Copper complexes of β-casomorphin (BCM7 BCM5, BCM4) and other sequence related peptides (Tyr-Pro-Leu-Gly-NH2, Tyr-Pro-Gly-Gly) 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 to be inferred and a rotational correlation time of 0.18 ns at 298 K to be evaluated for all complexes. All isomers (but two of BCM7) were shown to bind copper, and 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 3X-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 BCM7, 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. Interpretation of paramagnetic relaxation rates of 2Pro-Hα signals provided the corresponding isomeric probabilities in the metal-bound state. The observed change in the values of existential probabilities upon copper addition may be relevant for the biological role of copper as well as of other metal ions.
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