Redox linked protonation/deprotonation on the carboxylate of [Fe₄S₄(SCH₂CH₂COO^-)₄]^6- in aqueous micellar solutions

Abstract

[Fe₄S₄(SCH₂CH₂COO^-)₄]^6-,which acts as active center analogue of ferredoxin, demonstrates a proton coupled electron transfer phenomenon with <i>p</i>K_a^Ox at 3.0, 2.5, and 3.2 and <i>p</i>K_a^Red at 4.7,4.3, and 4.8, in 3% (<i>w/v</i>)<i> </i>aqueous SDS, TX- 100 and CTAB micelles. When the operating <i>p</i>H is between <i>p</i>K_a^Ox and <i>p</i>K_a^Red, the

observed redox Bohr effect exhibits a 60 to 70 mV per <i>p</i>H unit slope indicating one proton per electron coupled event. These low <i>p</i>K_a<i> </i>values arise because of the protonation/deprotonation of one of the four carboxylates. Also, protonation of the carboxylate leads to 120-140 mV positive shift in the mid -point potenti al. Formation of H bonding between the carboxylate and the neighbouring protonated core S is proposed as reason for the very low <i>p</i>K_a<i>.</i>