RT Abs (top) and 4.5 K MCD (bottom) spectra of as-isolated CDO (– - –), Fe(II)-CDO + Cys (- - -), and as-isolated CDO + Cys (—). Inset: VTVH MCD data (—) collected for Fe(II)-CDO + Cys at 30,580 cm−1 (position indicated by arrow) and 2.2, 4, 8, 15, 25 K. Open circles represent a theoretical fit of the data obtained with the parameters g = 2, S = 2, D = −13.0 cm−1, E/D = 0.23.