A model for interfacial electron transfer on colloidal melanin

Though melanins are involved in photochemical reactions (mainly of oxido-reductive type) in vitro and this activity is supposed to have biological implications, no satisfactory model of the reaction kinetics has so far been proposed. The main difficulty arises from the particulate structure of the insoluble melanins and the consequent necessity to describe their reactivity in the framework of heterogeneous chemistry, i.e., at the solid-liquid interface. Our paper presents a simplified model of the monoelectronic reduction reaction of dioxygen, based on well-established experimental facts and some reasonable assumptions: (1) surface adsorption of O₂ on colloidal melanins can be described by a Langmuir isotherm; (2) the kinetics of superoxide formation is photodependent and includes an interfacial electron-transfer process; (3) the photochemical behaviour of the single melanin granule can be described in terms of the electronic properties of amorphous semiconductor particles. Some satisfactory comparisons with experimental data and calculated values of the kinetic constants for the process are presented and discussed.     

Markov and semi-Markov option pricing models with arbitrage possibility

The aim of this paper is the presentation of new models for option pricing that are discrete in time and in the framework of Markov and semi-Markov processes as an alternative to the classical Cox–Rubinstein model, and that also allow the possibility of arbitrage. Both cases of European and American options are considered and possible extensions are given. © 1997 by John Wiley & Sons, Ltd.     

Polymorphism and Phonon Dynamics of α‐Quaterthiophene

Two 4T: Low‐frequency micro‐Raman spectroscopy coupled with lattice dynamics calculations is an invaluable tool for investigating polymorphism in organic semiconductors. The Raman spectra of the low‐temperature (LT) and high‐temperature (HT) polymorphs of α‐quaterthiophene (4T) are presented and interpreted (see picture). Raman mapping is applied to investigate the phase purity.

     

Measured Davydov splitting in oligothiophene crystals

The polarized absorption spectra of single crystals of oligothiophenes in a wide spectral range are reported. The experimental procedure is discussed, underlying several details which are relevant to obtain reliable spectra particularly for samples of increasing thickness. On the basis of these considerations, it has been possible to fully detect the transition to the upper Davydov exciton originating from the first molecular state. The position and shape of the main exciton peak in these materials are compared and discussed, taking into consideration the molecular arrangement and the longitudinal contribution which depends on the transition moment orientation. The Davydov splitting values as deduced from the experimental data at room temperature are also reported either for the first vibronic replica or for the electronic transition as a whole. The difference between the purely transverse and the measured Davydov splitting is discussed.     

Phonon dispersion of quasi-freestanding graphene on Pt(111)

High-resolution electron energy loss spectroscopy has been used to probe phonon dispersion in quasi-freestanding graphene epitaxially grown on Pt(111). Loss spectra clearly show different dispersing features related to both acoustic and optical phonons. The present results have been compared with graphene systems which strongly interact with the substrate, i.e. the nearly-flat monolayer graphene (MLG)/Ni(111) and the corrugated MLG/Ru(0001). We found that the phonon dispersion of graphene/Pt(111) reproduces well the behavior of pristine graphite. This could be taken as an indication of the negligible interaction between the graphene sheet and the underlying Pt substrate. The softening of out-of-plane modes observed for interacting graphene/metal interfaces does not occur for the nearly-free-standing graphene/Pt(111).     

Quenching of the excited states of 8-methoxypsoralen by synthetic Eumelanin

The interaction of 8-methoxypsoralen (8-MOP) with synthetic eumelanin was investigated using static and time-resolved fluoresence and pulsed photoacoustic calorimetry. Spectroscopic data indicate the absence of interaction in the ground state, whereas the singlet excited state of 8-MOP is quenched by the pigment; the average fluorescence lifetimes are independent of the melanin concentration, thus indicating a static mechanism. Photoacoustic data show that the quenching process involves an increased intersystem crossing probability, which is almost unaffected by the presence of oxygen, as expected for a molecule essentially acting as a type I photosensitizing agent.     

Dielectric tensor and optical spectra of quaterthiophene crystals

We present an approach to the microscopic calculation of the dielectric tensor near the resonances of mechanical Frenkel excitons in a vibronic coupling regime with one effective mode. The model, applied to the prototypical quaterthiophene crystal, is based on a discrete distribution of charges for the evaluation of the molecular transition moment, whilst the coulombic interactions responsible for the exciton delocalization in the crystal are calculated by the Ewald method. From the calculated tensor we simulate absorbance and reflectance spectra and by a direct comparison with the experimental ones, we identify the origin of some shoulders present in the spectra.     

Influence of structure on binding of chlorophylls to peptide ligands

Four classes of chlorophyll (Chl), a, b, c, and d, are involved in photosynthesis within cyanobacteria, algae, and plants. These classes have different evolutionary origins, chemical properties, and biological functions. Our results demonstrate that peptide-bound ligands provided by the imidazole group of histidine and the charge-compensated glutamate-arginine ion pair readily form coordination bonds with Chls a and d but do not interact significantly with Chls b and c. These ligands are apparently not sufficiently strong Lewis bases to displace strongly coordinated water from Chls b and c. These differences determine specificity of binding of Chls in light-harvesting complexes and play an important role in assembly of stable Chl-protein complexes, which has had a profound impact on the evolution of photosynthetic organisms.     

Photophysical properties of pyrene in interaction with the surface of melanin particles

Melanins perform their biological activity (photoprotection and light enhanced chemical reactivity) under the form of porous aggregates on which ions and neutral molecules can be adsorbed. For this reason, the photochemistry of natural and synthetic melanins must be investigated in the framework of the physico-chemical theory of the heterogeneous reactions and a detailed knowledge of the surface properties, is therefore, necessary. In this work, some surface characteristics of melanin particles have been investigated taking advantage of the photophysical behaviour of pyrene, a dye widely used in studies of the interface properties of micelles and colloidal semiconductors. Our fluorescence study has allowed to obtain valuable informations regarding the micro-environmental polarity of the melanin surface (that influences the vibronic structure of the emission spectra), the excimer formation, the lifetimes of the emissions and the kinetics of quenching by Cu2+.     

THE INTERACTION OF MELANIN WITH 8-METHOXYPSORALEN: EFFECT ON RADIATIVE and NONRADIATIVE TRANSITIONS. A FLUORESCENCE and PULSED PHOTOACOUSTIC STUDY

Abstract –The interaction of 8-methoxypsoralen (8-MOP) with synthetic eumelanin was investigated using static and time-resolved fluorescence and pulsed photoacoustic calorimetry. Due to the strong overlap of the absorption bands of melanin and 8-MOP, a method is presented to account for the systematic errors introduced by the optical filter effect exerted by each absorbing species in the fluorescence and the photoacoustic measurements. As a preliminary step to the understanding of the nonradiative behavior of the psoralen-melanin complexes, the photoacoustic parameters of 8-MOP in various solvents were determined. Spectroscopic data indicate the absence of interaction at the ground-state level, whereas the singlet excited state of 8-MOP is quenched by the pigment; the average fluorescence lifetimes are independent of the melanin concentration, thus indicating a static quenching mechanism. The photoacoustic data show that the quenching process involves an increased intersystem crossing probability, which is almost unaffected by the presence of oxygen, as expected for a molecule essentially acting as a type I photosensitizing agent.