Kinetic and thermodynamic analysis of induced-fit molecular recognition between tetraarylporphyrin and ubiquinone analogues

meso-Tetrakis(2-hydroxy-4-nonylphenyl)porphyrin (1), which was prepared as a host molecule for ubiquinone analogues, comprises four atropisomers, alpha alpha alpha alpha, alpha alpha alpha beta, alpha alpha beta beta, and alpha beta alpha beta, in a statistical ratio of 1:4:2:1, respectively. The atropisomerization is due to internal rotation about the C(aryl)-C(porphyrin) bonds and is observed even at room temperature, A rate constant for the rotation was determined (k = 1.73 x 10(-5) s(-1) in CHCl3 at 25 degrees C). UV/visible spectrophotometric titration of tetramethoxy-p-benzoquinone (3) against 1(alpha alpha alpha alpha) or 1(alpha alpha alpha beta) in CHCl3 at 25 degrees C showed 1:1 complexation with association constants of (9.4 +/- 0.1) x 10(3) and (3.5 +/- 0.1) x 10(2) M-1, respectively. Upon addition of 3 to a solution of 1 in CDCl3, the proportion of 1(alpha alpha alpha alpha) in the atropisomeric mixture increased and that of the other three isomers decreased over 500 h. At equilibrium the proportion of 1(alpha alpha alpha alpha) reached 78% in the presence of 3 (3 equiv); this showed that atropisomerization is induced by complexation with 3, An atropisomeric shift to 1(alpha alpha alpha alpha) was also observed upon addition of 2,3-dimethoxy-p-benzoquinone (4); however, the proportion of 1 alpha alpha alpha alpha was only 30% at equilibrium, since the difference in binding affinities between 1(alpha alpha alpha alpha)-4 and 1(alpha alpha alpha beta)-4 complexes is relatively small compared with that between 1(alpha alpha alpha alpha)-3 and 1(alpha alpha alpha beta)-3 complexes. Nickel and zinc porphyrins. 1.Ni and 1.Zn, also showed induced-fit interaction with 3. The atropisomeric shift to 1(alpha alpha alpha alpha).Ni upon addition of 3 was completed within Ih. whereas guest-induced atropisomerization in 1.Zn was much slower than in free base 1. The observed changes in the relative amounts of the four atropisomers are identical with the simulated changes calculated from multiple equilibrium systems by use of kinetic and thermodynamic parameters. This system is a suitable model for induced-fit molecular recognition in flexible enzymes.