We have matched the red absorption band measured at -196 C in a variety of chloroplast preparations with four major component curves representing forms of chlorophyll a having peaks at 661.6, 669.6, 677.1, and 683.7 nanometers. Chloroplast fractions enriched in one or the other of the two photochemical systems both contain these four major components, but system 1 preparations contain relatively more chlorophyll a 684. Chlorophyll a 677 and chlorophyll a 684 have greater bandwidths in system 1. Bands at longer wavelengths near 693 and 704 nanometers also often occur, but with far smaller heights than the above major bands. The longer wavelength bands are more common in system 1 than in system 2. In system 1 the half-widths of the four major bands in typical spectra average 11.3, 10.0, 10.3, and 10.8 nanometers while in system 2 they are 11.6, 9.8, 9.4, and 9.6 nanometers. Some spectra with sharper and some with wider bands were found, but the wavelengths were identical.The wide variations in the shape of the red absorption band of chlorophyll a in plants are due to the presence, in various proportions, of a number of forms of chlorophyll a with different absorption maxima in the 660 to 720 nm region. We wanted to determine the individual absorption spectra of these different forms of chlorophyll and to see if these forms are identical in various species. The question is whether there are only a few specific chlorophyll complexes always having constant absorption peak wavelengths and constant half-widths or if there are a large number of complexes with variable spectroscopic properties. With complex spectra it is possible, by a curve analysis, to determine the precise peak position, the half-widths, and the relative proportions of the components themselves.A long standing objective has been to see how far it is profitable to go in the interpretation of absorption spectra by fitting the data with the sums of simpler curves such as Gaussian components. We try here to define explicitly the potentialities and limits of this approach to the study of the native forms of chlorophyll.We have been particularly interested in the spectra of chloroplast fractions 1 and 2 that are enriched either in photosystem 1, which produces reducing power and has more long wavelength pigments, or in photosystem 2, which is directly responsible for 02 evolution and contains more chlorophyll b than does fraction 1.Various articles illustrate the diversity of opinions about the different forms of chlorophyll (1,2,6,14,15,20 half-width than Ca 680 and that in system 1 fractions of chloroplasts Ca 680 always had a greater half-width than in system 2 fractions (8, 9, 12, 13). Furthermore, the peak positions and the half-widths of both these hypothetical Ca 670 and Ca 680 components varied over a wide range of wavelengths when spectra of different preparations were compared. Such a variation in peak wavelengths and in halfwidth would result if each of the two assumed components were, in fact, composed of two separate chlorophyll forms ...

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In the measurement of absorption spectra, it is difficult to detect low intensity bands overlapped by bands of higher intensity. Their detection is greatly facilitated by measuring the first derivative of the transmission curve with respect to wavelength. A machine is under development in this laboratory to plot this derivative. A graphical study is here made of the behavior of the derivative curves for overlapping absorption bands of differing heights, widths and separation intervals and the derivatives are compared with the corresponding extinction and percent transmission curves. The curves here presented are intended as illustrations of the possibilities inherent in derivative spectrophotometry and for the identification of extinction or transmission curve types from measured derivatives.