The visible and ultraviolet absorption spectra of carotin and xanthophyll and the changes accompanying oxidation
1931; Volume: 7; Issue: 1 Linguagem: Inglês
10.6028/jres.007.011
ISSN2376-9815
Autores Tópico(s)Free Radicals and Antioxidants
ResumoThe absorption spectra of equal molecular concentrations of pure carotin and xanthophyll in alcohol-ether solution are studied throughout the visible and ultraviolet spectral range, and the changes in these spectra are followed as the pigments oxidize slowly in solution.The frequencies of the absorption bands for each pure unoxidized pigment appear to form a regular sequence extending throughout the observed spectral range and converging toward the lower frequencies in accordance with a simple parabolic law for each pigment.The imposition of a periodic variation in intens- ity on this progression of bands gives the appearance of three adjacent spectral regions of selective absorption which are partially resolved into overlapping component bands.During oxidation of the pigments both spectra pass through the same charac- teristic series of changes, which are such as to indicate two very definite stages in the oxidation process for each pigment.Carotin is not oxidized to xanthophyll in this process, as contended by some previous investigators.The stages of oxidation are closely related to the two stages of hydrogenation described by Zechmeister and his associates.The oxidation data confirm the contentions of Schertz and others that the different xanthophylls described by Tswett may represent distinct stages in the oxidation of a single pigment. CONTENTS PageI. Introduction .171 II.Methods and nomenclature , 173 III.Preparation and exposure of the solutions 174 1. Choice of solvent 174 2. Experimental procedure 175 IV.Absorption spectra of the pure pigments 176 V. Oxidation in solution 185 1. Experimental data 185 2. Test for oxidation 188 VI.Bearing of the oxidation experiments on the work of other investigators 188 VII.Concluding remarks 191 I. INTRODUCTION The wide distribution of the chloroplast pigments 1 in the tissues of plants and animals has attracted the attention of biologists since earliest times.Much progress has been made in the isolation, 1 The chloroplast pigments constitute a group of four pigments which are always found closely associated in the chloroplastids of plant cells.These pigments are carotin, xanthophyll, chlorophyll-a, and chloro- phyll-6.Other closely related pigments such as tycopin (an isomer of carotin), fucoxanthin, and some chlorophyll derivatives, are also found under certain conditions in the plastids of various plant cells.171-Bureau of Standards Journal of Research [voi.7identification, and classification of these pigments, but our knowledge of their significance, relationship, or function, in the metabolism of plant and animal life, has hardly progressed beyond the realms of speculation.A comprehensive discussion of the subject is contained in the books by Willstatter and Stoll, 2 Palmer, 3 and Spoehr. 4 Carotin and xanthophyll are the two yellow chloroplast pigments occurring in relatively large quantities along with chlorophyll in green plants.Carotin is a highly unsaturated hydrocarbon, C 40 H 56 , and one of the few naturally occurring " colored" hydrocarbons.Xanthophyll is very similar to carotin in its physical and chemical prop- erties, and has the atomic composition C 4oH56 2 .The molecular structure of these pigments is not definitely determined.Their isolation and the study of their physical and chemical properties is fraught with many difficulties, owing, in part, to the ease with which the pigments take up oxygen, either in solution or in the solid state.Some of their most characteristic properties undergo a marked alter- ation even in the earliest stage of oxidation.This is true of the spectral absorptive properties, which have long been a valuable aid in the identification and quantitative estimation of the pigments.Many attempts have been made by various investigators to explain the presence and mysterious associations of these yellow pigments.Thus, the close correlation between carotin pigmentation and vitamin A content of plant tissues and the presence of carotin in milk and xanthophyll in egg yolks has led to much controversial discussion of the probable significance of these associations.Hypothetical relationships between the chloroplast pigments and theories of the inter- conversion of these pigments in the process of photosynthesis have all been advanced from time to time.Usually these speculations have not been substantiated by careful and thorough experimentation.From the standpoint of molecular structure and possible function in the living plant the great affinity of carotin and xanthophyll for oxygen is a matter of considerable interest.It has been contendedthat these pigments may play a respiratory role or aid in the regulation of oxygen pressure in the plant cells.Aside from the academic and biological interest which may be attached to a study of the absorptive properties of the chloroplast pigments, the information thereby obtained is of value in the manufacture, purification, and bleaching of vegetable oils and other natural food products, such as flour, the color of which is a matter of considerable commercial importance.It is known that carotin, xanthophyll, and chlorophyll or its derivatives, are present in varying proportions in these materials and often play the chief role in deter- mining the color of the finished product.In the course of some studies of the chloroplast pigments, conducted at the Bureau of Plant Industry, United States Department of Agriculture, preparations of pure carotin and xanthophyll were made by Dr. F. M. Schertz, who also studied some of the physical and chemical properties 5 of these materials and applied colorimetric and spec- tropho tometrie methods in their quantitative estimation. 6Through
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