Conformational transitions in a Pro-Pro peptide on dissolution of single crystals

The peptide t-butyloxycarbonyl-α-aminoisobutyryl-L-prolyl-L-prolyl-N-methylamide has been shown to adopt an extended structure in the solid state. The Pro-Pro segment occurs in the poly-proline II conformation. On dissolution of single crystals at ～ 233°K, a single species corresponding to the all $\text{trans}$ peptide backbone is observed by 270 MHz ¹H NMR. On warming, $\text{trans}$ to $\text{cis}$ isomerization about the Pro-Pro bond is facilitated. Both $\text{cis'}$ (ψ ～?50°) and $\text{trans'}$ (ψ ～ 130°) rotamers about the Pro³ C^αCO bond are detectable in the Pro-Pro $\text{cis}$ conformer, at low temperature. These observations demonstrate unambiguously the large differences in the solid state and solution conformations of a Pro-Pro sequence.