Conformation of cyclo(Gly-L-Pro-L-Pro-Gly-L-Pro-L-Pro) 2 Mg 2+ complex crystallized from C 2 H 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 Mg(2+) 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 Mg(2+) relating the two peptide moieties. Cyclo(Gly-Pro-Pro-Gly-Pro-Pro)(2)Mg(ClO(4))(2). 4C(2)H(3)CN crystallizes in space group P3(1) with a = b = 15.744(4) A, c = 24.002(6) A, gamma = 120 degrees , 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.