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Friday, June 20, 2008

APT vs DEPT-135

Both the APT (Attached Proton Test) and DEPT (Distortionless Enhancement by Polarization Transfer) sequences are very commonly used to help assign 13C NMR spectra. Both experiments yield 13C NMR spectra where the number of attached protons (the multiplicity) is encoded in the phase of the 13C NMR signals. APT spectra have quaternary carbons, and methylene carbons phased negative and methine and methyl carbons phased positive. DEPT-135 spectra show no quaternary carbons and have methylene carbons phased negative and methine and methyl carbons phased positive. A modification to the DEPT-135 method (the DEPTQ-135) will also show quaternary carbons phased negative. Although the APT and DEPT methods provide similar information, the mechanism for multiplicity selection is different for each method. The multiplicity selection in APT experiments is based on 13C magnetization dephasing during a delay equal to the reciprocal of the average one-bond 13C - 1H coupling constant. Although DEPT experiments also employ a delay related to the average one-bond 13C - 1H coupling constant, the multiplicity selection is accomplished by adjusting the final 1H pulse (a DEPT-135 uses a 135 degree 1H pulse). Another major difference between the two techniques is that the DEPT technique transfers proton magnetization to carbon giving it a sensitivity advantage over the APT method. This is illustrated in the figure below which compares the 13C APT and DEPT-135 spectra of menthol. The signal-to-noise ratio is higher in the DEPT 135 spectrum.

5 comments:

John Lee said...

For the sensitivity of quaternary carbon, how do you compare APT with a standard 13C experiments and a DEPTQ?

Glenn Facey said...

John,
I do not know the answer off-hand. Check this paper by William Reynolds et. al. Magn. Res. Chem. vol 52, pp 195-201.

Glenn

Thomas Pettus said...

I think its better for DEPT due to NOE enhancement of quats

Ricardo M Borges said...

Can I suggest a topic in this line?
The real need for 13C acquisition; it’s always a theme of discussion. Should I collect edited HSQC + HMBC in high resolution or do I always need the 13C spectra?

Glenn Facey said...

Ricardo,
Thank you for your comment. Yes, I have had this discussion often. My position is that the edited HSQC and HMBC are all that is required. The 13C, 13C DEPT, or 13C APT are very time consuming techniques. The response I get from students is that the journal in which they seek to publish requires a 13C spectrum.
Glenn