Stereoelectronic Tuning of the Structure and Stability of the Trp Cage Miniprotein

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Abstract
Proline residues are critical structural elements in proteins, defining turns, loops, secondary structure boundaries, and polyproline helices. Control of proline conformation therefore may be used to define protein structure and stability. 4-Substituted proline derivatives may be used to control proline ring pucker, which correlates with protein main chain conformation. To examine the use of proline conformational restriction to tune globular protein stability, a series of peptides derived from the trp cage miniprotein was synthesized. Proline at residue 12 of the trp cage miniprotein, which adopts a Cγ-exo ring pucker in the NMR structure, was replaced with 4-substituted proline derivatives, including 4R derivatives favoring a Cγ-exo ring pucker and 4S derivatives favoring a Cγ-endo ring pucker. Eight trp cage peptides were synthesized, five of which included residues that are not commercially available, without requiring any solution phase chemistry. Analysis of the trp cage peptides by circular dichroism and NMR indicated that the structure and stability of the trp cage miniprotein was controllable based on the conformational bias of the proline derivative. Replacement of Pro12 with 4S-substituted proline derivatives that favor the Cγ-endo ring pucker destabilized the trp cage, while replacement of Pro12 with 4R-substituted proline derivatives that favor a Cγ-exo ring pucker resulted in increased α-helicity and thermal stability of the trp cage. The most stable trp cage derivatives contained benzoates of 4R-hydroxyproline, which also exhibited the most pronounced stereoelectronic effects in TYProxN model peptides. Overall, the stability of the trp cage was tunable by over 50 °C depending on the identity of the proline side chain at residue 12.