Abstract
The band shape of the Raman line attributed to the Fe 2+-N ϵ (His F8) stretching mode in deoxymyoglobin contains significant information on the nature of the Fe-His proximal linkage. Raman lines appearing close to it, however, obscure the true line profile. To isolate this from its accompanying lines we use its isotopic shift of ∼1 cm −1 when 56Fe in natural-abundance deoxymyoglobin is substituted by 54Fe. This enables us to isolate the true line shape. We have measured this line shape in sperm whale myoglobin dissolved in a 66% vol/vol glycerol/water solution for nine temperatures from 10 K to 300 K. The ν(Fe-His) band shows a complex temperature-dependent profile, with a shoulder on its high-frequency wing, which becomes more prominent with increasing temperature. Detailed analysis reveals that the band is composed of five distinct lines attributable to taxonomic conformational substates of the ν(Fe-His) linkage. These are in thermodynamic equilibrium above the glass transition temperature T f but freeze in into the thermodynamic distribution at T f for lower temperatures. Alternative models that try to explain the ν(Fe-His) band shape by either an anharmonic coupling of the ν(Fe-His) to a low-frequency heme doming mode or by conformational substates related to a Gaussian distribution of iron out-of-plane displacements are at variance with the distinct features observed experimentally.
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