Tertiary contact formation in alpha-synuclein probed by electron transfer

To explore tertiary contact formation in alpha-synuclein, a natively unfolded protein implicated in Parkinson's disease, we have measured the rates of reaction between a powerful electron donor, the tryptophan (W) triplet excited state, and an acceptor, 3-nitro-tyrosine (Y(NO2)) in six different variants, probing loop sizes between 15 and 132 residues. Electron transfer rates decrease with loop size with the fastest contact time of 140 ns for the N-terminal pair and the slowest of 1.2 mus for the N- to C-terminal pair. Diffusion coefficients ranging from approximately 2 x 10-6 to approximately 10-5 cm2 s-1 were extracted from simultaneous fits of the W to Y(NO2) electron (triplet excited state) and energy transfer (singlet excited state) kinetics.     

Selective ion-molecule reactions of lactams with dimethyl ether ions

The ion-molecule reactions of dimethyl ether ions CH₃OCH₃ ⁺ and (CH₃OCH₃)H⁺, and four- to seven-membered ring lactams with methyl substituents in various positions were characterized by using a quadrupole ion trap mass spectrometer and a triple-quadrupole mass spectrometer. In both instruments, the lactams were protonated by dimethyl ether ions and formed various combinations of [M + 13] ⁺, [M + 15] ⁺, and [M + 45] ⁺ adduct ions, as well as unusual [M + 3] ⁺ and [M + 16] ⁺ adduct ions. An additional [M + 47] ⁺ adduct ion was formed in the conventional chemical ionization source of the triple-quadrupole mass spectrometer. The product ions were isolated and collisionally activated in the quadrupole ion trap to understand formation pathways, structures, and characteristic dissociation pathways. Sequential activation experiments were performed to elucidate fragment ion structures and stepwise dissociation sequences. Protonated lactams dissociate by loss of water, ammonia, or methylamine; ammonia and carbon monoxide; and water and ammonia or methylamine. The [M + 16] ⁺ products, which are identified as protonated lactone structures, are only formed by those lactams that do not have an N-methyl substituent. The ion-molecule reactions of dimethyl ether ions with lactams were compared with those of analogous amides and lactones.     

The interaction of water and dibromine in the gas phase: an investigation of the complex H(2)O...Br(2) by rotational spectroscopy and ab initio calculations

The ground-state rotational spectra of eight isotopomers of a complex formed by water and dibromine in the gas phase were observed by pulsed-jet, Fourier transform microwave spectroscopy. The spectroscopic constants B(0), C(0), delta(J), delta(JK), chi(aa)(Br(x)) (x=i for inner, o for outer), [chi(bb)(Br(x))-chi(cc)(Br(x))] and M(bb)(Br(x)) were determined for H(2)O...(79)Br(79)Br, H(2)O...(81)Br(79)Br, H(2)O...(79)Br(81)Br, H(2)O...(81)Br(81)Br, D(2)O...(79)Br(81)Br and D(2)O...(81)Br(81)Br. For the isotopomers HDO...(79)Br(81)Br and HDO...(81)Br(81)Br, only (B(0) + C(0))/2, delta(J), the chi(aa)(Br(x)) and M(bb)(Br(x)) were determinable. The spectroscopic constants were interpreted on the basis of several models of the complex to give information about its geometry, binding strength and the extent of electronic rearrangement on complex formation. The molecule H(2)O...Br(2) has C(s) symmetry with a pyramidal configuration at O. The zero-point effective quantities r(O...Br(i))=2.8506(1) A and phi(0)=46.8(1), where phi is the angle between the C(2) axis of H(2)O and the O...Br-Br internuclear axis, were obtained under the assumption of monomer geometries unchanged by complexation. Ab initio calculations, carried out at the aug-cc-pVDZ/MP2 level of theory, gave the equilibrium values r(e)(O...Br(i))=2.7908 A and phi(e)=45.7 degrees and confirmed the collinearity of the O...Br-Br nuclei. The potential energy function V(phi), also determined ab initio, showed that the wavenumber required for inversion of the configuration at O in the zero-point state is only 9 cm(-1). By interpreting the Br nuclear quadrupole coupling constants, the fractions delta(O-->Br(i))=0.004(5) and delta (Br(i)-->Br(o))=0.050(2) of an electron were determined to be transferred from O to Br(i) and Br(i) to Br(o), respectively, when the complex is formed. The complex is relatively weak, as indicated by the small value k(sigma)=9.8(2) N m(-1) of the intermolecular stretching force constant obtained from delta(J). A comparison of the properties, similarly determined, of H(2)O...F(2), H(2)O...Cl(2), H(2)O...Br(2), H(2)O...BrCl, H(2)O...ClF and H(2)O...ICl is presented.     

