Fluorescence lifetimes and relative quantum yields of 124-kilodalton oat phytochrome in water and deuterium oxide solutions

Abstract

The lifetimes and relative quantum yields of the three fluorescence components of the P/sub r/ (red-absorbing) form of 124-kDa oat phytochrome in H/sub 2/O and D/sub 2/O solutions have been determined by single-photon-timing techniques. Lifetime and quantum yield of the main component, which is the shortest lived and reflects the photochromic properties of P/sub r/, are not affected by D/sub 2/O. The medium-lifetime component, which is attributable to a photochromic source behaving comparable to that responsible for the main component, is not affected either. Only the least occurring and longest lived component, which is nonphotochromic, i.e., unaffected by red irradiation, markedly increases in lifetime. Thus, its relative contribution to the total fluorescence yield increases in D/sub 2/O. The spectra of the individual fluorescence components, obtained by a global analysis of the decay traces from 124-kDa P/sub r/ in H/sub 2/O solution at different observation wavelengths, differ only slightly in wavelength. The authors conclude that the primary photoreaction in the P/sub r/ transformation to P/sub fr/ (far-red-absorbing form of phytochrome) of the main phytochrome component exhibiting the shortest lived fluorescence does not involve a proton transfer as has been suggested in the literature.