## 2 Experimental Section
### FTIR Microspectroscopy  
FTIR measurements were carried out at the IR end station, Synchrotron Light Research Institute (Public Organization), Nakhon Ratchasima, Thailand. Sample preparation was performed as previously described [19]. Briefly, mycoplasma-contaminated and -decontaminated HepG2 cells (at \(\sim\)80% confluence) were harvested by trypsin, centrifuged (540\(\times\)_g_, 5 min), washed twice by 0.85% sodium chloride (_uv/v_), and re-suspended in 10 \(\upmu\)L of 0.85% sodium chloride. Five microliters of resuspended cells were spotted onto a Low-e microscope slide (MirrIR, Kevley Technologies, Chesterland, OH, USA) and then vacuum-dried for 10 min in the desiccator. The spotted cells were quickly rinsed with distilled water to discard the remaining salts, and then vacuum-dried. The washed and dried cells were stored in a desiccator before analysis by FTIR microspectroscopy.  
The Bruker Hyperion 2000 microscope (Bruker Optics Inc., Ettlingen, Germany), equipped with a nitrogen-cooled MCT (HgCdTe) detector with a 36 \(\times\) IR objective and connected with Bruker Vertex 70 spectrometer, was used for IR data acquisition. The spectral acquisition was processed in reflection mode by collecting 64 scans, 68 \(\times\) 68 \(\upmu\)m aperture size with 4 cm\({}^{-1}\) resolution, covering the range of 4000-600 cm\({}^{-1}\). OPUS 6.5 (Bruker) software was used for data acquisition and instrument control.  
The significant variation between the data group was identified by principal component analysis (PCA) in the spectral range of 3050-2800 and 1800-900 cm\({}^{-1}\) by using Unscrambler 9.7 software (CAMO, Oslo, Norway). Data were processed by taking the second derivative, using the Savitzky-Golay algorithm with nine points of smoothing to minimize the effects of variable baselines. The processed data was normalized with extended multiplicative signal correction (EMSC) to account for differences in sample thickness and correcting for scattering artifacts. Six PCs were chosen for analysis and loading for each PC was plotted for each sample. The peak area integration was conducted by OPUS 6.5 (Bruker) software. After that, the primary spectral integrated peak area from (1) the lipid region: 3000-2800 cm\({}^{-1}\), (2) the protein region: 1700-1450 cm\({}^{-1}\), and (3) the nucleic acid region: 1280-900 cm\({}^{-1}\) were processed and represented as a histogram. The primary amide I spectral region (1550-1750 cm\({}^{-1}\)) was analyzed for the secondary protein structure by a curve-fitting analysis based on the Gaussian and Lorentzian algorithm.  
Curve fitting of FTIR spectra was applied to study the submolecular information contained in the 1550-1750 cm\({}^{-1}\) spectral interval (amide I region). The curve fitting process aims to study the absorption bands of a sample separately. The primary spectra are used to determine the number and the position (maximal IR intensity wavenumber) of the bands constituted in a spectral interval used in the classification model. The absorption bands are numbered 1 to 5 and designated as (1) turn, (2) \(\beta\)-sheet/turn, (3) \(\alpha\)-helix, (4) \(\beta\)-sheet 1, and (5) \(\beta\)-sheet 2, respectively. The area for every absorption band is expressed as a percentage of the total area of its corresponding spectral interval.