Pseudooctahedral complexes of vanadium(III): electronic structure investigation by magnetic and electronic spectroscopy

A variety of physical methods has been used to probe the non-Kramers, S = 1, V(III) ion in two types of pseudooctahedral complexes: V(acac)(3), where acac = anion of 2,4-pentanedione, and VX(3)(thf)(3), where thf = tetrahydrofuran and X = Cl and Br. These methods include tunable frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 95-700 GHz and fields up to 25 T) in conjunction with electronic absorption, magnetic circular dichroism (MCD), and variable-temperature variable-field MCD (VTVH-MCD) spectroscopies. Variable-temperature magnetic susceptibility and field-dependent magnetization measurements were also performed. All measurements were conducted on complexes in the solid state (powder or mull samples). The field versus sub-THz wave quantum energy dependence of observed HFEPR resonances yielded the following spin Hamiltonian parameters for V(acac)(3): D = +7.470(1) cm(-1); E = +1.916(1) cm(-1); g(x) = 1.833(4); g(y) = 1.72(2); g(z) = 2.03(2). For VCl(3)(thf)(3), HFEPR detected a single zero-field transition at 15.8 cm(-1) (474 GHz), which was insufficient to determine the complete set of spin Hamiltonian parameters. For VBr(3)(thf)(3), however, a particularly rich data set was obtained using tunable-frequency HFEPR, and analysis of this data set gave the folowing: D = -16.162(6) cm(-1); E = -3.694(4) cm(-1); g(x) = 1.86(1); g(y) = 1.90(1); g(z) = 1.710(4). Analysis of the VTVH-MCD data gave spin Hamiltonian parameters in good agreement with those determined by HFEPR for both V(acac)(3) and VBr(3)(thf)(3) and in rough agreement with the estimate for VCl(3)(thf)(3) (D approximately 10 cm(-1), |E/D| approximately 0.18), together with the finding that the value of D is negative for both thf complexes. The electronic structures of these V(III) complexes are discussed in terms of their molecular structures and the electronic transitions observed by electronic absorption and MCD spectroscopies.     

Superexcited state reconstruction of HCl using photoelectron and photoion imaging

The velocity-map imaging technique was used to record photoelectron and photofragment ion images of HCl following two-photon excitation of the E Sigma(+)(0+), V 1Sigma(+)(0+) (nu=9,10,11) states and subsequent ionization. The images allowed us to determine the branching ratios between autoionization and dissociation channels for the different intermediate states. These branching ratios can be explained on the basis of intermediate state electron configurations, since the configuration largely prohibits direct ionization in a one-electron process, and competition between autoionization and dissociation into H* (n=2)+Cl and H+Cl*(4s,4p,3d) is observed. From a fit to the vibrationally resolved photoelectron spectrum of HCl+ it is apparent that a single superexcited state acts as a gateway to autoionization and dissociation into H+Cl*(4s). Potential reconstruction of the superexcited state to autoionization was undertaken and from a comparison of different autoionization models it appears most likely that the gateway state is a purely repulsive and low-n Rydberg state with a (4Pi) ion core.     

