Chapter 2
Synthesis of In-Tether Chiral Center
Peptides and Their Biophysical
Properties Study
2.1 Introduction
The majority of protein-protein interactions (PPIs) involve α-helices and are usually
untreatable with small molecules due to their large interaction areas and shallow
surfaces [1–3]. Therefore, the development of constrained peptides that can modulate
PPIs by enhancing the helicity of short peptides is important [4–20]. Many strategies
have been developed and successfully applied [21–25]. Early approaches to stabilize
an α-helix involved both polar and physiological labile linkages, such as disulfide
bonds [7] and lactam bridges [5, 26]. A significant advance in this field was made
by applying ring-closing metathesis to construct constrained peptides [27]. Verdine,
Walensky, and others developed a hydrocarbon staple that significantly increased the
peptide’s α-helical content, proteolytic resistance, and enhanced biological activities
[10, 28–30]. This strategy has been used to perturb protein-protein interactions such
as intracellular Bcl-2 [28], MDM2 [31, 32], and extracellular EGFR [33]. Arora
et al. developed a hydrogen bond surrogate system that was also broadly utilized
[34–37]. Due to the therapeutic potential of constrained peptides, alternative peptide
‘stapling’ methods were developed, including hydrozone by Satterthwait [8], azobenzene by Woolley [9], thioether by Spatola [12], perfluoroaryl formation by Pentelute
[18], oxime formation by Brown [38], and other cysteine-based S N Ar by Qin and
others [39, 40]. These strategies are summarized in Scheme 2.1. These special linkers
increase peptide backbone rigidity that leads to a more rigid helical conformation.
The solved crystal structures of estrogen receptor-α (ER-α) and E3 ubiquitin ligase
MDM2 with their peptide ligands show that the hydrocarbon tethers contribute to
target binding. This is due to that the tether interacts with the hydrophobic region
surrounding the binding pocket [31, 41]. Incorporating a modification site on the
tether could lead to versatile applications. This concept was first demonstrated by
Dawson et al. [42]. They built a carboxyl group into the linker for further oxime
ligation to be used for fluorescent molecule labeling. Smith et al. utilized the inverse
electron demand Diels–Alder reaction of the S, S-tetrazine for peptide stapling and
© Springer Nature Singapore Pte Ltd. 2021
K. Hu, Development of In-Tether Carbon Chiral Center-Induced
Helical Peptide, Springer Theses,
https://doi.org/10.1007/978-981-33-6613-8_2
25
Synthesis of In-Tether Chiral Center
Peptides and Their Biophysical
Properties Study
2.1 Introduction
The majority of protein-protein interactions (PPIs) involve α-helices and are usually
untreatable with small molecules due to their large interaction areas and shallow
surfaces [1–3]. Therefore, the development of constrained peptides that can modulate
PPIs by enhancing the helicity of short peptides is important [4–20]. Many strategies
have been developed and successfully applied [21–25]. Early approaches to stabilize
an α-helix involved both polar and physiological labile linkages, such as disulfide
bonds [7] and lactam bridges [5, 26]. A significant advance in this field was made
by applying ring-closing metathesis to construct constrained peptides [27]. Verdine,
Walensky, and others developed a hydrocarbon staple that significantly increased the
peptide’s α-helical content, proteolytic resistance, and enhanced biological activities
[10, 28–30]. This strategy has been used to perturb protein-protein interactions such
as intracellular Bcl-2 [28], MDM2 [31, 32], and extracellular EGFR [33]. Arora
et al. developed a hydrogen bond surrogate system that was also broadly utilized
[34–37]. Due to the therapeutic potential of constrained peptides, alternative peptide
‘stapling’ methods were developed, including hydrozone by Satterthwait [8], azobenzene by Woolley [9], thioether by Spatola [12], perfluoroaryl formation by Pentelute
[18], oxime formation by Brown [38], and other cysteine-based S N Ar by Qin and
others [39, 40]. These strategies are summarized in Scheme 2.1. These special linkers
increase peptide backbone rigidity that leads to a more rigid helical conformation.
The solved crystal structures of estrogen receptor-α (ER-α) and E3 ubiquitin ligase
MDM2 with their peptide ligands show that the hydrocarbon tethers contribute to
target binding. This is due to that the tether interacts with the hydrophobic region
surrounding the binding pocket [31, 41]. Incorporating a modification site on the
tether could lead to versatile applications. This concept was first demonstrated by
Dawson et al. [42]. They built a carboxyl group into the linker for further oxime
ligation to be used for fluorescent molecule labeling. Smith et al. utilized the inverse
electron demand Diels–Alder reaction of the S, S-tetrazine for peptide stapling and
© Springer Nature Singapore Pte Ltd. 2021
K. Hu, Development of In-Tether Carbon Chiral Center-Induced
Helical Peptide, Springer Theses,
https://doi.org/10.1007/978-981-33-6613-8_2
25