Kathleen McFadden's Analysis of Boc Methionine (2-(tert-butoxycarbonylamino)-4-methylsulfanyl-butanoic acid)


Figure 1.0 HNMR of2-(tert-butoxycarbonylamino)-4-methylsulfanyl-butanoic acid (Boc-Methionine)


Figure 1.1 H NMR Spectra of CD3OD the Solvent which was used in this experiment


Figure 1.2 While the fact that a carboxylic acid was present in this compound suggests that broad peak would be absorbed around 11-12 ppm, due to the CD3OD solvent which was used in this experiment, an exchange was occurring between the carboxyl group and the solvent which caused a peak for CD3OH to be observed here.


Figure 1.3 Demonstrates a quartet between 4.20 and 4.30. This likely corresponds to the diastereotopic hydrogen located on the chiral carbon. This cause the two attached hydrogens to be split separately as they are not equivalent, leading to a doublet of doublets as observed here.


Figure 1.4 This peak was not expected given the structure of Boc-Methionine. However, this compound was solvated in CD3OD and this peak is typically observed in the NMR spectra of CD3OD.


Figure 1.5 The chiral center in this molecule caused all of the hydrogens to be different and thus split each other (due to the diasterotopic nature of this molecule). The hydrogens on the carbon attached directly to the sulfur was slightly more deshielded than the CH2 group next to it, thus the position of this multiplet at 2.560. The six peaks are due to the three attached hydrogens—leading to a doublet of a doublet of a doublet as observed here.


Figure 1.6 Demonstrates a fairly strong singlet at about 2.085 ppm. This corresponds to the sp3 hybridized carbon (methyl group) located next to the sulfur group. A methyl group located next to sulfur typically experiences a chemical shift of 2.1 which is very close to the value observed in this spectrum.


Figure 1.7 This multiplet is due to the CH2 group attached directly to the chiral carbon. This is close enough to split four different hydrogens leading to the doublet of a doublet of a doublet of a doublet (eight expected peaks). This demonstrates a few more peaks than expected which may be due to additional splitting of the hydrogens with each other.


Figure 1.8 Demonstrates a singlet at 1.650. This singlet is negligible in size and may be due to contamination of the presence of solvent.


Figure 1.9 Demonstrates a very large (180 arbitrary units), well defined peak at about 1.43 ppm. This corresponds to the t-butyl group which was not significantly deshielded leaving it well within the range for sp3 hybridized hydrogens (typically 0.9-1.5 ppm). Each of the methyl groups was equivalent which explains the lack of splitting.


Figure 1.10 Demonstrates a miniscule peak at about 1.224 ppm. This was not visible until magnification occurred and may be residual background noise as all other hydrogen groups were accounted for. It was likely a residual byproduct of the reaction which was performed to produce the Boc-Methionine product being analyzed. This impurity would also account for the fact that the –OH group was observed in the spectra.


Figure 1.11 Demonstrates a strong singlet at 0.00 ppm. This corresponds to the Tetramethylsilane (TMS) which was likely added to the sample to give a “zero” reference point. Since silicon is less electronegative than carbon, TMS protons are highly deshielded causing organic protons to absorb downfield of this signal. This pattern was observed in this HNMR.

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