Strong positive allosterism which appears in molecular recognition with cerium(IV) double decker porphyrins: Correlation between the number of binding sites and Hill coefficients

Cerium(IV) double decker porphyrins bearing one-to-four pairs of 4-pyridyl groups (3a, 3a', 3bp, 3bd, 3c, and 3d) were synthesized from tetraarylporph yrins bearing mono-, bis-, tris-, and tetrakis(4-pyri dyl) groups. In 3b be aring two pairs of 4-pyridyl groups, there exist two isomers in which the 4 -pyridyl groups are either proximal or distal (3bp and 3bd, respectively). In a mixed solvent of dichloromethane: ethyl acetate (30:1 v/v), 3a' bearin g one pair of 4-pyridyl groups and three pairs of phenyl groups did not int eract with any dicarboxylic acids whereas 3d bearing four pairs of 4-pyridy l groups interacted only with dicarboxylic acid guests with a dimethylene s pacer [e.g., BOC-L-aspartic acid (L-4) and (1R,2R)-cyclohex ane-1,2-dicarbo xylic acid ((1R,2R)-5)]. Interestingly, the complexation process monitored by CD spectroscopy showed a positive homotropic allosterism which satisfied the Hill equation giving constants K = 2.63x1011M-4 and n = 3.9 for L-4 an d K = 2.75x109 M-4 and n = 4.0 for (1R,2R)-5. The continuous variation plot s (Job plots) also supported the formation of the 1:4 3d/dicarboxylic acid guest complexes. The results consistently indicate that four pairs of 4-pyr idyl groups in 3d allosterically bind guests. In 3d, the two porphyrin ring s can still rotate, but once the rotation is suppressed by the first guest binding, the subsequent binding of the second, third and fourth guests can occur cooperatively. This is the origin of the present positive homotoropic allosterism. A similar positive homotropic allosterism was also observed f or 3bp and 3bd with n = 1.5 and 1.7, respectively and 3c with n = 3.0. The X-ray crystallographic study of the 3d .[(1R,2R)-5]4 complex showed that th e two porphyrin planes are warped outward to relax the electrostatic repuls ion and chirally twisted. The two carboxylic acid groups form intermolecula r hydrogen bonds (but not intramolecular bridge-type hydrogen bonds) with t he pyridyl groups because of the close packing effect of rigid host 3d and rigid guest (1R,2R)-5. In conclusion, this is a rare example of positive ho motropic allosterism in an artificial system which is frequently seen in na ture where the biological events must be efficiently regulated in response to signals.