γ‐Azaproline Confers pH Responsiveness and Functionalizability on Collagen Triple Helices

Isonitrile-Proline - A Versatile Handle for the Chemoselective Derivatization of Collagen Peptides

Functional groups that allow for chemoselective and bioorthogonal derivatization are valuable tools for the labelling of peptides and proteins. The isonitrile is such a group but synthetic methods for its incorporation into peptides by solid-phase peptide synthesis are not known. Here, we introduce (4S)- and (4R)-isonitrileproline (Inp) as building blocks for solid-phase peptide synthesis. Conformational studies of (4S)- and (4R)-Inp and thermal stability analysis of Inp-containing collagen triple helices revealed that the isonitrile group exerts a stereoelectronic gauche effect. We showcase the value of Inp for bioorthogonal labelling by derivatization of Inp-containing collagen model peptides (CMPs). Dual labelling with a pair of bioorthogonal reactions of a CMP containing Inp and azidoproline residues further highlights the versatility of the new isonitrile-containing amino acids.

Predicting Collagen Triple Helix Stability through Additive Effects of Terminal Residues and Caps

Collagen model peptides (CMPs) consisting of proline-(2S,4R)-hydroxyproline-glycine (POG) repeats have provided a breadth of knowledge of the triple helical structure of collagen, the most abundant protein in mammals. Predictive tools for triple helix stability have, however, lagged behind since the effect of CMPs with different frames ([POG]_n , [OGP]_n , or [GPO]_n ) and capped or uncapped termini have so far been underestimated. Here, we elucidated the impact of the frame, terminal functional group and its charge on the stability of collagen triple helices. Combined experimental and theoretical studies with frame-shifted, capped and uncapped CMPs revealed that electrostatic interactions, strand preorganization, interstrand H-bonding, and steric repulsion at the termini contribute to triple helix stability. We show that these individual contributions are additive and allow for the prediction of the melting temperatures of CMP trimers.

Triple-Helix-Stabilizing Effects in Collagen Model Peptides Containing PPII-Helix-Preorganized Diproline Modules

Collagen model peptides (CMPs) serve as tools for understanding stability and function of the collagen triple helix and have a potential for biomedical applications. In the past, interstrand cross-linking or conformational preconditioning of proline units through stereoelectronic effects have been utilized in the design of stabilized CMPs. To further study the effects determining collagen triple helix stability we investigated a series of CMPs containing synthetic diproline-mimicking modules (ProMs), which were preorganized in a PPII-helix-type conformation by a functionalizable intrastrand C2 bridge. Results of CD-based denaturation studies were correlated with calculated (DFT) conformational preferences of the ProM units, revealing that the relative helix stability is mainly governed by an interplay of main-chain preorganization, ring-flip preference, adaptability, and steric effects. Triple helix integrity was proven by crystal structure analysis and binding to HSP47.