ULTRAVIOLET RESONANCE RAMAN SPECTRA OF PHYTOCHROME: A COMPARISON OF THE ENVIRONMENTS OF TRYPTOPHAN SIDE CHAINS BETWEEN RED LIGHT‐ABSORBING AND FAR‐RED LIGHT‐ABSORBING FORMS

Ultraviolet resonance Raman spectra of phytochrome in the red light‐absorbing form (Pr) and the far‐red light‐absorbing form (Pfr) are reported. The spectra excited at 240‐nm provide structural information about the protein part of phytochrome. The protein contains only a very small amount of β‐sheet structure and most of the tyrosine side chains are located in hydrophobic environments. Indole rings of tryptophan (Trp) interact with neighboring groups in the Pr form and these interactions become weaker with the conversion from Pr to Pfr. Some Trp side chains of Pfr are surrounded by aliphatic groups but such is not the case in Pr. These changes in the environment occur at the same time as changes in orientation of Trp side chains. Our observations suggest that interactions between Trp residues and the tetrapyrrolic chromophore occur in the Pr form and that the strength of these interactions diminishes in the Pfr form.