Abstract

Blue copper proteins function as mobile electron carriers in a wide variety of biological systems,¹ Ultrafast pump-probe techniques have been used to examine active site dynamics in three of these charge-transfer proteins: spinach and poplar plastocyanin,¹ two photosynthetic proteins, and ceruloplasmin,² a protein of vertebrate blood plasma. Pulses of 16 fs duration from a Ti:sapphire laser centered at 770 nm were used to pump and interrogate electronic and vibrational excitation and relaxation associated with the optically active Cu-S_cysreine ligand-to-metal charge transfer. Figure 1 shows that the wavelength- integrated pump-probe signal of poplar plastocyanin exhibits a rapid decay with superimposed oscillations. These contributions correspond, respectively, to decay of ground state bleach (with a time constant of ~300 ft, similar to results for spinach plastocyanin and ceruloplasmin) and to modulations of the transition dipole by vibrational coherences of protein modes coupled to the electronic excitation. The most prominent oscillation in this signal has a frequency of~375 cm⁻¹. Wavelength-resolved signals of spinach plastocyanin and ceruloplasmin show a much more intense oscillation at ~ 500 cm⁻¹ that is not observed in poplar plastocyanin. By contrast, resonance Raman spectra of these plastocyanins show several vibrations between 350 and 450 cm⁻¹, but they show little or no intensity around 500 cm^−1,3 This 500 cm⁻¹ mode may be an excited-state frequency that has been upshifted relative to its ground state analog. The excited state surface must be coupled to the ground state or another electronic state along the nonradiative decay coordinate to show the observed rapid excited state decay.

© 1997 Optical Society of America