Solubility of Pyropheophorbide-a Methyl Ester and Its with Copper(II) Complex in Aqueous Systems of Tetradecyltriphenylphosphonium Bromide with Protomicelles Formation

The aggregation behavior of pyropheophorbide-a methyl ester and methyl ester Cu(II) pyropheophorbide-a in organic, water-organic, and aqueous media in the presence of tetradecyltriphenylphosphonium bromide (3) was studied by spectrophotometry. The molar extinction coefficients of the monomers were determined in DMSO. In mixed DMSO–water solutions with a water content of 30 wt % and higher, aggregates of compound 2 with a bathochromic shift of the absorption bands were observed. In aqueous media containing surfactants, experiments were carried out using saturated solutions of pyropheophorbide-a methyl ester and methyl ester Cu(II) pyropheophorbide-a under conditions of thermodynamic equilibrium with precipitation of the substance. The solubilization of pyropheophorbide-a methyl ester and methyl ester Cu(II) pyropheophorbide-a was shown to begin in the premicellar region of surfactants, where protomicelles are formed with the participation of aggregates of pyropheophorbide-a methyl ester and methyl ester Cu(II) pyropheophorbide-a acting as nanoadsorbents, and to be most effective in the presence of conventional micelles. Based on the experimental data, the limiting value of the solubilizing ability of tetradecyltriphenylphosphonium bromide toward pyropheophorbide-a methyl ester and methyl ester Cu(II) pyropheophorbide-a was calculated. This value increases upon transition from pyropheophorbide-a methyl ester to methyl ester Cu(II) pyropheophorbide-a and, for the latter, upon transition to pH 7.2.