1,720,967 research outputs found
Photoacoustic studies of non-radiative relaxation of excited-states in melanin
No full textPhotoacoustic measurements made at various chopping frequencies on dense acqueous melanin suspensions have revealed a significant dependence upon pH and redox state. It is shown that such behaviour, differing from the simple predictions of the Rosencwaig-Gersho theory, can be explained in terms of finite carrier diffusion and recombination times. The implications of these findings with respect to the amorphous semiconductor model proposed to describe the dynamic role of epidermal melanin are discussed. From the experimental data, values of physical parameters were calculated which allow a qualitative correlation between chemical states and electronic behaviour and, consequently, some aspects of the molecular biology of the melanosome, founded on a rigorous base
Quenching of the excited states of 8-methoxypsoralen by synthetic eumelanin
No full textThe 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
Binding of chemicals to melanins re-examined: Adsorption ofsome drugs to the surface of melanin particles
No full textThis work presents a first attempt to study the interaction of some drugs with melanins, realistically considered as solid aggregates of
primary particles. This situation appears similar to the adsorption of organic molecules onto the surface of colloidal absorbers, as active
carbon, zeolites or titanium dioxide. We have applied some of the most popular theoretical models used in technological applications with the
aim to give a more realistic picture of the melanin–drug interaction responsible for some observed side effects in vivo. Moreover, this
approach can simplify the problem of the search of the physical parameters dominating the binding processes, by reducing the phenomenon
to a simple physisorption/chemisorption, at least in a first approximation.
We have studied the binding to melanin of gentamicin, methotrexate and chlorpromazine, molecules with different physico-chemical and
structural characteristics. Our study demonstrates the possibility to fit experimental adsorption data with Langmuir, Freundlich, Tempkin and
Dubinin–Radushkevich equations. In such a way we obtain binding parameters useful to characterize the drug–surface interaction in terms
of energy and of mean affinity.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Melanin; Drugs; Adsorption isotherms; Sorption kinetics; Surface interaction; Chemisorptio
The interaction of melanin with 8-methoxypsoralen: effect on radiative and nonradiative transitions. A fluorescence and pulsed photoacoustic study
No full textThe 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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