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 most accurately describes the process that occurs when visualizing the formation of the Compound 2 reaction mixture on a 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 most accurately describes the process that occurs when visualizing the formation of the Compound 2 reaction mixture on a thin-layer chromatography plate?

Quizplus · Accepted Answer

11ecba2d_12a8_5908_a3bf_21f72df6908c_MD0008_00 Thin-layer chromatography (TLC) is a technique that separates mixture components based on the polarity of the molecules in the mixture. The mixture components can be visualized under UV light if the molecules contain UV chromophores, including conjugated systems, double or triple bonds, and carbonyls. The chromophore absorbs UV light, which excites electrons to a higher energy state (excited state). The formation of Compound 2 was monitored by TLC, and the conjugated double bonds in tryptophan, Fmoc-Cl, and Compound 2 each absorb UV light on the TLC plate. The chromophore electrons jump to a higher energy excited state as the light is absorbed, and the compounds are visible as black spots on the TLC plate. (Choices A and C) Although bound electrons do absorb UV light, the electrons are excited from a lower energy state to a higher energy state rather than falling back to the ground state from a higher energy level or staying at the same energy level. (Choice D) Bound electrons do move to an excited state when absorbing UV light. If an electron is moving to an excited state, it goes to a higher energy level rather than a lower energy level. Educational objective: When UV chromophores (conjugated systems, double and triple bonds, carbonyls) absorb UV light, energy from the UV light excites electrons to a higher energy excited state.

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