Fe4S4(Z-cys-Gly-OMe)4] [NMe4I2 (1) and [Fe,S,(Z-~ys-Gly-Ala-OMe)~] [NMe,], (2) were prepared from [FedS4(S-t-Bu),] [NMe4I2 and the corresponding peptides as a model of P. aerogenes Fd. A positive shift in redox potential(120 mV) of 2 was observed in dichloromethane at 233 K. No shift was detected in the case of 2 in the dimethylformamide or 1 in CH2C12 even at low temperatures. The results strongly suggest that the shift of redox potential is caused by the NH---S hydrogen bond between Ala N H and Cys S in 2. The CD spectra and IR spectra of 2 also support a peptide folding induced by the NH---S bond.

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The support of the NSF (Grant No. CHE-79-18217) and NIH RCDA (Grant No. 5 KG4 GM 00227) is gratefully acknowledged. We wish to thank Dr. N. D. Chasteen for providing the program eprpow and for helpful comments concerning its use, Ronald D. Guiles for adapting the program to the CDC computer at San Francisco State and for carrying out the EPR spectra simulations, and Dr. Rollie J. Myers for providing access to the Calcomp plotter at the

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