SQUID MAGNETIZATION STUDY OF THE INFRARED-INDUCED SPIN TRANSITION IN THE S-2 STATE OF PHOTOSYSTEM-II - SPIN VALUE ASSOCIATED WITH THE G = 4.1 EPR SIGNAL

The Mn-4 complex which is involved in water oxidation in photosystem I I is known to exhibit three types of EPR signals in the S-2 state, one of the five redox states of the enzyme cycle: a multiline signal (spi n 1/2) signals at g > 5 (spin 5/2), and a signal at g = 4.1 (spin valu e 3/2 or 5/2). The multiline and g = 4.1 signals are those the most re adily observed. The relative proportions of the g = 4.1 signal and of the multiline signal are affected by many biochemical treatments inclu ding the substitution of Ca2+ and Cl- which are two essential cofactor s for O-2 evolution. The state responsible for the multiline signal ca n also be converted, reversibly, to that responsible for the g = 4.1 s ignal upon the absorption of near-IR light at around 150 K. These infr ared-induced effects are confined to the Mn4 cluster, and no other red ox change occurs in the enzyme. Here, we have used the IR-induced phot ochemistry 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 SQU ID magnetometer below 20 K and at magnetic fields less than or equal t o 5.5 T. Simulations of experimental data provide strong indication th at 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 Mn-III (from the spin 2 to spin 1) of the Mn-4 cluster.