Универзитетска библиотека 'С. Марковић', Београд (УБСМ)
  • Termalna analiza melanina : doktorska disertacija
    Simonović, Branislav
    The effect of heat as a natural agent on structural shanges and functions of melanin is especially interesting with respect to a well known melanin protective function against heat. The aim of the ... present work was to investigate melanin thermal properties and their change in this light using metghods of thermal analysis. Besides thermal analyses (DSC, TGA, DTA, dilatometry) other physico-chemical methods were also used for this purpose, such as: photoacoustic spectroscopy (PAS), IR spectroscopy, NMR and mass spectroscopy as well as adsorption-desorption measurements. Investigations were performed on synthetic melanins obtained both anzymatically and autooxidatively (from tyrosine, L-DOPA, tryptophane, pyrocatechol and tyramin) on chemically modified synthetic melanins (partially or fully esterified and etherified) and on natural melanins (from banana skin, human hair and bovine eyes). General characteristic of all DSC thermograms of all melanins investigated in the temperature interval from 218 to 773 K is appearance of one well defined endothermal peak (or several overlapping peaks) in the biologically important temperature region of 321 - 353 K, as well as of another very slightly expressed peak in the region of 411 - 465 K. One sharp exothermal peak with a maximum at 625 - 710 K begins to appear above 450 K. The characteristic temperatures of endothermal and eagzothermal peaks in thermograms enable utilization of DSC method for melanin identification. Enthalpies of endothermal peaks and thermal stability of the investigated melanins determined from DSC measurements point out to parallelism between certain natural and synthetic melanins. Repeated recordings of DSC thermograms of the same melanin sample in the temperature interval of 218-453K has shown slow thermal relaxation. Endothermal peak`s enthalpy is affected by the time interval between two successive recordings of DSC thermograms but their exact quantitative relation could not be established. Appearance of slow thermal relaxation has also been proved by dilametric measurements as well as by measuring longitudinal and transversal NMR relaxation times. In repeated experiments with the same sample thermogravimetric studies have shown melanin mass loss in the endothermal peak temperature region pointing out to the presence of thermochemical reactions at temperatures up to 453K. The results of simultaneous DTA, TGA and mass spectrometric analysis lead us to ascribe endothermal peak to water and ammonium loss from melanin whereas exotermal peak in our opinion is mainly due to the loss of carbon dioxide and only partly to that of water ammonium. Evolving of carbon dioxide from melanin begins at considerably lower temperatures (350-360K) than so far reported in the literature. Presenceof thermochemical reactions is also proved by PAS and IR spectra of heated melanin samples. Origin of the endothermal peak has also been established from the above results. The endothermal peak in DSC thermograms originates from water and ammonium desorption as well as from melanin structural changes caused by loss of water. Because of simultaneous occurance of both processes it is difficult to determine exactly their separate contributions to the total enthalpy. The loss of water also leads to weakening and partial breaking of hydrogen bonds between planar layers in melanin. Significant dilatation as measured in hydrated and partly hydrated melanins as well as slight dilatation of dehydrated melanin both prove our assumption on the important role of water in the formation of hydrogen bonds between planar layers. This also proves that not only adsorbed but also more strongly bound water exist in melanin structure. The results of adsorption-desorption measurements showing higher adsorption capacities of samples evacuated at higher temperatures than of those evacuated at ambient temperatures support the above conclusion. Melanin specific surfaces have been detrmined (BET method) from nitrogen adsorption at liquid nitrogen temperatures as well as from water adsorption and it`s enthalpy.
    Врста грађе - дисертација ; u
    Издавање и производња - Beograd : [s.n.], 1989
    Језик - српски
    COBISS.SR-ID - 3326479

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Magacin
РД 0000013331
ИН: 003091843 Magacin
РД 13331
ИН: 003091843 		слободно - за читаоницу
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