Passage
Fluorescent amino acids are useful for labeling proteins because they can be visualized spectroscopically, facilitating the study of protein structure and interactions. The noncanonical amino acid dansylalanine, a fluorescent amino acid that is sensitive to the hydrophobicity of its environment, is a good candidate to be incorporated into proteins because of its small size and fluorescent capabilities. Dansyl chloride and Compound 1 were used to synthesize dansylalanine, as shown in Reaction 1. Dansylalanine absorbs maximally at 340 nm and emits at approximately 540 nm.
Reaction 1 Synthesis of dansylalanineA leucyl-tRNA synthetase from Escherichia coli was engineered to aminoacylate bacterial tRNAs containing a CUA anticodon with dansylalanine. After verifying that the tRNA/synthetase pair does not interact with endogenous amino acids or tRNAs, it was transfected into the yeast Saccharomyces cerevisiae along with a plasmid encoding a mutated human superoxide dismutase (hSOD) . Using this system, dansylalanine was genetically incorporated into hSOD in place of either Gln-16 or Trp-33, and expressed in S. cerevisiae.Position 16 is located on the surface of hSOD at the N-terminus of a β-strand, and position 33 lies in the center of a strand within a β-barrel. To probe changes in the structure of hSOD, researchers exposed the purified protein to guanidinium chloride (GdmCl) to denature the protein at concentrations ranging 0-4.5 M. The experiment was conducted in a 35 mM sodium phosphate buffer at pH 7.2. The fluorescence of hSOD with dansylalanine at either position 16 or 33 was monitored at each GdmCl concentration, and the results of the unfolding experiment are shown in Figure 1.
Figure 1 Plot of dansylalanine fluorescence at different concentrations of GdmCl
Adapted from Summerer D, Chen S, Wu N, Deiters A, Chin JW, Schultz PG. A genetically encoded fluorescent amino acid. Proc Natl Acad Sci USA. 2006.
-In the synthesis of dansylalanine, the Boc group on Compound 1 most likely functions to:

Question

Passage
Fluorescent amino acids are useful for labeling proteins because they can be visualized spectroscopically, facilitating the study of protein structure and interactions. The noncanonical amino acid dansylalanine, a fluorescent amino acid that is sensitive to the hydrophobicity of its environment, is a good candidate to be incorporated into proteins because of its small size and fluorescent capabilities. Dansyl chloride and Compound 1 were used to synthesize dansylalanine, as shown in Reaction 1. Dansylalanine absorbs maximally at 340 nm and emits at approximately 540 nm.

Reaction 1 Synthesis of dansylalanineA leucyl-tRNA synthetase from Escherichia coli was engineered to aminoacylate bacterial tRNAs containing a CUA anticodon with dansylalanine. After verifying that the tRNA/synthetase pair does not interact with endogenous amino acids or tRNAs, it was transfected into the yeast Saccharomyces cerevisiae along with a plasmid encoding a mutated human superoxide dismutase (hSOD) . Using this system, dansylalanine was genetically incorporated into hSOD in place of either Gln-16 or Trp-33, and expressed in S. cerevisiae.Position 16 is located on the surface of hSOD at the N-terminus of a β-strand, and position 33 lies in the center of a strand within a β-barrel. To probe changes in the structure of hSOD, researchers exposed the purified protein to guanidinium chloride (GdmCl) to denature the protein at concentrations ranging 0-4.5 M. The experiment was conducted in a 35 mM sodium phosphate buffer at pH 7.2. The fluorescence of hSOD with dansylalanine at either position 16 or 33 was monitored at each GdmCl concentration, and the results of the unfolding experiment are shown in Figure 1.

Figure 1 Plot of dansylalanine fluorescence at different concentrations of GdmCl
Adapted from Summerer D, Chen S, Wu N, Deiters A, Chin JW, Schultz PG. A genetically encoded fluorescent amino acid. Proc Natl Acad Sci USA. 2006.
-In the synthesis of dansylalanine, the Boc group on Compound 1 most likely functions to:

Quizplus · Accepted Answer

11ecba2d_1275_8971_a3bf_5dbdd348a4db_MD0008_00 Organic synthesis requires reactions at specific functional groups to make the target compound. However, some molecules have multiple functional groups that could react under the given conditions. Protecting groups are added to organic molecules to modify the functional group by masking it. Protecting groups prevent unwanted reactions under given reaction conditions. They tend to be stable to oxidation and reduction conditions and prevent nucleophilic reactions from occurring. There are particular protecting groups for each type of functional group. After a reaction is complete, the protecting group can then be removed to reveal the original functional group. The addition of dansyl chloride to Compound 1 requires an amine nucleophile to attack the electrophilic sulfur atom in dansyl chloride. Because there are two amines in Compound 1, the one bonded to the α-carbon needs to be protected so it will not react. The Boc group provides steric hindrance to prevent the amino group from acting as a nucleophile. (Choice B) Although the Boc protecting group is used to protect a functional group from reacting, the C-terminus (also called the carboxyl group) does not have a Boc protecting group attached to it. (Choices C and D) The N-terminus and the carboxyl group of the amino acid are present in both Compound 1 and dansylalanine. Therefore, neither can participate in the reaction as a leaving group; instead, the chloride in dansyl chloride is the leaving group in Reaction 1. Educational objective: Protecting groups are organic molecules added to functional groups to mask them and keep them from reacting under a given set of reaction conditions. There are different protecting groups for different functional groups.

Passage Fluorescent Amino Acids Are Useful for Labeling Proteins Because They