Metalation of Hibiscus-derived chlorophyll: Spectroscopic elucidation of a cobalt-porphyrin complex

Chlorophyll is a naturally occurring chlorin macrocycle capable of interacting with transition metal ions. In this study, chlorophyll extracted from Hibiscus sabdariffa leaves was treated with cobalt(II) under controlled conditions to investigate possible metal-induced modifications. Considering that native chlorophyll contains a central Mg²⁺ ion, partial demetalation to pheophytin-like intermediates is expected prior to interaction with cobalt. UV-Vis spectroscopy revealed a bathochromic shift of the Soret band (≈432 → ≈448 nm) and reorganization of the Q-band region (≈662 → ≈642 nm), indicating perturbation of the macrocyclic π-electron system. FT-IR spectra showed subtle changes in porphyrinic skeletal vibrations, suggesting modification of the macrocyclic environment. However, these features alone are not sufficient to confirm metal coordination. ¹H and ¹³C NMR spectra exhibited significant signal broadening due to the paramagnetic nature of cobalt(II), limiting detailed structural interpretation. The NMR data are therefore considered supportive but not definitive evidence. Overall, the results suggest cobalt-associated modification of Hibiscus-derived chlorophyll. However, definitive structural characterization requires further studies, including purification and high-resolution mass spectrometry. These NMR signals in the downfield region alone do not serve as unequivocal proof for the metal coordination with the macrocyclic chlorophyll and indicative of metal‑cobalt modification of the chlorophyll.