Biomedical subjects
R Blankenship
Publications and source records attributed to R Blankenship.
Electron spin polarization in photosynthesis and the mechanism of electron transfer in photosystem I. Experimental observations.
Transient electron paramagnetic resonance (EPR) methods are used to examine the spin populations of the light-induced radicals produced in spinach chloroplasts, photosystem I particles, and Chlorella pyrenoidosa. We observe both emission and enhanced absorption within the hyperfine structure of the EPR spectrum of P700+, the photooxidized reaction-center chlorophyll radical (Signal I). By using flow gradients or magnetic fields to orient the chloroplasts in the Zeeman field, we are able to influence both the magnitude and sign of the spin polarization. Identification of the polarized radical and P700+ is consistent with the effects of inhibitors, excitation light intensity and wavelength, redox potential, and fractionation of the membranes. The EPR signal of the polarized P700+ radical displays a 30% narrower line width than P700+ after spin relaxation. This suggests a magnetic interaction between P700+ and its reduced (paramagnetic) acceptor, which leads to a collapse of the P700+ hyperfine structure. Narrowing of the spectrum is evident only in the spectrum of polarized P700+, because prompt electron transfer rapidly separates the radical pair. Evidence of cross-relaxation between the adjacent radicals suggests the existence of an exchange interaction. The results indicate that polarization is produced by a radical pair mechanism between P700+ and the reduced primary acceptor of photosystem I. The orientation dependence of the spin polarization of P700+ is due to the g-tensor anisotropy of the acceptor radical to which it is exchange-coupled. The EPR spectrum of P700+ is virtually isotropic once the adjacent acceptor radical has passed the photoionized electron to a later, more remote acceptor molecule. This interpretation implies that the acceptor radical has g-tensor anisotropy significantly greater than the width of the hyperfine field on P700+ and that the acceptor is oriented with its smallest g-tensor axis along the normal to the thylakoid membranes. Both the ferredoxin-like iron-sulfur centers and the X- species observed directly by EPR at low temperatures have g-tensor anisotropy large enough to produce the observed spin polarization; however, studies on oriented chloroplasts show that the bound ferredoxin centers do not have this orientation of their g tensors. In contrast, X- is aligned with its smallest g-tensor axis predominantly normal to the plane of the thylakoid membranes. This is the same orientation predicted for the acceptor radical based on analysis of the spin polarization of P700+, and indicates that the species responsible for the anisotropy of the polarized P700+ spectrum is probably X-. The dark EPR Signal II is shown to possess anisotropic hyperfine structure (and possibly g-tensor anisotropy), which serves as a good indicator of the extent of membrane alignment.
Chemically induced dynamic electron polarization in chloroplasts at room temperature: evidence for triplet state participation in photosynthesis.
A transient electron paramagnetic resonance emission is observed after flash excitation of chloroplasts at room temperature. The spectrum of the emission signal is centered at g = 2.0037 and has a linewidth deltaHpp = 4G (4 X 10(-4) tesla). Inhibitor studies and chemical oxidation indicate that the signal is associated with Photosystem I, but the spectrum and kinetics indicate that it is neither P700 nor an iron-sulfur protein. The emission signal rises with the 2-musec time response of the instrument, and decays during the actinic flash. The emission signal is produced on only the first of a pair of strong flashes separated by 100 musec, indicating that the precursor has not been regenerated in that time. The results are discussed with reference to the two currently accepted mechanisms for chemically induced dynamic electron polarization: the radical pair and the photochemical triplet. For several reasons the photochemical triplet mechanism is the more attractive of the two. It is suggested that at room temperature the primary photochemistry of photosystem I proceeds via a triplet state of chlorophyll, and that the species giving rise to the emission signal is the primary electron acceptor of Photosystem I.