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

Abstract

Proline derivatives bearing substituents at Cγ are valuable tools for biological and materials investigations. However, the stereochemistry at Cγ can produce undesired steric or stereoelectronic interactions. Here, we introduce γ‐azaproline (γ‐azPro), which lacks a stereogenic center at Cγ, as a pH‐responsive and functionalizable proline analogue that can adapt to its environment. Conformational analyses by NMR spectroscopy and DFT calculations revealed that the imidazolidine ring of γ‐azPro is flexible. Incorporation of γ‐azPro into collagen model peptides (CMPs) produced pH‐responsive triples helices and triple helices that can be easily functionalized.

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

“…However, the ethoxy groups in Ac‐ (EtO) 2 ‐ProM1 ‐OMe were found to be radially oriented, pointing away from the triple helix axis. In addition to existing methods for triple‐helix functionalization, the (HO) 2 ‐ProM1 scaffold might be used for double functionalization by O‐alkylation. In conclusion, the steric demand of the linker group was determined to be crucial for the thermal stability of ProM‐modified CMPs.…”

“…As steric repulsion within the triple helix could explain this result, [39] the DFT-optimized structures of Ac-(EtO) 2 -ProM1-OMe and Ac-(EtO) 2 -ProM2-OMe were aligned with (PPG) 10 to describe the steric situation in the corresponding collagen model peptides.T he alignments (Figure 7) showed that the ethoxy groups in Ac-(EtO) 2 -ProM2-OMe indeed clash with apreceding carbonyl group of the same strand and ap roline ring of an eighboring strand. However,t he ethoxy groups in Ac-(EtO) 2 -ProM1-OMe were found to be radially oriented, pointing away from the triple helix axis.Inaddition to existing methods for triple-helix functionalization, [41][42][43] the (HO) 2 -ProM1 scaffold might be used for double functionalization by O-alkylation. In conclusion, the steric demand of the linker group was determined to be crucial for the thermal stability of ProM-modified CMPs.…”

Alkylation of γ‐Azaproline Creates Conformationally Adaptable Proline Derivatives for pH‐Responsive Collagen Triple Helices

“…[13] The K t/c values at basic pH are 1.3 andt husl ower than that of the non-alkylated derivative 1 (K t/c = 3.3). [12] Upon protonation, the trans/cis ratios of 2 and 3 increase( K t/c = 2.2 for Ac-g-azPro(Et)-OMe; K t/c = 1.7 for Ac-g-azPro(Me)-OMe) and are slightly larger than that of g-azPro 1 (K t/c = 1.4).…”

“…Reductivea minationo fA c-g-azPro-OMe 1 provided the methylateda nd ethylated g-azPro derivatives 2 and 3 for conformational analysis andc omparison to 1. [12] NMR spectroscopic analyses of 2 and 3 in D 2 Ou nder basic (pH 11) and acidic conditions (pH 2) gave trans/cis amide bond ratios in the range of 1.3-2.2 that are typical for pseudoprolines ( Figure 2b). [13] The K t/c values at basic pH are 1.3 andt husl ower than that of the non-alkylated derivative 1 (K t/c = 3.3).…”

“…[6, [8][9][10][11] We recently introduced g-azaproline( g-azPro)a saproline analog that embodies the advantages of Amp-such as pHsensitivity and the ability to be functionalized through acylation-but subverts stereochemical tuning due to the absence of as tereogenic centeri nt he g-position( Figure 1b). [12] We hypothesized that alkylation of g-azProb yr eductive amination should produce functionalized g-azPro derivatives that are still pH-responsive ( Figure 1b). Additionally,a lkylation of the N g center should providei nsights into the inversion of the stereochemistry at the N g atom (Figure 1c).…”