The protein haemerythrin is responsible for binding and carrying 0, in some invertebrates. Each protein molecule has two Fe'+ ions that are in very close proximity and work together to bind one molecule of 0, The Fe, O, group of oxygenated haemerythrin is coloured and has an electronic absorption band at 500 nm. The resonance Raman spectrum of oxygenated haemerythrin obtained with laser excitation at 500 nm has a band at 844 cm-I that has been attributed to the 0-0 stretching mode of bound 160,
(a) Why is resonance Raman spectroscopy and not infrared spectroscopy the method of choice for the study of the binding of 0, to haemerythrin? (b) Proof that the 844 cm-1 band arises from a bound 0, species may be obtained by conducting experiments on samples of haemerythrin that have been mixed with 180" instead of 160,. Predict the fundamental vibrational wave number of the 180_180 stretching mode in a sample of haemerythrin that has been treated with 180.
(c) The fundamental vibrational wave numbers for the 0-0 stretching modes of02, 02 (super oxide anion), and O~- (peroxide anion) are 1555, 1107, and 878 cm-1, respectively. Explain this trend in terms of the electronic structures of 02' 02' and O2/2. Hint: Review Section 11.4. What are the bond orders of 02' 02", and O2/2? (d) Based on the data given above, which of the following species best describes the Fe2O2 group of haemerythrin: Fe2/2 +O2' Fe2+ Fe3+02, or Fe 3/2 + O 2/2? Explain your reasoning.
(e) The resonance Raman spectrum of haemerythrin mixed with 160180 has two bands that can be attributed to the 0-0 stretching mode of bound oxygen. Discuss how this observation may be used to exclude one or more of the four proposed schemes (5-8) for binding of O, to the Fe2 site of haemerythrin.

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