Photoreversible absorbance changes in solutions of allophycocyanin purified from Fremyella diplosiphon: Temperature dependence and quantum efficiency.

Preparations of allophycocyanin isolated from the alga Fremyella diplosiphon show light-induced optical absorbance changes that suggest the presence of a photoconvertible component [Formula: see text] similar to the algal pigments described by J. Scheibe [(1972) Science 176, 1037-1039]. At pH < 4 the allophycocyanin has an absorption maximum at 620 nm. Red illumination causes a loss of absorbance in the red, centered at 620 nm, and subsequent green illumination restores the lost absorbance. We have studied this photoconversion at temperatures between 200 K and 307 K, analyzing the results in terms of photostationary states established under red (640 nm) and green (550 nm) light. As the temperature was lowered to 260 K, the state P(r) became progressively favored; the reaction P(r) --> P(g) induced by red light was attenuated but the reaction P(g) --> P(r) induced by green light was not. Decreasing the temperature from 260 K to 200 K had no further effect. Two distinct and simple models can account for this curious temperature dependence. By analyzing the kinetic and steady-state data, with reasonable estimates of the molar extinction coefficients of P(r) and P(g), we computed quantum efficiencies greater than 15% for the photoconversion at 300 K. We deduced that a conversion of "all P(r)" to "all P(g)" should produce a fractional absorbance change DeltaA/A at 620 nm equal to 0.1. If the chromatic adaptation response of intact F. diplosiphon shows the unusual temperature dependence reported here, the system P(r) right harpoon over left harpoon P(g) will be implicated in mediating this response.