Table 1.

Predicted and observed masses for cluster-bound forms of FNR

FNR species	Predicted mass (Da)*	Average observed mass (Da)†	ΔMass (Da)‡
Monomeric FNR
[4Fe-4S]2+	29,897	29,898	+1
[3Fe-4S]1+	29,842	29,843	+1
[3Fe-3S]3+	29,808	29,811	+3
[3Fe-3S]1+	29,810	29,811	+1
[2Fe-4S]2+	29,785	29,784	−1
[2Fe-3S]2+	29,753	29,752	−1
[2Fe-2S]2+	29,721	29,720	−1
Dimeric FNR
[4Fe-4S]2+/[4Fe-4S]2+	59,794	59,796	+2
[4Fe-4S]2+/[3Fe-4S]1+	59,739	59,740	+1
[3Fe-4S]1+/[3Fe-4S]1+	59,684	59,685	+1
[3Fe-3S]3+/[3Fe-4S]1+	59,650	59,653	+3
[3Fe-3S]1+/[3Fe-4S]1+	59,652	59,653	+1

^*The difference in predicted mass depending on the overall charge on the cluster is caused by charge compensation of cluster binding. A lower overall cluster charge would result in more protons remaining bound and consequently, a higher mass (26, 34).

^†The average observed mass is derived from at least four independent experiments, with SD of ±1 Da.

^‡The difference between the average observed and predicted masses.