Primary quantum yield and volume change of phytochrome-A phototransformation determined by laser-induced optoacoustic spectroscopy

The primary quantum yield, phi(I700), for the photoconversion of the red-absorbing form of phytochrome, P-r, to the set of primary intermediates, I-700(i), was redetermined by laser-induced optoacoustic spectroscopy at very low excitation laser fluences. The phi(I700) value obtained is in the range of phi(Pfr) reported for the complete phototransformation P-r --> P-fr (J. M. Kelly and C. Lagarias, 1985, Biochemistry 24, 6003-6010). An energy level of ca 150 kJ/mol was found for the intermediates I-700(i), i.e. ca 85% of the 0-0 level of P-r. Furthermore, a molecular expansion of 7 mL/mol (equivalent to 11 Angstrom(3)/molecule) was determined for the P-r --> I-700(i) transformation. It reflects the protein reorganization induced by the geometrical photoisomerization of the chromophore, which results in changes of bonding interactions, in particular between the chromophore and its protein surrounding.