The association behavior of protoporphyrin IX was investigated at different pH by absorption spectroscopy and gel chromatography. Protoporphyrin IX existed as monomer in acidic water, as aggregate in neutral water, and as dimer in basic water. Poly(N-vinylpyrrolidone) tended to suppress dimerization and aggregation of protopophyrin IX.

Gel chromatography of chlorophyllin (Cu–Na salt) in water was compared with that in aqueous poly(N-vinylpyrrolidone) (PVP) solution. A shoulder at 430 nm in the absorption spectra, which was observed in all fractionated PVP solution system, is due to the monomer of chlorophyllin (Cu–Na salt). PVP tends to suppress aggregation of chlorophyllin (Cu–Na salt).