Conformation of cyclo(Gly-L-Pro-L-Pro-Gly-L-Pro-L-Pro)2Mg2+ complex crystallized from C2H3CN solution
Abstract
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.
