susceptible to fluorescence measurements, two derivatives labeled with 2-aminobenzoic acid (Abz), N epsilon B29-Abz-insulin and [AbzB1]insulin, were prepared. First, the Abz-labeled insulins were shown by CD spectroscopy to exhibit conformational properties and self-association as well as the T-->R transition similar to the native hormone. By means of time-resolved fluorescence measurements, not only metal-ion induced hexamerization was observable in aqueous solution: The T-->R allosteric transition of the hexamer was shown to be accompanied by a diminution of its hydrodynamic radius. Second, structure and rotational dynamics of the labeled insulins were investigated in reversed micelles. In sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reversed micelles, the main-chain conformation is similar to that in aqueous solution according to CD spectroscopy in the far-UV, whereas the weak circular dichroism in the near-UV is indicative of reduced aromatic contacts as well as of the absence of quaternary structure, and the CD spectra show the same shape as found for proteins in an intermediate state of folding referred to as the "molten globule". Fluorescence anisotropy decay measurements of N epsilon B29-Abz-insulin in reversed micelles of AOT, cetyltrimethylammonium bromide, and alpha-L-1,2-dioctanoylphosphatidylcholine showed that the internal mobility of the solubilizate is reduced compared to that in aqueous solution and that the rotational mobility of the labeled insulin decreases with decreasing micellar size. With respect to the immobilization, insulin interacts in a stronger way with the anionic than with the cationic or zwitterionic amphiphile; an integration into the amphiphile monolayer, however, could be ruled out in all cases. In conclusion, the results reveal an evident influence of amphiphiles on the structure and rotational dynamics of insulin. Further investigations should be focused on this finding also with regard to the possible importance of lipid-insulin interactions in vivo.

Analysis of the bimolecular reduction of ferricytochrome c by ferrocytochrome b 5 through mutagenesis and molecular modelling

Site-directed mutagenesis has been used to produce variants of cytochrome c in which selected structural or functional properties of this protein are altered that have been implicated previously in contributing to the rate at which ferricytochrome c is reduced by ferrocytochrome b 5. In total, 18 variants have been studied by kinetic and electrochemical methods to assess the contributions of thermodynamic driving force, surface charge and hydrophobic interactions, and redox-linked structural reorganization of the protein to the rate of electron transfer between these two proteins under conditions where the reaction is bimolecular. While some variants (those at position-38) appear to affect primarily the driving force of the reaction, other appear to influence the rearrangement barrier to electron transfer (those at positions-67 and -52) while the interface between electron donor and acceptor centers is the principal effect of substitutions for a conserved aromatic heme contact residue at the surface of the protein (position-82). Interpretation of these results has been facilitated through the use of energy minimization calculations to refine the hypothetical models previously suggested for the cytochrome c- cytochrome b 5 precursor complex on the basis of Brownian dynamics simulations of the bimolecular encounter event.

A New Helicopodand: Molecular Recognition of Dicarboxylic Acids with High Diastereoselectivity

AbstractThe helicopodand (PM)‐2 is prepared following the photocyclodehydrogenation route to helicenes (Scheme). At the ends of a [7]helicene backbone, this acyclic receptor (‘podand’) possesses a H‐bonding recognition site shaped by two convergent N‐(pyridin‐2‐yl)carboxamide (CONH(py)) units. In the crystal of diethyl [7]helicene‐2,17‐dicarboxylate ((PM)‐3), a direct synthetic precursor of 2, molecules of the same chirality form stacks, and two stacks of opposite chirality are interlocked in a pair having average face‐to‐face aromatic contacts of 3.82 Å between benzene rings of different enantiomers (Fig. 2). In contrast, two conformations are observed in the crystal structure of 2, one with both CONH(py) residues pointing with their H‐bonding centers NH/N away from the binding site (‘out‐out’) and a second (‘in‐out’) with one of the two CONH(py) residues pointing towards the binding site (‘in’; Fig. 4). While no H‐bonding network propagates throughout the crystal, enantiomers of 2 in the different conformations ‘out‐out’ and ‘in‐out’ form H‐bonded pairs that are further stabilized by a H‐bond to one molecule of CHCl3. In the productive ‘in‐in’ conformation, 2 forms stable 1:1 complexes with α,ω‐dicarboxylic acids in CHCl3, and a diastereoselectivity in complexation of Δ(ΔG°) = 1.4 kcal mol−1 is measured for two substrates differing only in the (E)/(Z)‐configuration at their double bond (see Table 2). A comprehensive force‐field molecular‐modeling study suggests that only the (E)‐derivative possesses the correct geometry for a ditopic four‐fold H‐bonding interaction between its two COOH residues and the two CONH(py) groups in 2 (Fig. 5). With N,N′‐bis [(benzyloxy)carbonyl]‐L‐cystine, the formation of diastereoisomeric complexes with (PM)‐2 is observed (Fig. 7).

