Effects of Charge and Intramolecular Structure on the Lipophilicity of Nitrophenols Ve ´ ronique Chopineaux-Courtois, ² Fre ´ de ´ ric Reymond, ² Ge ´ raldine Bouchard, ‡ Pierre-Alain Carrupt, ‡ Bernard Testa, ‡ and Hubert H. Girault* ,² Contribution from the Laboratoire d’Electrochimie, Ecole Polytechnique Fe ´ de ´ rale de Lausanne, CH-1015 Lausanne, Switzerland, and Institut de Chimie The ´ rapeutique, Section de Pharmacie, UniVersite ´ de Lausanne, CH-1015 Lausanne, Switzerland ReceiVed October 14, 1998 Abstract: The lipophilicity of a series of phenolic compounds was studied in the 1,2-dichloroethane (1,2- DCE)/water system. Cyclic voltammetry at the ITIES was used to study the transfer characteristics of the charged species, and their partition coefficient was deduced from their formal transfer potential. For the neutral species, log P DCE values were measured by a two-phase pH-metric method. The results are compared to those previously obtained in octanol/water and by linear solvation energy relationships (LSER) in the two solvent systems. It is shown that nitrophenols with intramolecular H-bonding deviate from the solvatochromic equation for the 1,2-DCE/water system, and discrepancies between both approaches are discussed on the basis of conformational and steric effects. When charged however, all the species have approximately the same partition coefficients because the effect of the intramolecular H-bond disappears and the differences in measured lipophilicity arise from the variation of the intramolecular charge delocalization due to resonance equilibria. Some biological implications of these properties are discussed. I. Introduction Nitro-substituted phenols are weak hydrophobic acids for which the toxicity is due primarily to their ability to uncouple the cellular energy production. 2,4-Dinitrophenol (2,4-DNP), 3-nitrophenol (3-NP), and, to a lesser degree, 2-nitrophenol (2- NP) and 4-nitrophenol (4-NP) increase the ATPase activity of mammalian skeletal muscle in the presence of different cations. 1 2,4-DNP also acts as uncoupler for oxidative phosphorylation in mitochondria, 2 and in the presence of Mg 2+ , it mimics the effect of actin. The study of their lipophilicity is of particular interest to understand their mechanisms of skin permeability and hence their biological activity. 3 Phenolic compounds may be trans- ported across the skin barrier by diffusion through the append- ages, hair follicles, sweat glands, and sebaceous glands or by diffusion through the stratum corneum itself. 4 Until now, the intrinsic partition coefficient of phenols has been determined in the reference n-octanol/water system, 5 and their permeability has been classified. 4 However, the target sites of many toxic compounds are biological membranes constituted of distinct hydrophilic and hydrophobic regions, where both neutral and charged species may interact with the membrane. 6 Thus, studies taking only neutral species into consideration to investigate the partitioning behaviors can be too restrictive. The partition of ionic species has been taken into account only recently in the evaluation of the lipophilicity of drugs and compounds of pharmaceutical interest. Then it has been possible to measure the partitioning of charged species by studying the variation of the distribution coefficient of ionizable compounds as a function of pH. Electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) appears as a new efficient technique in the field of lipophilicity to study the transfer and partition mechanisms of ionic compounds. 7,8 At thermodynamic equi- librium, the distribution of an ion is determined by the equality of its electrochemical potentials in the two phases, and it is given by the Nernst equation at the ITIES, 9 which can be expressed in terms of the partition coefficient log P I (log P of ion) as where Δ o w φ is the applied Galvani potential difference between the organic (o) and the aqueous (w) phase, c I is the concentration of the ion I, and Δ o w φ I 0′ is the formal transfer potential of I. In dilute solutions, the formal ion-transfer potential is related to the standard Gibbs energy of transfer by ΔG tr,I 0,wfo represents the difference of standard Gibbs energy of solvation between water and the organic phase and P I 0′ repre- ² Ecole Polytechnique Fe ´de ´rale de Lausanne. ‡ Universite ´ de Lausanne. (1) Salerno, V. P.; Ribeiro, A. S.; Dinucci, A. N.; Mignaco, J. A.; Sorenson, M. M. Biochem. J. 1997, 324, 877-884. (2) Ohkouchi, T.; Kakutani, T.; Senda, M. Bioelectrochem. Bioenerg. 1991, 25, 71-80. (3) Jetzer, W. E.; Huq, A. S.; Ho, N. F. H.; Flynn, G.; Duraiswamy, N.; Condie, L. J. Pharm. Sci. 1986, 75, 1098-1103. (4) Roberts, M. S.; Anderson, R. A.; Swarbrick, J.; Moore, D. E. J. Pharm. Pharmacol. 1978, 30, 486-490. (5) Huq, A. S.; Ho, N. F. H.; Husari, N.; Flynn, G. L.; Jetzer, W. E.; Condie, L. J. Arch. EnViron. Contam. Toxicol. 1986, 15, 557-566. (6) Petty, H. R. Molecular Biology of Membranes. Structure and Function; Plenum Press, New York, 1993. (7) Reymond, F.; Steyaert, G.; Carrupt, P.-A.; Testa, B.; Girault, H. H. HelV. Chim. Acta 1996, 79, 101-117. (8) Ohkouchi, T.; Kakutani, T.; Senda, M. Bioelectrochem. Bioenerg. 1991, 25, 81-89. (9) Girault, H. H. Charge Transfer across Liquid/Liquid Interfaces; Plenum Press: New York, 1993; Vol. 25, pp 1-62. log P I ) log ( c I o c I w 29 ) zF RT ln 10 (Δ o w φ - Δ o w φ I 0′ ) (1) Δ o w φ I 0′ ) -ΔG tr,I 0,wfo z I F ) -RT ln 10 z I F log P I 0′ (2) 1743 J. Am. Chem. Soc. 1999, 121, 1743-1747 10.1021/ja9836139 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/17/1999