In the literature there exist two conflicting pieces of data concerning the thermodynamics of the Cu(2+)-methionine enkephalin complex. One study utilized 1H nuclear magnetic resonance (NMR) to study the solution phase Cu(2+)-enkephalin binding and reported a strong interaction over a wide pH range. Another study utilized potentiometry to measure a log beta 101 of 4.6, a value representative of a weak metal-ligand complex. In order to resolve this discrepancy a 300 MHz 1H NMR is used to study the protonation sequence of methionine enkaphalin, a bioactive peptide, and its complexation with seven diamagnetic metals (Ba2+, Ca2+, Zn2+, La2+, Mg2+, Sr2+ and Cd2+).