Naringin is a naturally occurring flavonoid that is responsible for the characteristic bitter flavour in citrus fruits like grapefruit and pomelo. Structurally it is an O-glycoside of naringenin (Figure 1).
Figure 1. Structures of naringenin (left) and naringin (right)
Although the structure of naringin is quite complex for a small molecule, the NMR spectra obtained (1H,1H COSY, multiplicity-edited 1H,13C HSQC_ME and 1H,13C HMBC) on a Spinsolve system at 90 MHz are well resolved and allow straightforward assignments. As examples, we show the 1H,1H COSY and 1H,13C HSQC-ME spectra (Figures 2 and 3).
Figure 2. Structure of naringin, full range 1H,1H COSY and expansion of the main sugar region of the 1H,1H COSY
The COSY (Figure 2) shows that the two protons in position 32 have different chemical shifts because they are diastereotopic; their close spatial proximity to both the carbonyl group in the ring and the phenol group enhances the shift difference further. A further feature of low field COSY spectra is the presence of cross peaks across four or more bonds, indicated here by the correlation between H30 and H34/35.
Figure 3. 1H,13C HSQC-ME and structure of naringinin
The HSQC in Figure 3 features DEPT-like multiplicity editing, with CH and CH3 groups appearing as positive (red) and CH2s as negative (blue)