Ground vs. excited state electron transfer: adsorbed monolayers and trimers in solution

Transient emission spectroscopy has been used to probe the rate of photoinduced electron transfer between metal centres within a novel trimeric complex [[Os(bpy)2(bpe)2][Os(bpy)2Cl]2]4+, where bpy is 2,2'-bipyridyl and bpe is trans-1,2-bis-(4-pyridyl)ethylene. Transient emission experiments on the trimer, and on [Os(bpy)2 (bpe)2]2+ in which the [Os(bpy)2 Cl]+ quenching moieties are absent, reveal that the rate of photoinduced electron transfer (PET) across the bpe bridge is 1.3 +/- 0.1 x 10(8) s(-1). Investigations into the driving forces for oxidation and reduction of the electronically excited state within the trimer indicate that quenching of the [Os(bpy)2 (bpe)2]2+ centre within the trimer involves electron transfer from the [bpe Os(bpy)2 Cl]+ centres to the electronically excited state with a driving force of -0.3 eV. Monolayers of the complex, [Os(bpy)2 bpe pyridine]2+, have been formed by spontaneous adsorption onto platinum microelectrodes and used to probe the dynamics of electron transfer across the trans-1,2-bis-(4-pyridyl)ethylene bridge in the ground state. These monolayers are stable and exhibit well defined voltammetric responses for the Os2+/3+ redox reaction. Cyclic voltammograms recorded at high scan rates can be accurately modelled according to a non-adiabatic electron transfer model based on the Marcus theory using a standard heterogeneous electron transfer rate constant, k(o), of 3.1 +/- 0.2 x 10(4) s(-1) and a reorganization energy of 0.4 +/- 0.1 eV. This rate constant is a factor of approximately two orders of magnitude smaller than that found for photoinduced electron transfer across the same bpe bridge for identical driving forces. This significant difference is interpreted in terms of both the nature of the orbitals involved in electrochemically and optically driven electron transfer, as well as the strength of electronic coupling between two molecular components as opposed to a molecular component and a metal electrode.     

Biogenesis of betalaine. Biotransformation of dopa and tyrosine in betalaminic acid part of the betanins

During studies on the biogenesis of betalains (I) in cactus fruits (Opuntia sp.). DL -dopa-1-[¹⁴C] and -2-[¹⁴C] were incorporated into betanin (III) which was obtained radiopure after crystallization. The specific activity remained constant after conversion to betanidin (IV) and to a neobetanidin derivative (IX). Reaction of radiobetanin with proline afforded indicaxanthin (V) carrying more than 90% of the radioactivity. Dopa (VI) is thus an efficient precursor for betalamic acid (VIII) but not for cyclodopa (VII). Decarboxylation of radiobetanidin and radioindicaxanthin showed that the carboxyl group of dopa remained a carboxyl group in the biotransformation to betalamic acid. It is concluded that the aromatic ring of dopa is cleaved and that re-cyclization involving the nitrogen generates the dihydropyridine moiety. Under the same conditions mevalonic acid, aspartic acid and phenylalanine showed low incorporations. Studies with beet seedlings and DL -dopa-1-[¹⁴C], -2-[¹⁴C] and DL -tyrosine-1-[¹⁴C] afforded similar results but with low incorporations.     

Radiative relaxation quantum yields for synthetic eumelanin

We report absolute values for the radiative relaxation quantum yield of synthetic eumelanin as a function of excitation energy. These values were determined by correcting for pump beam attenuation and emission reabsorption in both eumelanin samples and fluorescein standards over a large range of concentrations. Our results confirm that eumelanins are capable of dissipating >99.9% of absorbed UV and visible radiation through nonradiative means. Furthermore, we have found that the radiative quantum yield of synthetic eumelanin is excitation energy dependent. This observation is supported by corrected emission spectra, which also show a clear dependence of both peak position and peak width on excitation energy. Our findings indicate that photoluminescence emission in eumelanins is derived from ensembles of small chemically distinct oligomeric units that can be selectively pumped. This hypothesis lends support to the theory that the basic structural unit of eumelanin is oligomeric rather than heteropolymeric.     

The number of Kekulé structures and the thermodynamic stability of conjugated systems

The dependence of Hückel π-electron energies, E_π, on the basic graph theoretical parameters N (the number of vertices), ν (the number of edges) and ASC (the algebraic structure count) is explored. The form with the ASC enters E_π is established and an equation for E_π is developed. It is shown how the early and apparent success of the (resonance) theory rested on the fortunate fact that all Kekulé structures for benzenoid hydrocarbons and acyclic polyenes have the same parity. The significance of ASC in determining chemical stability and reactivity is dicussed briefly.