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

Copper complexes of beta -casomorphin peptides (BCM-7, BCM-5, BCM-4) were i nvestigated by EPR and NMR in DMSO-d(6) solutions. Speciation of copper amo ng many of the possible isomers was apparent. Computer simulations of low a nd 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 ro tational correlation time of 0.18 ns at 298 K to be evaluated for all compl exes. All isomers of BCM-4 and BCM-5 were shown to bind copper, but the res ulting structures were strictly determined by the conformational state of ( 2)Pro. The trans, rather than the cis, conformation was shown to allow bind ing of the deprotonated (3)Phe-NH; the terminal amino and carboxylate group s provided the other binding groups in all cases. Structures were obtained by constrained molecular dynamics using copper-proton distances obtained fr om paramagnetic nuclear relaxation rates. In the case of BCM-7, only the ci s-cis-trans and/or the cis-cis-cis isomers were not binding copper. The con formational state of each Pro was shown to drive formation of the copper-ni trogen bond within the immediately adjacent residue, leading to the complex having four co-ordinated nitrogens in the case of the trans-trans-trans is omer.