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Degree Name

Doctor of Philosophy




Tetraalkylammonium salts of Fe(,n)S(,n)(SPh)(,4) ('2-) (n = 2,4), which are synthetic analogues of 2Fe-2S and 4Fe-4S sites in proteins, can;be prepared in >80% yields from 4/1/1 PhSH/FeCl(,3)/S in a medium consisting of 90/5/5 vol % (pH (TURN) 8) aqueous buffer/CH(,3)CN/Triton X-100. For this system the sole assembly pathway appears to be; Fe(SPh)(,4) ('2-) (--->) Fe(,2)S(,2)(SPh)(,4) ('2-) (--->) Fe(,4)S(,4)(SPh)(,4) ('2-), which suggests an analogous assembly pathway for 4Fe-4S sites in ferredoxins. At lower PhSH/Fe mole ratios or at lower pH the Fe(,4)(SPh)(,10) ('2-) (--->) Fe(,4)S(,4)(SPh)(,4) ('2-) pathway appears to occur, but to a lesser extent in aqueous Triton than in organic solvents. High yields of the pre-;cursors Fe(SPh)(,4) ('2-) and Fe(,4)(SPh)(,10) ('2-) are also achieved in the aqueous-based detergent medium. Evidence from ('1)H and ('19)F NMR indicates that the equilibrium Fe(,4)(SPh)(,10) ('2-) + 6PhS('-) = 4 Fe(SPh)(,4) ('2-) is established prior to addition of S. The two most prominent unique features in the 90/5/5 medium are (i) the ability;of detergent aggregates in the presence of relatively hydrophobic R(,4)N('+) to shield Fe(,2)S(,2)(SPh)(,4) ('2-) from water, thereby inhibiting the rate of conversion to Fe(,4)S(,4)(SPh)(,4) ('2-), and (ii) the facility with which the Fe(,4)(SPh)(,10) ('2-) = Fe(SPh)(,4) ('2-) equilibrium can be shifted. Unique feature i leads to the ability to achieve essentially quantitative pro- duction of either Fe(,2)S(,2)(SPh)(,4) ('2-) or Fe(,4)S(,4)(SPh)(,4) ('2-) from the same 4/1/1 PhSH/FeCl(,3)/S reagent ratio. Unique feature ii leads to the production of substantial proportions of Fe(,2)S(,2)(SPh)(,4) ('2-) upon reac- tion of Fe(,4)(SPh)(,10) ('2-) with S, due to a shift of the above equilibrium to the right. Unique feature ii also leads to larger proportions of Fe(,4)S(,4)(SPh)(,4) ('2-) at pHs below 6 (below the pK(,a) of PhSH), apparently due to a shift of the above equilibrium to the left. These two unique;features suggest means by which ferredoxins could exert both kinetic and thermodynamic control over assembly of 2Fe-2S and 4Fe-4S sites.



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William Charles Stevens



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114 pages