The protein haemerythrin (Her) is responsible for binding and carrying O2 in some invertebrates. Each protein molecule has two Fe2+ ions that are in very close proximity and work together to bind one molecule of O2, form ing an Fe2O2 group. (a) The Raman spectrum of oxygenated haemerythrin
obtained with laser excitation at 500 nm has a band at 844 cm-1 which has been attributed to the O-O stretching mode of bound 16O2. Why is Raman spectroscopy and not infrared spectroscopy the method of choice for the study of the binding of O2 to haemerythrin? (b) Proof that the 844 cm-1
band in the Raman spectrum of oxygenated haemerythrin arises from a bound O2 species may be obtained by conducting experiments on samples of haemerythrin that have been mixed with 18O2, instead of 16O2 . Predict the fundamental vibrational wavenumber of the 18O-18O stretching mode in a sample of haemerythrin that has been treated with 18O2. (c) The fundamental vibrational wavenumbers for the O-O stretching modes of O2, O2- (superoxide anion), and O22-( peroxide anion) are 1555, 1107, and 878 cm-1 respectively. (i) Explain this trend in terms of the electronic structures of O2, O2-, and O22-. (ii) What are the bond orders of O2, O2-, and O22-? (d) Based on the data given in part (c), which of the following species best describes the Fe2O2 group of haemerythrin: Fe2+2O2, Fe2+Fe3+O2-, or Fe3+2O2 2-? Explain your reasoning . (e) The Raman spectrum of haemerythrin mixed with 16O18O has two bands that can be attributed to the O-O stretching mode of bound oxygen. Discuss how th is observation may be used to exclude one or more of the four proposed schemes as shown for binding of O2 to the Fe2 site of haemerythrin.
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