NH-S hydrogen bonds

Some of the complexes were found to exhibit redox potentials that shifted with temperature (230-300 K). The complexes with Cys-Gly-Ala and Cys-Gly-Ala-Cys sequences were found to be particularly susceptible to temperature variations in CH2CI2 (13,14) with positive shifts at (ca. 0.10 V). These shifts were rationalized by the formation of NH—S hydrogen-bonded conformers (see Fig. 5), which are expected to stabilize the reduced forms of the complexes Such a hydrogen-bonded structure has been deduced from the X-ray analysis of several bacterial Fds (26). 

Adman, E. T., Watenpaugh, K. D., and Jensen, L. H. (1975). NH---S hydrogen bonds in Peptococcus aerogenes ferrodoxin, Clostridium pasteurianum rubredoxin and Chromatium high potential iron protein. Proc. Natl. Acad. Sci. U.S.A. 72,4854—4858. 

Some detailed comparisons of the protein environments around the HiPIP and Fd clusters have been made.769,770 It is noteworthy that the HiPIP cluster is more deeply buried (about 4.5 A) than is the case for the clusters in the other iron-sulfur proteins. All iron-sulfur proteins for which structural data are available, with the exception of the three-iron protein from Azotobacter vinelandii, have hydrogen bonding between the cysteine sulfur in the iron-sulfur cluster and the backbone peptide link. It appears that there is an approximate correlation between the number of NH S hydrogen bonds in the environment of a cluster and its redox potential. In HiPIP, these hydrogen bonds become more linear and shorten on reduction of the cluster. It is possible, therefore, that the oxidation states of the cluster may be controlled by the geometries of the hydrogen bonds.770... 

Ueno, T., Kousumi, Y., Yoshizawa-Kumagaye, K., Nakajima, K., Ueyama, N., Okamura, T., and Nakamura, N. (2001) Role of a-Helix Conformation Cooperating with NH—S Hydrogen Bond in the Active Site of Cytochrome P-450 and Chloroperoxidase Synthesis and Properties of [MIII(OEP)(Cys-Helical Peptide)] (M = Fe and Ga), J. Am. Chem. Soc. 120, 12264-12273. 

Besides the NH—S hydrogen bonding, an effect of chelation is expected to arise from the variation of Fe-S torsion angles. The presence of this effect was suggested from the 1H NMR data of [Fe2S2(Z-cys-X-Y-cys-OMe)2]2 , where Z-Cys-X-Y-Cys-OMe (Z and OMe are substituents at the Cys residues) provides two contact-shifted... 

NH—S hydrogen bonding is supported by measurements in a low dielectric solvent such as dichloromethane (47). However, the difference from the dielectric constants of DMF and dichloromethane is negligible, and it is likely therefore that the NH—S hydrogen bonds are stabilized in dichloromethane by the conformational freezing of Z-Cys-Gly-Ala-OMe at the lower temperature, as shown in Fig. 12. The stronger solvation of DMF results in disruption of the NH—S hydrogen bonds. 

The preferential formation of NH—S hydrogen bonding in less polar solvents is also supported by the results of the solvent dependence of the LMCT absorption maxima of [Fe4S4(Z-cys-Gly-Ala-OMe)4]2 or [Fe4S4(Z-cys-Gly-0Me)4]2. The former complex has maxima at 406 nm in DMF and 390 nm in dichloromethane, but the latter complex shows absorption maxima at 402 nm in DMF and 402 nm in dichloromethane. Absence of the effect of the hydrogen bonding in the complex with Z-Cys-Gly-OMe is evident. 

Cys-Gly-Ala provides a / -II like conformer with NH—S hydrogen bonding. Bulkiness of a substituent R group on an amino acid residue adjacent to the Cys residue prevents the Ala NH group from forming NH—S hydrogen bonds (45, 47). Consequently, it is concluded that the identity of the amino acid residues following the coordinated cysteine residue controls the electrochemical as well as the spectroscopic properties of the [4Fe-4S] core. 

The synthetic [2Fe-2S] model complex of the 20-peptide complex exhibits two LMCT absorption maxima at 423 and 461 nm in DMF, maxima which are near to those of the native plant-type ferredoxin (423 and 466 nm) (69). Two redox couples for — 3/—2 were observed at — 0.64 V versus SCE and at —0.96 V versus SCE in DMF. One of them is very close to the value (—0.64 V versus SCE) of native ferredoxin. The 20-peptide complex containing invariant sequences Cys-A-B-C-D-Cys-X-Y-Cys and Leu-Thr-Cys-Val possesses all essential factors for a model of the active site except for the peptide conformation. The positive-shifted redox potential of the 20-peptide complex in DMF is undoubtedly due to the interactions between the Fe2S22+ core and adjacent amino-acid residues, giving rise to NH--S hydrogen bonding. 

Since it has been shown that the structures of cytochromes P450 have at least one NH- -S hydrogen bond at the active... 

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