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

Abstract

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 ΔA/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 ⇌ P_g will be implicated in mediating this response.