Similarly, in the case of H-Tyros, the parent species [C52H50FeN6O7] + started the degradation by a double loss of 71 m/z. The final ionized species was [C42H50FeN6O2] + (m/z 727.3) (Fig. ). This is because the conjugation of tyrosine to hemin is weaker than that of tyramine. For instance, the Soret band shifted and became broader with a new shoulder at a lower wavelength in the spectra of all compounds in buffer and culture medium. This relates to the spontaneous dimerization of these compounds in the aqueous solutions. The blue shift of the Soret band was more prominent in the case of hemin (406 to 394 nm), followed by H-Tyra (402 to 395 nm), with slight shift in H-Tyros from (404 to 400 nm). In contrast, H-Styr solution showed a red shift from (402 to 420 nm). This was supported by the broad bands within the charge transfer region. The formation of a π-π dimer between the unligated faces of two five-coordinate iron-porphyrins in these cases is considered to be the predominant mechanism without involvement of the Fe(III) centres . This was confirmed in the previous section studying hemin aggregation. In addition, owing to the conjugated benzene rings to the hemin macrocycle, there is an increased probability for the π-π stacking-mediated dimerization to take place.