Fig. 2.
4.2-K/53-mT Mössbauer spectra of samples in which 
was reacted with Ar-NHOH or Ar-NH2. In all cases, the red, blue, and green lines illustrate the fractional contributions of the reference spectra of 
, 
, and 
, respectively, to the experimental spectrum, as described in the text. (Left) A solution of 
(1.2 mM Fe2) was mixed with 0.5 equivalent volume of buffer solution containing 1.8 mM O2 (O2/Fe2 = 0.75). This solution was allowed to react at 5 °C for 0.11 s to permit accumulation of 
. (A) The solution was then directly freeze-quenched. (B and C) The solution was then mixed with one-sixth equivalent volume of an O2-free solution of 3.6 mM Ar-NHOH (Ar-NHOH/O2 = 1), and this solution was allowed to react for 45 ms (B) or 400 ms (C) prior to being freeze-quenched. D and E are the difference spectra B-A and C-B, respectively. The black line in D is the sum of the contributions of 
(-28%) and 
(28%). The black line in E is the difference spectrum B-A for comparison. (Middle) A solution of 
(1.8 mM) was mixed with two equivalent volumes of a buffer solution containing 1.8 O2 mM O2 (O2/Fe2 = 2). This solution was allowed to react at 5 °C for 110 ms to permit accumulation of 
. (F) The reaction was then directly freeze-quenched. (G and H) The solution was then mixed with one-sixth equivalent volume of an O2-free buffer solution containing 3.6 mM Ar-NHOH (Ar-NHOH/O2 = 0.5), and this reaction was allowed to proceed at 5 °C for 45 ms (G) or 2 s (H) prior to being freeze-quenched. I and J are the difference spectra G-F and H-G, respectively. The black line in J is difference spectrum C–B, scaled by a factor of -0.8 for comparison. (Right) A solution of 
(1.2 mM) was mixed with 0.5 equivalent volume of a buffer solution containing 1.8 mM O2 (O2/Fe2 = 0.75), and the reaction was allowed to proceed at 5 °C for 110 ms to permit accumulation of 
. (K) The reaction was directly freeze-quenched. (L) The resulting solution was then mixed with one-sixth equivalent volume of an O2-free buffer solution containing 0.91 mM Ar-NH2 (Ar-NH2/O2 = 0.25), and the reaction was allowed to proceed for 4 s before being freeze-quenched. M is the difference spectrum L–K. The black line in M is the sum of the contributions of 
(-29%), 
(12%), and 
(17%).