A water-soluble Chl a/b-protein complex, CP668, from Chenopodium album converts to another form of protein complex, CP743, upon light illumination. Structural changes of pigments and proteins upon photoconversion were studied using resonance Raman (RR) and Fourier transform infrared (FTIR) spectroscopies. RR spectra of CP668 and CP743 and a light-induced FTIR difference spectrum showed that the macrocyle C=C bands of Chl a in CP668 considerably changed upon conversion to the pigment (not chemically identified yet) in CP743. The C=C band pattern of the RR spectrum of CP743 was similar to that of bacteriochlorophyll a, suggesting that the conjugated system of the CP743 pigment resembles a bacteriochlorin ring. Judging from the C=O frequencies, the 131-keto C=O groups of Chl a and b in CP668 are free from hydrogen bonding, whereas the 132-ester C=O groups of both Chl a and b and the 7-formyl C=O of Chl b in CP668 are hydrogen bonded. Upon conversion to CP743, interactions of the 131-keto and 132-ester C=O groups were basically unaffected, demonstrating no drastic changes around these C=O groups. FTIR spectra in the amide I' region of CP668 and CP743 in D2O buffer showed a peak at 1,633 cm−1, which represents a major component of β-sheet conformation. Second-derivative spectra of the amide I' bands as well as a light-induced FTIR difference spectrum suggested that drastic change in the protein conformation does not occur upon photoconversion.
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Noguchi et al. (1999) studied this question.
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