Similar to the role that hemoglobin plays in human blood, the protein hemocyanin transports and delivers O2 in mollusks. Hemocyanin has a di-copper center, rather than an Fe-heme center, that serves as the site of O2 binding and release. In early experiments performed before any crystal structures could be obtained, researchers relied on spectroscopy to determine the structure of the O2-carrying active site. They found that they were able to reconstitute the protein with O2 via the following reaction: The oxidized Cu2(O2) protein is blue and gives rise to the bluish color of mollusks. Researchers used resonance Raman spectroscopy to explore the structure of the oxidized protein by exciting the UV-visible absorption band responsible for its blue color (at 570nm ) to enhance the Raman feature observed at 744cm1. When they reconstituted the protein with 18O2 and repeated the resonance Raman experiment, they saw this band shift to 708cm1. Is this observation most consistent with assigning the vibrational feature to an OO or a CuO stretch