Thermodynamics of porphyrin dimerization in aqueous solutions

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Abstract
The dimerization equilibrium of deuteroporphyrin IX and of mesoporphyrin IX in aqueous solutions were studied by fluorimetric techniques over the 0.01-1 .mu.M concentration range, where dimerization is the dominant aggregation process. Deuteroporphyrin IX was studied at several temperatures over the range 22.degree.-37.degree. C, and mesoporphyrin at 25.degree. and 37.degree. C. The magnitudes determined for the dimerization equilibrium constants (25.degree. C, neutral pH, phosphate-buffered saline) are 2.3 .times. 106 M-1 and 5.4 .times. 106 M-1 for the deutero and meso derivatives, respectively. The meso, deutero and hemato species tested show a similar temperature effect, namely, dimerization decreasing with increasing temperature, indicating the involvement of a negative enthalpy change. Van''t Hoff isochore of the dimerization constants determined for deuteroporphyrin IX was linear within the temperature range of 22.degree.-37.degree. C, allowing the calculation of the thermodynamic parameters. For deuteroporphyrin dimerization, those were .DELTA.G0 = -36.4 kJ .cntdot. mol-1; .DELTA.H0 = -46.0 kJ .cntdot. mol-1 and .DELTA.S0 = -32.2 J .cntdot. K-1 .cntdot. mol-1 (at neutral pH, 25.degree. C, phosphate-buffered saline), showing the process to be enthalpy-driven. Similar trends were found for porphyrin species other than those studied here. These data fit with a hypothesis giving a major role to the solvent in driving porphyrins to aggregate in aqueous solution. The magnitudes and directions of the energetic changes fit better with the expectation of the solvophobic force theory predicting enthalpy-driven association, than with the classic hydrophobic bonding, predicting the association to be entropy-driven.