Electronic Properties and Conformation Analysis of Phytochromobilins, Chromophore in Phytochrome

Quantum-chemical investigation on the conformation analysis and electronic properties of phytochromobilins (PCBs), an open chain tetrapyrrole chromophore of phytochrome are performed. The PCB chromophore have two stable forms, which occur photoisomerization by visible light absorption. Structures for two stable forms, Pr and Pfr isomers were fully optimized by using semiempirical AM1, PM3 methods, ab initio HF/3-21G(d), and B3LYP/6-31G(d) methods. The potential energy curves with respect to the change of single torsion angle are obtained by using semiempirical methods. ab initio HF, and DFT calculations. It is shown that the conformations of the isomers are compromised between the steric repulsion interaction and the degree of the conjugation. Electronic properties of the molecules were obtained by applying the optimized structures and geometries to the Zindo/S method. Absorption wavelengths are predicted by Zindo/S analysis. The wave-lengths which are calculated from optimized geometries by HF/3-21G(d) and B3LYP/6-31G(d) is reported. The absorption wavelength on the optimized geometries by B3LYP/6-31G(d) is much longer than that by HF/ 3-21G(d) level. The absorption wavelengths of Pfr form are longer than that of corresponding Pr form in the same torsion angle because of conjugation length difference. The absorption wavelengths of isomers with perpendicular linkage are shorter than those of planar linkage.