Transformation of a Mononitrosyl Iron Complex to a [2Fe-2S] Cluster by a Cysteine Analogue

Reversible modification of iron-sulfur
clusters by nitric oxide
acts as a genetic switch in a group of regulatory proteins. While
the conversion of [Fe-S] clusters to iron-nitrosyls has been widely
studied in the past, little is known about the reverse process, the
repair of [Fe-S] clusters. Reported here is a system in which a mononitrosyl
iron complex (MNIC), (PPN)­[Fe­(StBu)3(NO)] (1), is converted to a [2Fe-2S] cluster,
(PPN)2[Fe2S2(SCH2CH2C­(O)­OMe)4] (2). This conversion requires
only the addition of a cysteine analogue, 3-mercaptomethylpropionate
(MMP), at room temperature without the need for any other reagents.
The identity of 2 was confirmed spectroscopically, chemically,
crystallographically, and analytically. Mass spectrometry and 34S labeling studies support that the bridging sulfides in 2 derive from the added MMP, the cysteine analogue. The NO
lost during the conversion of 1 to 2 is
trapped in a dinitrosyl iron side product, (PPN)­[Fe­(SCH2CH2C­(O)­OMe)2(NO)2] (4). The present system implies that MNICs are likely intermediates
in the repair of NO-damaged [2Fe-2S] clusters and that cysteine is
a viable molecule responsible for the destabilization of MINCs and
the formation of [2Fe-2S] clusters.