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 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.
Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, D. C. 20375.
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