Models for the Change in Chromophore Structure of Allophycocyanin Corresponding to Its Observed Reversible Change in Absorbance

Abstract

On the basis of the ZDO approximation of LCAO–ASMO–SCF–CI theory of π-electrons, the optical absorption of phycocyanobilin is calculated, by taking its interaction with a point-charge and a point-dipole into account. It is thus shown that the reversible change in absorbance of allophycocyanin from Anabaena cylindrica, which was observed by Murakami et al., can be reproduced according to the following model: when allophycocyanin changes its tertiary structure due to the change in its physico-chemical environment, the terminal pyrrole ring D of one of the two phycocyanobilins in allophycocyanin rotates around the single and double bonds of its adjacent methyne bridge.