Characterization of resting versus stimulated saliva fingerprints using Middle-Infrared Spectroscopy assisted by Principal Component Analysis

Fourier Transform Mid-Infrared spectroscopy with Attenuated Reflection combined with multivariate data Principal Component Analysis have been applied for the discrimination of 12 couples of resting and stimulated saliva obtained from healthy volunteers. The resting saliva samples were collected before eating, while stimulated saliva, after chewing stimulation with parafin and preserved in Natrium azide 1%, deproteinized by methanol, before analysis. The spectra were compared by identifying the main absorption regions and quantitatively, by measuring the area peaks for each group of saliva samples. The Principal Component Analysis was performed discriminate differences among samples, either based on raw FTIR or second derivative spectra. The region 1000-1700 cm-1 corresponded to specific absorptions of saccharides and aminoacids/peptides/proteins where higher intensities for stimulated saliva against resting saliva (samples 1-8) while samples 9-12 showed similar or even decreased intensities after stimulation. In the region 2000 to 2200 cm-1, corresponding to specific absorptions of azides, thiocyanates and lipids, there were noticed higher intensities for stimulated saliva (1-4 and 7) against resting saliva samples, while samples 5,6,8, 9-12 showed similar or even decreased intensities after stimulation. The stimulated saliva were grouped similarly and clustered in an intermediate region, between positive PC-1 and neutral PC-2 region, being more represented by a stronger PC-2 component, while resting saliva was represented by a positive PC-1. Therefore, FTMIR spectra are able to discriminate between the composition differences between resting and stimulated saliva samples. Further studies are needed to discriminate better among the characteristics of physiological and pathological saliva samples.