Ultraviolet absorption spectrum change induced by the interaction between chloroplast coupling factor 1 and ADP or ATP

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Abstract
Changes in the ultraviolet absorption spectrum took place when coupling factor 1 of spinach chloroplasts (CF1) was mixed with ATP or ADP. The difference spectrum had a maximum at about 278 nm and a minimum at about 250 nm. The profile of the difference spectrum inidcates a shift in the absorption spectrum of the ADP or ATP bound to CF1. The ratio of the maximal absorbance change at about 278 nm to the absorbance of CF1 at that wavelength was 0.027–0.048. The molar concentration of nucleotide required to give the maximal absorbance change was 2–3 times that of CF1 when Ca2− or Mg2+ was present. AMP induced no significant spectral change. When ADP was added, the absorbance change reached a plateau within a few minutes if Mg2− or Ca2+ was present. The absorbance change induced by ATP reached a plateau within a few minutes only when Ca2+ was present. In the presence of Mg2+, it reached a plateau of nearly the same level, but at a slower rate. The absorbance change was reduced in the presence of EDTA. PPi effectively inhibited the absorbance change induced by ADP and Mg2+.