Primary Quantum Yield and Volume Change of Phytochrome‐A Phototransformation Determined by Laser‐Induced Optoacoustic Spectroscopy*

The primary quantum yield, Φ1700, for the photoconver‐sion of the red‐absorbing form of phytochrome, Pr, to the set of primary intermediates, Ii700, was redetermined by laser‐induced optoacoustic spectroscopy at very low excitation laser fluences. The Ii700 value obtained is in the range of ΦPfr reported for the complete phototransformation Pr→ Pfr (J. M. Kelly and C. Lagarias, 1985, Biochemistry24, 4003–6010). An energy level of ca 150 kJ/ mol was found for the intermediates Ii700, i.e. ca 85% of the 0–0 level of Pr. Furthermore, a molecular expansion of 7 mL/mol (equivalent to 11 Å3/molecule) was determined for the Pr→ Ii700 transformation. It reflects the protein reorganization induced by the geometrical pho‐toisomerization of the chromophore, which results in changes of bonding interactions, in particular between the chromophore and its protein surrounding.