Simultaneous observation of the O - O and Fe - O₂ stretching modes in oxyhemoglobins

Understanding of the chemical nature of the dioxygen moiety of oxyhemoglobin is crucial for elucidation of its physiological function. In the present work, direct Raman spectroscopic observation of both the Fe - O₂ and O - O stretching modes unambiguously establishes the vibrational characteristics of the oxygen-bound heme moiety in the hemoglobins of Chlamydomonas eugametos and Synechocystis PCC6803. In addition to providing the resonance Raman assignment of the O - O stretching mode (1136 cm^-1 for Chlamydomonas, 1133 cm^-1 for Synechocystis) in an oxyhemoglobin with an iron-porphyrin, this study also reports unusually low frequencies for the Fe - O₂ stretching modes (554 cm^-1). The effect of strong hydrogen bonding to the bound oxygen is confirmed by changes in the frequency of the Fe - O₂ stretching mode on mutation of distal residues. These findings suggest an enzymatic function rather than an oxygen transport role for these hemoglobins.