Structure and conformation of linear peptides

The crystal structure of t-Boc-glycyl-L-phenylalanine (C₁₄H₂₂N₂O₅, molecular weight = 298) has been determined. Crystals are monoclinic, space group P2₁, with a = 7.599(1)Å, b = 9.576(2), c = 12.841(2), β = 97.21(1)°, Z = 2, D_m = 1.149, D° = 1.168 g · cm^-3. Trial structure was obtained by direct methods and refined to a final R-index of 0.064 for 1465 reflections with I> 1s?. The peptide unit is trans planar and is nearly perpendicular to the plane containing the urethane moiety. The plane of the carboxyl group makes a dihedral angle of 16.0° with the peptide unit. The backbone torison angles are ω₀ = - 176.9°, ø₁ = - 88.0°, ψ₁ = - 14.5°, ω1 = 176.4°, ø₂ = - 164.7° and ψ2 = 170.3°. The phenylalanine side chain conformation is represented by the torsion angles χ₁ = 52.0°, χ₂ = 85.8°.