No abstract

The stable tyrosine radical YD. (tyrosine 160 in the D2 polypeptide) in photosystem II (PSII) exhibits nonexponential electron spin-lattice relaxation transients at low temperature. As previously reported, the tetranuclear Mn complex in PSII significantly enhances the spin-lattice relaxation of YD.. However, in Mn-depleted PSII membranes, the spin-lattice relaxation transients of YD. are also nonexponential, and progressive power saturation (P 1/2) experiments show that it does not behave like an isolated tyrosine radical. A model is developed to treat the interaction of two paramagnets in a rigid lattice at a fixed distance apart but with a random orientation in a magnetic field. This model describes the spin-lattice relaxation of a radical in proximity to another paramagnetic site in terms of three relaxation rate constants: the "intrinsic" relaxation rate, the relaxation rate due to scalar exchange, and the relaxation rate due to dipole-dipole interactions. The intrinsic and the scalar exchange relaxation rates are isotropic and together contribute a single rate constant to the spin-lattice relaxation transients. However, the dipolar relaxation rate is orientation dependent. Each orientation contributes a different dipolar relaxation rate constant to the net spin-lattice relaxation rate constant. The result is a superposition of single-exponential recoveries, each with a different net rate constant, causing the observed saturation-recovery transients to be non-(single)-exponential. Saturation-recovery relaxation transients of YD. are compared with those of a model tyrosine radical, generated by UV photolysis of L-tyrosine in a borate glass. From this comparison, we conclude that scalar exchange does not make a significant contribution to the spin-lattice relaxation of YD. in Mn-depleted PSII. We account for the nonexponential relaxation transients obtained from YD. in Mn-depleted PSII membranes in terms of dipolar-induced relaxation enhancement from the non-heme Fe(II). From simulations of the spin-lattice relaxation transients, we obtain the magnitude of the magnetic dipolar interaction between YD. and the non-heme Fe(II), which can be used to calculate the distance between them. Using data on the non-heme Fe(II) in the reaction center of Rhodobacter sphaeroides to model the non-heme Fe(II) in PSII, we calculate a YD.-Fe(II) distance of greater than or equal to 38 A in PSII. This agrees well with the distance predicted from the structure of the bacterial reaction center.

The binding of several primary amines to the O2-evolving center (OEC) of photosystem II (PSII) has been studied by using low-temperature electron paramagnetic resonance (EPR) spectroscopy of the S2 state. Spinach PSII membranes treated with NH4Cl at pH 7.5 produce a novel S2-state multiline EPR spectrum with a 67.5-G hyperfine line spacing when the S2 state is produced by illumination at 0 degrees C [Beck, W. F., de Paula, J. C., & Brudvig, G. W. (1986) J. Am. Chem. Soc. 108, 4018-4022]. The altered hyperfine line spacing and temperature dependence of the S2-state multiline EPR signal observed in the presence of NH4Cl are direct spectroscopic evidence for coordination of one or more NH3 molecules to the Mn site in the OEC. In contrast, the hyperfine line pattern and temperature dependence of the S2-state multiline EPR spectrum in the presence of tris(hydroxymethyl)aminomethane, 2-amino-2-ethyl-1,3-propanediol, or CH3NH2 at pH 7.5 were the same as those observed in untreated PSII membranes. We conclude that amines other than NH3 do not readily bind to the Mn site in the S2 state because of steric factors. Further, NH3 binds to an additional site on the OEC, not necessarily located on Mn, and alters the stability of the S2-state g = 4.1 EPR signal species. The effects on the intensities of the g = 4.1 and multiline EPR signals as the NH3 concentration was varied indicate that both EPR signals arise from the same paramagnetic site and that binding of NH3 to the OEC affects an equilibrium between two configurations exhibiting the different EPR signals.(ABSTRACT TRUNCATED AT 250 WORDS)

We have employed two third-order femtosecond spectroscopic methods, stimulated-photon-echo peak-shift (3PEPS) and transient-grating (TG) spectroscopy, to characterize solvation dynamics and interexciton-state radiationless decay in the α subunit of C-phycocyanin and in allophycocyanin. The α subunit contains a single phycocyanobilin chromophore in an isolated protein-matrix environment. Allophycocyanin contains exciton-coupled pairs of phycocyanobilins in the same type of binding site found in the α subunit. The results show that both systems exhibit a biphasic solvation response:  the inertial phase, arising from librational motions of the amino acids or included water molecules in the phycocyanobilin-binding site, contributes a 80−100-fs component to the 3PEPS profile and appears as a rapidly damped 72-cm-1 modulation of the TG signal; the diffusive phase, arising from collective protein-matrix motions, contributes a component in the TG signal and 3PEPS profile on the 5−20-ps time scale. Both systems exhibit nearly instantaneous (16-fs) components in the 3PEPS profiles that arise from intrachromophore vibrational modes. The 3PEPS profile observed with allophycocyanin exhibits additional fast decay components, with time constants of 56 and 220 fs, that apparently report the contributions to electronic dephasing arising from radiationless decay between imperfectly correlated exciton states. The TG signal evidences vibrational relaxation in the lower exciton state and incoherent energy transfer between the chromophores in a given pair. The results present complementary details on solvation and interexciton-state radiationless decay dynamics that were first observed in this laboratory using time-resolved pump−probe and anisotropy methods.

Vibrational coherence was observed following excitation into the lowest-energy spin-allowed 4 A 2 / 4 T 2 ligand-field absorption of Cr(acac) 3 . The transient kinetics were fit to a rapidly damped 164 cm À1 oscillatory component, the frequency of which is not associated with the ground state of the molecule. The signal is assigned as an excited-state vibrational coherence; the timescale of the event suggests that this vibrational coherence is retained during the 4 T 2 / 2 E intersystem crossing that immediately follows 4 A 2 / 4 T 2 excitation. DFT calculations indicate that the 164 cm À1 oscillation likely corresponds to a combination of Cr-O bond stretching in the ligand-field excited state as well as large amplitude motion of the ligand backbone. This hypothesis is supported by ultrafast timeresolved absorption measurements on Cr(t-Bu-acac) 3 (where t-Bu-acac is the monoanionic form of 2,2,6,6-tetramethyl-3,5-heptanedione) -an electronically similar but more sterically encumbered molecule -which exhibits a 4 T 2 / 2 E conversion that is more than an order of magnitude slower than that observed for Cr(acac) 3 . These results provide important insights into the nature of the reaction coordinate that underlies ultrafast excited-state evolution in this prototypical coordination complex.