Ultraviolet Raman spectroscopy indicates fast (less than 7 ns) R----T-like motion in hemoglobin.

Raman spectra of oxy- and deoxyhemoglobin obtained with 218 and 200 nm pulsed (7 ns) laser excitation show changes (loss of 880 cm-1 tryptophan band intensity, increase in the 830/850 cm-1 tyrosine doublet intensity ratio) which are attributed to the aromatic contacts (Trp beta 37-Tyr alpha 140 and Tyr alpha 42-Asp beta 99) that are specific to the T quaternary structure. At high concentration (2 mM in heme) HbCO shows the same spectral signatures as HbO2. As the HbCO concentration is decreased, however, the spectra approach those shown by deoxy-Hb. This dilution effect is attributable to photolysis, which increases with decreasing concentration. The results imply that the HbCO photoproduct shows the same aromatic environments as does deoxy-Hb. Thus, T-like contacts are apparently formed at the alpha 1 beta 2 interface within 7 ns of photolysis, a time short compared to the spectral alterations of the heme group (approximately 100 ns, approximately 1 microsecond, and approximately 20 microseconds) which have previously been attributed to tertiary and quaternary relaxations.

The interaction of alkali metal cations with aromatic molecules in complexes of the type M[AlMe3X]�aromatic, M[Al2Me6X]�aromatic, and related

The crystal structures of 13 compounds of the form M[Al2Me6X]·aromatic and related have been examined in order to learn about the M+...aromatic approach. Four types of interactions have been discerned: (1) metal...aromatic, (2) metal...aromatic...metal, (3) aromatic...metal...aromatic, and (4) no metal...aromatic contact. It was found that the closest K+...C(aromatic) and Cs+...C(aromatic) separations are essentially equal after a correction for the difference in metal radii. The strength of the K+...aromatic attraction was found to be sufficient to move the K+ ion 0.3 A out of the plane of the crown ether in two complexes of dibenzo-18-crown-6.

Thermodynamic Properties of Atactic Polypropylene Solutions

The heats of mixing of atactic polypropylene at infinite dilution were measured at 25°C in linear alkanes, aromatic hydrocarbons, cyclohexane, chloroform, and carbon tetfachloride. For some solvent systems excess volume and chemical potential measurements were also performed at 25°C. The results were treated by the new Flory theory for polymer solutions. The exchange interaction parameter X12 increases with an increase in the chain length of n-alkanes and decreases with the introduction of aliphatic groups into benzene. These changes can be interpreted well in terms of the difference in force field between methyl–methylene contacts or aliphatic–aromatic contacts. The observed excess volumes are negative for n-alkanes and positive for rather globular cyclohexane, chloroform, carbon tetrachloride, and benzene, which is consistent with the prediction by the theory. However, the calculated excess volumes are less than those observed for all systems studied. Agreement between experimental and theoretical χ-parameters can be achieved by the introduction of the exchange entropy parameter Q12 and adjusting the contact surface ratio (s2/s1).

Electronic Properties of Quaterrylene, C40H20

Abstract Quaterrylene (C40H20) is a dark green-coloured aromatic hydrocarbon and is extraordinarily stable, as it can be heated to over 500°C without decomposition. The semiconduction of the aromatic hydrocarbon possesses a substantial anisotropy by a factor of 107∼108; the resistivity in the direction parallel to the b axis is 106Ωcm. (for a single crystal) or 105Ωcm. (for a surface-type cell) and that perpendicular to the b axis is 1013Ωcm. However, their energy gaps are around 0.6 eV. From its pulsed photoconduction of the thin film, a drift mobility has been estimated in the order 10−3 cm2/V. sec. However, the rectifying effect of the photoconduction can scarcely be recognized; that is to say, it seems that the mobilities of the holes and electrons in crystal are almost the same. Further, its photovoltage between aluminium and quaterrylene has a value of 0.1∼0.3 V., while the ionization potential of the hydrocarbon is 4.9 eV. judging from the threshold value of the spectral response of the photoelectric emission. From these results, a energy level diagram has been introduced for the system of the aromatic hydrocarbon-aluminium contact when a band-type model is employed as the organic semiconductor.