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Title: The photo-oxidation of chlorophyll and related compounds
Author: Laurie, Catherine M.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1952
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Examination has been made of systems of chlorophyll and related compounds, as deposited on various solids, by direct measurement of the pressure changes involved on illumination by visible light in oxygen. Chlorophyll and carotene show a pressure decrease under such conditions which corresponds, by a rate extrapolation method, to a molecular ratio of pressure decrease to pigment concentration of unity; after evacuation, increased rates are in evidence. The view that such behaviour can be ascribed to a simple oxidation which is reversible in nature, appeared to be borne out by the results obtained at low oxygen pressures (< 0.5 mm.); thus for chlorophyll it was shown that there exists a pressure limit below which in general the rate of oxidation becomes very slow. Similar behaviour was observed with pheophytin. In the presence of phosphoric oxide, however, no marked pressure limit was found, while in this pressure region a variation in rate for apparently similar total pressures was evident, which could only be explained by the presence of some retarding reaction. At high oxygen pressures (100 mm.) extrapolated pressure decrease values were observed in the presence of soda lime and /or phosphoric oxide, which corresponded to molecular ratios greater than unity, the maximum ratio obtained for chlorophyll being approximately six. Similarly increased ratios were found with carotene. The general features of the oxidations were shown to be the same when the pigments were adsorbed on glass as on thallous bromide, rates at high oxygen pressures then being slower; the thallous bromide is regarded as a photosensitiser. A colorimetric method of estimating the amount of peroxidic oxygen present after the illumination of such pigment systems in oxygen was developed. For chlorophyll the amount of hydroperoxide found after oxidations in the absence of adsorbents was some 80% of the pressure decrease, if that decrease be considered as oxygen, while in the presence of adsorbents approximately 15% was found; these results are consistent with the assumption of one molecule of oxygen present in the product as hydroperoxide in each case. In consideration of the evidence presented, it is believed that the simple reversible oxide theory cannot be satisfactorily supported. A possible general mechanism involving consecutive reactions has been proposed to account for the main features of the photo-oxidation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available