MOLECULAR RECOGNITION .9. SIMULTANEOUS HYDROGEN-BONDING AND METAL COORDINATION INTERACTIONS IN THE 2-POINT FIXATION OF AMINO-ACIDS WITH A BIFUNCTIONAL METALLOPORPHYRIN RECEPTOR

The organometallic acetone-Rh(III) derivative of 5,15-irans-bis(2-hydroxy-1-naphthyl)octaethylporphyrin (1b) reversibly forms two-point adducts in CHC13 with oaminobenzoic acid (2b, a rigid β-amino acid) and its methyl ester (2a) via simultaneous Rh(III)-NH2- coordination and OH-CO2R hydrogen bonding (R = H or CH3). The weaker interaction, hydrogen bonding (HB), was characterized from spectroscopic as well as thermodynamic viewpoints: for adduct lb-2a, KHB = 9.5 and ΔG°HB = -1.3 kcal/mol at 288 K (ΔH°HB = -2.8 kcal/mol and ΔS°HB = -5.3 cal/mol-K); for adduct 1b-2b, KHB = 107 and ΔG°HB = -2.7 kcal/mol at 288 K (ΔH°HB = -3.6 kcal/mol and ΔS°HB = -3.0 cal/mol-K), where KHB is the selectivity factor due to the hydrogen bonding, α-Amino esters also form similar two-point adducts; for the phenylalanine methyl ester adduct, KHB = 42 and ΔG°HB = -2.1 kcal/mol at 288 K. The importance of hydrogen bonding, however, sharply decreases in going from flexible β- through γ- to ϵ-amino ester adducts. The intermolecular hydrogen bonding of 1b and methyl acetate (KHB = 0.38 M-1 and ΔG°HB = 0.56 kcal/mol at 288 K) (ΔH°HB = -2.4 kcal/mol and ΔS°HB = -10.2 cal/mol-K) is a weak interaction. On the other hand, the hydrogen bonding in adduct lb-2a is essentially intramolecular in nature and takes place much more readily owing to a less unfavorable entropy change; it thus makes a significant contribution to the stability and selectivity of the resulting adduct. Such α-amino acids as phenylalanine, leucine, isoleucine, and 2-aminohexanoic acid (norleucine) are readily extracted from neutral aqueous solutions into CHC13 upon formation of similar two-point lb-(amino acid) adducts, in marked contrast to alanine, serine, and 6-aminohexanoic acid, which are not extractable; sufficient lipophilicities of amino acids as well as intramolecular hydrogen bonding in the adducts play crucial roles. Selective transport of lipophilic α-amino acids through a CHC13 liquid membrane was also achieved with 1b as a carrier. © 1993, IEEE. All rights reserved. © 1990, American Chemical Society. All rights reserved.