HMQC VGSE

Let us start our analysis with the carbon resonance at 43 ppm in the HMQC spectrum, which is a —CH2— 

FIGURE 5.31 The DQF COSY spectrum of VGSE. Correlation lines are drawn in and some assignments are given as an aid. 

The serine residue can be accounted for by starting with the carbon resonance at 62 ppm. DEPT indicates another methylene group, and the HMQC shows that it correlates with coincident protons resonating at 3.85 ppm. These protons overlap with another proton. Careful line drawing in the DQF-COSY (or more easily in the TOCSY spectrum) suggests correlation with a proton at 4.48 ppm. This proton shows a correlation to a carbon atom at 56 ppm in the FIMQC spectrum. That proton also shows a correlation to an amide proton at 

40 ppm in the COSY. Like glycine, the serine residue cannot be the N-terminus. 

---

Compared to caryophyllene oxide and lactose, the HMQC spectrum for the tetra-peptide appears relatively simple (Figure 5.33). Indeed, VGSE has only 10 carbon atoms with attached protons and the spectrum shows correlations to nine carbons. Actually, there are 10 correlations as can be seen in the inset of the shielded methyl portion of the spectrum. Let us summarize the complementary information up to and including the HMQC spectrum. 

For VGSE, the HMQC is the perfect accompaniment to the DQF-COSY and TOCSY for assigning all of the protons and nearly all of the carbons (except the carbonyls). This collection of spectra, however, does not allow us to assign or confirm the order of amino acids in the peptide, an immensely important task. 

We end our discussion of VGSE by comparing the ROESY correlations of the amide protons with the corresponding interactions in COSY and TOCSY. Figure 5.35 shows comparable sections of each spectrum. We have the seen the COSY and TOCSY portions earlier and showed how these spectra, along with the HMQC, can be used for intra-residue assignments. The ROESY correlations, on the other hand,... 

---