1. 4R- and 4S-iodophenyl hydroxyproline, 4R-pentynoyl hydroxyproline, and S-propargyl-4-thiolphenylalanine: conformationally biased and tunable amino acids for bioorthogonal reactions
Christina R Forbes, Anil K Pandey, Himal K Ganguly, Glenn P A Yap, Neal J Zondlo Org Biomol Chem. 2016 Feb 21;14(7):2327-46. doi: 10.1039/c5ob02473k. Epub 2016 Jan 25.
Bioorthogonal reactions allow the introduction of new functionalities into peptides, proteins, and other biological molecules. The most readily accessible amino acids for bioorthogonal reactions have modest conformational preferences or bases for molecular interactions. Herein we describe the synthesis of 4 novel amino acids containing functional groups for bioorthogonal reactions. (2S,4R)- and (2S,4S)-iodophenyl ethers of hydroxyproline are capable of modification via rapid, specific Suzuki and Sonogashira reactions in water. The synthesis of these amino acids, as Boc-, Fmoc- and free amino acids, was achieved through succinct sequences. These amino acids exhibit well-defined conformational preferences, with the 4S-iodophenyl hydroxyproline crystallographically exhibiting β-turn (ϕ, ψ~-80°, 0°) or relatively extended (ϕ, ψ~-80°, +170°) conformations, while the 4R-diastereomer prefers a more compact conformation (ϕ~-60°). The aryloxyproline diastereomers present the aryl groups in a highly divergent manner, suggesting their stereospecific use in molecular design, medicinal chemistry, and catalysis. Thus, the 4R- and 4S-iodophenyl hydroxyprolines can be differentially applied in distinct structural contexts. The pentynoate ester of 4R-hydroxyproline introduces an alkyne functional group within an amino acid that prefers compact conformations. The propargyl thioether of 4-thiolphenylalanine was synthesized via copper-mediated cross-coupling reaction of thioacetic acid with protected 4-iodophenylalanine, followed by thiolysis and alkylation. This amino acid combines an alkyne functional group with an aromatic amino acid and the ability to tune aromatic and side chain properties via sulfur oxidation. These amino acids provide novel loci for peptide functionalization, with greater control of conformation possible than with other amino acids containing these functional groups.
2. 3-Aminopyrrolidine-4-carboxylic acid as versatile handle for internal labeling of pyrrolidinyl PNA
Nisanath Reenabthue, Chalothorn Boonlua, Chotima Vilaivan, Tirayut Vilaivan, Chaturong Suparpprom Bioorg Med Chem Lett. 2011 Nov 1;21(21):6465-9. doi: 10.1016/j.bmcl.2011.08.079. Epub 2011 Aug 23.
Conformationally restricted pyrrolidinyl PNAs with an α/β-dipeptide backbone consisting of a nucleobase-modified proline and a cyclic five-membered amino acid spacer such as (1S,2S)-2-aminocyclopentanecarboxylic acid (ACPC) (acpcPNA) can form very stable hybrids with DNA with high Watson-Crick base pairing specificity. This work aims to explore the effect of incorporating 3-aminopyrrolidine-4-carboxylic acid (APC), which is isosteric to the ACPC spacer, into the acpcPNA. It is expected that the modification should not negatively affect the DNA binding properties, and that the additional nitrogen atom in the APC should provide a handle for internal modification. Orthogonally-protected (N(3)-Fmoc/N(1)-Boc and N(3)-Fmoc/N(1)-Tfa) APC monomers have been successfully synthesized and incorporated into the acpcPNA by Fmoc-solid-phase peptide synthesis. T(m), UV and CD spectroscopy confirmed that the (3R,4S)-APC could substitute the (1S,2S)-ACPC spacer in the acpcPNA with only slightly decreasing the stability of the hybrids formed between the modified acpc/apcPNA and DNA. In contrast, the (3S,4R) enantiomer of APC caused substantial destabilization of the hybrids. Furthermore, a successful on-solid-support internal labeling of the acpc/apcPNA via amide bond formation between pyrene-1-carboxylic acid or 4-(pyrene-1-yl) butyric acid and the pyrrolidine nitrogen atom of the APC spacer has been demonstrated. Fluorescence properties of the pyrene-labeled acpc/apcPNAs are sensitive to their hybridization states and can readily distinguish between complementary and single-mismatched DNA targets.
3. (2S,4R)- and (2S,4S)-perfluoro-tert-butyl 4-hydroxyproline: two conformationally distinct proline amino acids for sensitive application in 19F NMR
Caitlin M Tressler, Neal J Zondlo J Org Chem. 2014 Jun 20;79(12):5880-6. doi: 10.1021/jo5008674. Epub 2014 Jun 6.
(2S,4R)- and (2S,4S)-perfluoro-tert-butyl 4-hydroxyproline were synthesized (as Fmoc-, Boc-, and free amino acids) in 2-5 steps. The key step of each synthesis was a Mitsunobu reaction with perfluoro-tert-butanol, which incorporated a perfluoro-tert-butyl group, with nine chemically equivalent fluorines. Both amino acids were incorporated in model α-helical and polyproline helix peptides. Each amino acid exhibited distinct conformational preferences, with (2S,4R)-perfluoro-tert-butyl 4-hydroxyproline promoting polyproline helix. Peptides containing these amino acids were sensitively detected by (19)F NMR, suggesting their use in probes and medicinal chemistry.
