Structure and conformation of linear peptides

L-tyrosyl-L-tyrosine crystallizes as a dihydrate in the orthorhombic system, space group C222₁, with a = 12.105(2), b = 12.789(2), c = 24.492(3) Å, Z = 8. The structure was solved by direct methods and refined to a final R-value of 0.059 for 1740 observed reflections. The molecule exists as a zwitterion, the peptide unit is trans planar, and the backbone torsion angles correspond to an extended conformation, with e₁ = 149.4°, e₂ = - 161.2°, e₂ = 158.3°. The values of the side-chain torsion angles (χ₁, χ₂) are (- 58.8°, - 63.1°) for the first tyrosine and (- 171.7°, - 116.5°) for the second. The planes of the aromatic rings are nearly parallel (dihedral angle of 6.1°), and their centers are separated by 10.9 Å. The carboxyl plane forms a dihedral angle of 23.8° with the plane of the peptide bond.