*3.6. FTIR Spectra*

Figure 1 shows the FTIR spectra of whole pig brain and its lipid. The whole pig brain and its lipids have "fingerprint" spectra in the wavenumber range of 2700 to 3500 cm<sup>−</sup>1, which are originated from stretch vibrations of CH, NH, and OH groups and related to their chemical compositions [21]. In the range of 400 to 1800 cm<sup>−</sup>1, the spectra of whole pig brain and lipid fraction differed, indicating the presence of distinct chemical bonds and functional groups in the samples. In the whole pig brain, the various bands revealed the presence of water, protein, lipid, and carbohydrate. Some peaks may, however, be overlapping due to the combination of various compositions. According to Guillén and Cabo [68] and Lerma-García et al. [69], the presence of the vibration OH stretching of water allows the first band (3600–3200 cm<sup>−</sup>1) to function. The peaks at 2850–2950 cm<sup>−</sup><sup>1</sup> correspond to the CH stretching bonds of methyl and methylene. The peaks at 1738 cm<sup>−</sup><sup>1</sup> are due to the presence of triglyceride functional groups (C=O stretching). The amide I peak at 1655 cm<sup>−</sup><sup>1</sup> (C=O stretching/hydrogen bonding paired with CN stretch and CCN deformation), the amide II peak at 1547 cm<sup>−</sup><sup>1</sup> (NH ending mixed with CN stretching), and the amide III peak at 1452 cm<sup>−</sup><sup>1</sup> (CN stretching and NH deformation) are all characteristic of proteins [70,71]. The amide A and amide B peaks were also identified at 3282 and 2922 cm<sup>−</sup>1, respectively [63]. Finally, carbohydrates are responsible for the peaks in the 900–1200 cm<sup>−</sup><sup>1</sup> range [72], with the most conspicuous saccharide band about 1370 cm<sup>−</sup><sup>1</sup> [21].

In the spectra of lipids, due to the purity of the lipid and their lipid composition (e.g., triglyceride, phospholipid, cholesterol, sphingomyelin, and cerebrosides), the peaks in the range 400 to 1800 cm<sup>−</sup><sup>1</sup> were different from those of the original whole pig brain. Attributed to the existence of ester groups, Krafft et al. [21] found that the neutral lipid triacylglyceride showed a peak at 1729 cm<sup>−</sup>1. The existence of phospholipids could explain the presence of a high peak at 835 cm<sup>−</sup>1. It has been reported that phosphatidic acid, the parent component of phospholipids, has an 860 cm<sup>−</sup><sup>1</sup> band [21]. Several phospholipid varieties, including phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylinositol (PI), were discovered in the brain, each with a different spectrum, due to differences in functional head groups or fatty acid residues [21]. According to Krafft et al. [21], the cholesterol spectrum has multiple sharp bands ranging from 400 to 1200 cm<sup>−</sup>1, with the most intense ones at 429, 548, 608, and 702 cm<sup>−</sup>1. Furthermore, the peak at 1440 cm<sup>−</sup><sup>1</sup> is attributed to cholesterol CH distortion. In the spectra of sphingomyelin, choline bands are positioned at 718 and 875 cm<sup>−</sup>1, the same as in PC, because sphingomyelin has a PC residue linked to the ceramide backbone [21].

**Figure 1.** Fourier transform infrared (FTIR) spectra of the freeze-dried pig brain and pig brain lipid.
