Passage
The N-terminus of an amino acid must be protected with 9-fluorenylmethoxycarbonyl (Fmoc) during solid state peptide synthesis to avoid side reactions (Scheme I) .
Scheme IThe Fmoc protecting group is typically removed with piperidine (Scheme II) , a heterocyclic secondary amine.
Scheme IIBecause the use of piperidine is restricted, structurally similar amines were examined as possible replacements. To determine whether piperidine analogues were suitable alternatives for Fmoc removal, deprotection of Compound 2 was carried out with several amine bases (Scheme III) .
Scheme IIICompound 2 was prepared from L-tryptophan and Fmoc-Cl, and the reaction was monitored by thin-layer chromatography (TLC) and visualized by ultraviolet light. The deprotections were done with piperidine and each analogue using a microwave set to 155 watts at 75 °C for 15 seconds. The products of the deprotections were analyzed by high-performance liquid chromatography (HPLC) and mass spectrometry; the retention times of the products were compared to tryptophan and Compound 2 standards.
Adapted from O.P. Luna et al, "Deprotection Reagents in Fmoc Solid Phase Peptide Synthesis: Moving Away from Piperidine?." Molecules. ©2016 MDPI.
-Which of the following statements provides the most plausible explanation for why UV light was used to visualize the thin-layer chromatography plate?

Question

Passage
The N-terminus of an amino acid must be protected with 9-fluorenylmethoxycarbonyl (Fmoc) during solid state peptide synthesis to avoid side reactions (Scheme I) .

Scheme IThe Fmoc protecting group is typically removed with piperidine (Scheme II) , a heterocyclic secondary amine.

Scheme IIBecause the use of piperidine is restricted, structurally similar amines were examined as possible replacements. To determine whether piperidine analogues were suitable alternatives for Fmoc removal, deprotection of Compound 2 was carried out with several amine bases (Scheme III) .

Scheme IIICompound 2 was prepared from L-tryptophan and Fmoc-Cl, and the reaction was monitored by thin-layer chromatography (TLC) and visualized by ultraviolet light. The deprotections were done with piperidine and each analogue using a microwave set to 155 watts at 75 °C for 15 seconds. The products of the deprotections were analyzed by high-performance liquid chromatography (HPLC) and mass spectrometry; the retention times of the products were compared to tryptophan and Compound 2 standards.
Adapted from O.P. Luna et al, "Deprotection Reagents in Fmoc Solid Phase Peptide Synthesis: Moving Away from Piperidine?." Molecules. ©2016 MDPI.
-Which of the following statements provides the most plausible explanation for why UV light was used to visualize the thin-layer chromatography plate?

Quizplus · Accepted Answer

11ecba2d_12a7_e3d7_a3bf_f5caa57dbee9_MD0008_00 Thin-layer chromatography (TLC) is a technique that separates components of a mixture based on polarity, and can be used to monitor reactions. The stationary phase (TLC plate) is made of a polar material, usually silica (SiO₂). Therefore, polar molecules will have a greater affinity for the TLC plate and travel a smaller distance up the plate than nonpolar molecules. After separation of the reaction mixture, the components are visualized. Molecules that have UV chromophores, which include double and triple bonds, carbonyls and conjugated systems, can be visualized with UV light. The aromatic rings of the indole group on the tryptophan side chain and the Fmoc group are made up of conjugated systems of double bonds, and therefore can absorb UV light. Because tryptophan, Fmoc-Cl, and compound 2 are visible on a TLC plate under UV light and differ in relative polarity, the addition of the Fmoc protecting group to tryptophan can be monitored by TLC. (Choice A) Distillation and gas chromatography, not TLC, separate compounds based on their boiling points. They do not use UV light as a means of detection. (Choice B) Tryptophan and Compound 2 have different molecular weights and will give different molecular ion peaks, but molecular ion peaks result from ionization in mass spectrometry rather than irradiation with UV light. (Choice C) Absorption of infrared light rather than UV light causes stretching vibrations and rotations of the amino acid and Fmoc carbonyl bonds. Educational objective: Thin-layer chromatography is a technique that separates mixture components based on polarity, and can be used to monitor reactions. After separation, the mixture components are visualized, usually by UV light. Molecules with UV chromophores (double and triple bonds, carbonyls, conjugated systems) can be visualized with UV light.

Passage the N-Terminus of an Amino Acid Must Be Protected with with 9-Fluorenylmethoxycarbonyl