15: TOCSY
- Page ID
- 332815
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)TOCSY Spectroscopy*
The TOCSY (TOtal Correlation SpectroscopY, aka HoHaHa) experiment is similar to the COSY experiment. However, cross peaks are observed not only for 3J-coupled nuclei, but also between nuclei that are connected by a long-range coupling. This makes it useful for identifying the larger interconnected networks of spin couplings.
In the COSY experiment, most protons that are more than three chemical bonds apart give no cross signal because the 4J coupling constants are close to 0.
- List some examples of when you would expect to see 4J correlations in COSY.
The TOCSY experiment correlates ALL protons of a spin system. No matter how far.
Therefore, not only the 3J signals are visible (which also appear in a COSY spectrum) but also additional signals that originate from the interaction of all protons of a spin system.
* All spectra are either from SDBS (Japan National Institute of Advanced Industrial Science and Technology) or simulated.
Comparison of COSY and TOCSY Spectra
- On the structure drawn below, include arrows showing COSY correlations (3JHH):
- On the structure drawn below, include arrows showing all TOCSY correlations:
- Compare your predictions with the spectra below. Do they match?
- Which atoms are NOT correlated in either COSY or TOCSY?
TOCSY to Distinguish Spin Systems in Macromolecule
TOCSY is often used to identify discrete building blocks within a molecule such as an amino acid in a longer peptide.
The COSY of an amino acid in a peptide will show correlations between the amide NH and the side chain
- Add additional correlations for the TOCSY of this amino acid (alanine).
These TOCSY patterns and shifts for amino acids are well documented.
TOCSY of Amino Acids (META: missing link)
TOCSY to Distinguish a Spin System
In the macromolecular peptide structure, each amino acid is a unique spin system that does not have 3JHH coupling to other amino acids in the structure.
- Circle three unique spin systems in this structure:
The amide proton has a single correlation to the alpha carbon in each COSY spectrum, but it has a TOCSY correlation to each atom in the amino acid.
For this reason, many people look at the region containing the amide protons (F2) vs the region for aliphatic protons (F1) in a TOCSY.
- In the following TOCSY of the tripeptide, draw a line through each spin system.
- Using the chemical shift information (TOCSY of Amino Acids ), identify which line corresponds to which amino acid.
TOCSY to Analyze a Mixture
TOCSY help distinguish spin systems from different molecules.
Consider the following example of a mixture of 2-pentenal and 5-epoxyhexene. The proton and COSY spectra of this mixture show considerable peak overlap in some regions, making peak assignments difficult.*
*Note: the actual spectra are even more complex than the simulated spectra.
- Use the TOCSY spectrum to determine which peaks belong to 2-pentenal and which belong to 5-epoxyhexene.
