cis–trans Isomerization of β-casomorphin peptides bound to copper(II): integration of EPR and NMR studies

Abstract

Copper complexes of β-casomorphin peptides (BCM-7, BCM-5, BCM-4) were investigated by EPR and NMR in DMSO-d₆ solutions. Speciation of copper among many of the possible isomers was apparent. Computer simulations of low and room temperature EPR allowed the number of co-ordinated nitrogens in the major species (2 for BCM-4 and BCM-5, 4 for BCM-7) to be inferred and a rotational correlation time of 0.18 ns at 298 K to be evaluated for all complexes. All isomers of BCM-4 and BCM-5 were shown to bind copper, but the resulting structures were strictly determined by the conformational state of ²Pro. The trans, rather than the cis, conformation was shown to allow binding of the deprotonated ³Phe-NH; the terminal amino and carboxylate groups provided the other binding groups in all cases. Structures were obtained by constrained molecular dynamics using copper–proton distances obtained from paramagnetic nuclear relaxation rates. In the case of BCM-7, only the cis-cis-trans and/or the cis-cis-cis isomers were not binding copper. The conformational state of each Pro was shown to drive formation of the copper–nitrogen bond within the immediately adjacent residue, leading to the complex having four co-ordinated nitrogens in the case of the trans-trans-trans isomer.