Conformation of cyclo(Gly-L-Pro-L-Pro-Gly-L-Pro-L-Pro)(2)Mg complex crystallized from CH(3)CN solution

The cyclic hexapeptides (Gly-L-Pro-L-Pro-Gly-L-Pro-L-Pro) in the (peptide—Mg—peptide)²⁺ complex have nearly identical asymmetric conformations. Each has two cis Pro-Pro linkages and lacks any intraring hydrogen bonds. The Mg²⁺ ion forms six ligands in a regular octahedral array with the carbonyl oxygen atoms of the two Gly residues and one Pro residue of each peptide. The “sandwich” complex has an approximate 2-fold rotation axis through the Mg²⁺ relating the two peptide moieties. Cyclo(Gly-Pro-Pro-Gly-Pro-Pro)₂Mg(ClO₄)₂· 4C²H₃CN crystallizes in space group P3₁ with a = b = 15.744(4) Å, c = 24.002(6) Å, γ = 120°, and Z = 3. A highlight of the structure determination is the ready location of the Mg self-vector in a Harker section and the development of the entire structure by use of the tangent formula starting with the known position of the Mg atom.