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Serine-proline (Ser-Pro) backbone-modified dipeptide analogues are powerful tools to investigate the role of cis-trans isomerization in the regulation of the cell cycle and transcription. These studies have previously been limited to synthetic peptides, whose synthesis is a challenge for larger peptides due to the compounding yield loss incurred in each step. We now introduce a method for the aminoacylation of tRNA with dipeptides and dipeptide analogs to permit the installation of cis- and trans-locked Ser-Pro analogues into full-length proteins. To that end, we synthesized the 3,5-dinitrobenzyl (DNB)-activated esters of a native Ser-Pro dipeptide and its cis- and trans-locked alkene analogs. Murakami et al. created the DNB flexizyme (dFx), a ribozyme that acylates tRNA with DNB esters of amino acids to permit unnatural amino acids to be incorporated into proteins. A tRNA from yeast that recognizes the amber stop codon, along with the dFx flexizyme, were generated by in vitro transcription with T7 RNA polymerase. dFx was used to successfully catalyze the chemical misacylation of truncated amber tRNA with the Ser-Pro-DNB activated dipeptide. This method allows the introduction of non-native Ser-Pro dipeptide mimics into full-length proteins by in vitro transcription-translation. © 2024 Wiley Periodicals LLC. Basic Protocol 1: Synthesis of 3,5-dinitrobenzyl activated esters of Ser-Pro Basic Protocol 2: Preparation of truncated amber tRNA Basic Protocol 3: Acylation of amber-tRNA by the dFx flexizyme Basic Protocol 4: PAGE electrophoresis of tRNA SerPro ., (© 2024 Wiley Periodicals LLC.)

Subunit-selective inhibition of N -methyl-d-aspartate receptors (NMDARs) is a promising therapeutic strategy for several neurological disorders, including epilepsy, Alzheimer's and Parkinson's disease, depression, and acute brain injury. We previously described the dihydroquinoline-pyrazoline (DQP) analogue 2a ( DQP-26 ) as a potent NMDAR negative allosteric modulator with selectivity for GluN2C/D over GluN2A/B. However, moderate (<100-fold) subunit selectivity, inadequate cell-membrane permeability, and poor brain penetration complicated the use of 2a as an in vivo probe. In an effort to improve selectivity and the pharmacokinetic profile of the series, we performed additional structure-activity relationship studies of the succinate side chain and investigated the use of prodrugs to mask the pendant carboxylic acid. These efforts led to discovery of the analogue ( S )-(-)- 2i , also referred to as ( S )-(-)- DQP-997 - 74 , which exhibits >100- and >300-fold selectivity for GluN2C- and GluN2D-containing NMDARs (IC 50 0.069 and 0.035 μM, respectively) compared to GluN2A- and GluN2B-containing receptors (IC 50 5.2 and 16 μM, respectively) and has no effects on AMPA, kainate, or GluN1/GluN3 receptors. Compound ( S )-(-)- 2i is 5-fold more potent than ( S )- 2a . In addition, compound 2i shows a time-dependent enhancement of inhibitory actions at GluN2C- and GluN2D-containing NMDARs in the presence of the agonist glutamate, which could attenuate hypersynchronous activity driven by high-frequency excitatory synaptic transmission. Consistent with this finding, compound 2i significantly reduced the number of epileptic events in a murine model of tuberous sclerosis complex (TSC)-induced epilepsy that is associated with upregulation of the GluN2C subunit. Thus, 2i represents a robust tool for the GluN2C/D target validation. Esterification of the succinate carboxylate improved brain penetration, suggesting a strategy for therapeutic development of this series for NMDAR-associated neurological conditions.

We report the first experimental evidence for a fluoro-alkene amide isostere participating in n→π* donation, which stabilizes the collagen triple helix. Of the three amide positions in canonical collagen-like peptides, Gly-Pro, Pro-Hyp, and Hyp-Gly, triple helix stability stands to benefit from substitution of only the isomerizable 3° Gly-Pro amide bond with a trans -locked fluoro-alkene. A ( Z )-fluoro-alkene isostere of Gly- trans -Pro was synthesized, and its effect on the thermostability of a collagen-like peptide triple helix was measured. The mixture of enantiomers, Boc-Gly-Ψ[( Z )CFC]-L/D-Pro-OH, was synthesized in 8 steps with 27% overall yield, and the Fmoc-Gly-Ψ[( Z )CFC]-L/D-Pro-Hyp-OBn diastereomers were separated. The Gly-Ψ[( Z )CFC]-Pro isostere installed in a collagen-like peptide forms a stable triple helix. By CD, the thermal melting ( T m ) value of the fluoro-alkene peptide was +42.2 ± 0.4 °C, and the T m value of the control peptide was +48.4 ± 0.5 °C, a difference in stability of Δ T m -6.2 °C. Deshielding of the fluorine nucleus in the 19 F NMR spectra is evidence of a stabilizing n→π* electronic interaction.

Collagen is the most abundant human protein, with the canonical sequence (Gly-Pro-Hyp) n in its triple helix region. Cis-trans isomerization of the Xaa-Pro amide has made two of these amide bonds the target of alkene replacement: the Gly-Pro and the Pro-Hyp positions. The conformations of Gly-Pro and Pro-Pro (as a Pro-Hyp model) fluoro-, chloro-, and proteo-alkene mimic models were investigated computationally to determine whether these alkenes can stabilize the polyproline type II (PPII) conformation of collagen. Second-order Møller-Plesset (MP2) calculations with various basis sets were used to perform the conformational analyses and locate stationary points. The calculation results predict that fluoro- and chloro-alkene mimics of Gly-Pro and Pro-Pro can participate in n→π* donation to stabilize PPII conformations, yet they are poor n→π* acceptors, shifting the global minima away from PPII conformations. For the proteo-alkene mimics, the lack of significant n→π* interactions and unstable PPII-like geometries explains their known destabilization of the triple helix in collagen-like peptides.