A Graphical Approach to Hyperfine Spectra Analysis

Coremans et al. (1997) obtained ESEEM spectra for the HisNj of histidine that is coordinated to the copper ion in azurin. The profile of these spectra resemble 

The hyperfine splitting parameters of the two azurin HisNs were determined via multi-frequency cw-EMR studies by Antholine et al. (1993), yielding the following principal values for the hyperfine tensor (in MHz)  

For the pair, and the respective isotropic (contact) interactions of 26.5 and 18.8 MHz. The exact cancellation condition for the two nitrogen nuclei would therefore occur at v= Via, or 13 and 9.4 MHz, respectively, and the field required to promote the nuclear Zeeman energy to 13 MHz is 4.25T, which is a 122-GHz experiment at g=2.05. The corresponding field and spectrometer operating frequency that would cross levels of the more weakly coupled nitrogen is 3.07T (88 GHz). The predicted splitting of the Am/ =2 transition of the m/ spin manifold at 4.2r is approximately 52 MHz, and the three ground states are therefore within the excitation bandwidth of a 15-ns microwave pulse, which is routinely achieved. 

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