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.