KINETIC AND THERMODYNAMIC ANALYSIS OF INDUCED-FIT MOLECULAR RECOGNITION BETWEEN TETRAARYLPORPHYRIN AND UBIQUINONE ANALOGS

meso-Tetrakis(2-hydroxy-4-nonylphenyl)porphyrin (1), which was prepare d as a host molecule for ubiquinone analogues, comprises four atropiso mers, alpha alpha alpha alpha, alpha alpha alpha beta, alpha alpha bet a 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 temperatu re, 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 titrat ion of tetramethoxy-p-benzoquinone (3) against 1(alpha alpha alpha alp ha) or 1(alpha alpha alpha beta) in CHCl3 at 25 degrees C showed 1:1 c omplexation 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 solutio n of 1 in CDCl3, the proportion of 1(alpha alpha alpha alpha) in the a tropisomeric mixture increased and that of the other three isomers dec reased over 500 h. At equilibrium the proportion of 1(alpha alpha alph a 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 o f 2,3-dimethoxy-p-benzoquinone (4); however, the proportion of 1 alpha alpha alpha alpha was only 30% at equilibrium, since the difference i n binding affinities between 1(alpha alpha alpha alpha)-4 and 1(alpha alpha alpha beta)-4 complexes is relatively small compared with that b etween 1(alpha alpha alpha alpha)-3 and 1(alpha alpha alpha beta)-3 co mplexes. Nickel and zinc porphyrins. 1.Ni and 1.Zn, also showed induce d-fit interaction with 3. The atropisomeric shift to 1(alpha alpha alp ha 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 atropisome rs are identical with the simulated changes calculated from multiple e quilibrium systems by use of kinetic and thermodynamic parameters. Thi s system is a suitable model for induced-fit molecular recognition in flexible enzymes.