Copper(II)-Binding Ability of Stereoisomeric cis- and trans-2-Aminocyclohexanecarboxylic Acid-L-Phenylalanine Dipeptides. A Combined CW/Pulsed EPR and DFT Study

With the aim of an improved understanding of the metal-complexation properties of alicyclic beta-amino acid stereoisomers, and their peptides, the complex equilibria and modes of coordination with copper(II) of L-phenylalanine (F) derivatives of cis/trans-2-aminocyclohexanecarboxylic acid (c/tACHC), i.e. the dipeptides F-c/tACHC and c/tACHC-F, were investigated by a combination of CW and pulsed EPR methods. For the interpretation of the experimental data, DFT quantum-chemical calculations were carried out. Simulation of a pH-dependent series of room-temperature CW-EPR spectra revealed the presence of EPR-active complexes ([Cu(aqua)](2+), [CuL](+), [CuLH-1], [CuLH-2](-), and [CuL2H-1](-)), and an EPR-inactive species ([Cu2L2H-3](-)) in aqueous solutions for all studied cases. [CuLH](2+) was included in the equilibrium model for the c/tACHC-F-copper(II) systems, and [CuL2], together with two coordination isomers of [CuL2H-1](-), were also identified in the F-tACHC-copper(II) system. Comparison of the complexation properties of the diastereomeric ligand pair F-(1S,2R)-ACHC and F-(1R,2S)-ACHC did not reveal significant differences. Considerably lower formation constants were obtained for the trans than for the cis isomers for both the F-c/tACHC and the c/tACHC-F pairs in the case of [CuLH-1] involving tridentate coordination by the amino, the deprotonated peptide, and the carboxylate groups. A detailed structural analysis by pulsed EPR methods and DFT calculations indicated that there was no significant destabilization for the complexes of the trans isomers. The lower stability of their complexes was explained by the limitation that only the conformer with donor groups in equatorial equatorial ring positions can bind to copper(II), whereas both equatorial-axial conformers of the cis isomers are capable of binding. From a consideration of the proton couplings obtained with X-band H-1 HYSCORE, H-2 exchange experiments, and DFT, the thermodynamically most stable cyclohexane ring conformer was assigned for all four [CuLH-1] complexes. For the F-cACHC case, the conformer did not match the most stable conformer of the free ligand.