Direct evidence of the metal-free nature of sirohydrochlorin in desulfoviridin

We have obtained direct evidence that the majority of the sirohydrochlorin chromophore in the dissimilatory sulfite reductase desulfoviridin from Desulfovibrio gigas , is not associated with any metal. The evidence comes from resonance Raman measurements of native and deuterated samples of desulfoviridin. The breathing mode v 4 (or v * 4 ) at 1336 cm −1 in the native enzyme is downshifted to 1326 cm −1 upon deuteration. This mode is not sensitive to deuteration if a metal is present at the center of the chromophore inside protein or in solution. The results also establish the existence of exchangeable core hydrogen(s) at the pyrrolic nitrogen(s).