SQUID Magnetization Study of the Infrared-Induced Spin Transition in the S2 State of Photosystem II: Spin Value Associated with the g = 4.1 EPR Signal

Abstract

The Mn4 complex which is involved in water oxidation in photosystem II is known to exhibit three types of EPR signals in the S2 state, one of the five redox states of the enzyme cycle: a multiline signal (spin 1/2), signals at g > 5 (spin 5/2), and a signal at g = 4.1 (spin value 3/2 or 5/2). The multiline and g = 4.1 signals are those the most readily observed. The relative proportions of the g = 4.1 signal and of the multiline signal are affected by many biochemical treatments including the substitution of Ca2+and Cl- which are two essential cofactors for O2 evolution. The state responsible for the multiline signal can also be converted, reversibly, to that responsible for the g = 4.1 signal upon the absorption of near-IR light at around 150 K. These infrared-induced effects are confined to the Mn4 cluster, and no other redox change occurs in the enzyme. Here, we have used the IR-induced photochemistry of the Mn4 cluster to measure the changes in magnetization occurring upon interconversion of the state responsible for the spin 1/2 state and the g = 4.1 state. Measurements were performed with a SQUID magnetometer below 20 K and at magnetic fields ≤5.5 T. Simulations of experimental data provide strong indication that the spin value of the state responsible for the g = 4.1 state is 5/2. Results are discussed in terms of a model implying an IR-triggered spin conversion of the MnIII (from the spin 2 to spin 1) of the Mn4 cluster.