Journal of Alzheimer’s Disease xx (20xx) x–xx DOI 10.3233/JAD-181008 IOS Press 1 FTIR Spectroscopy of Cerebrospinal Fluid Reveals Variations in the Lipid:Protein Ratio at Different Stages of Alzheimer’s Disease Joanna Depciuch a , Izabela Zawlik b,c,∗ , Marzena Skrzypa b , Justyna Paj˛ ak b , Natalia Potocka b , Kornelia Lach b , Halina Bartosik-Psujek c,d , Anna Koziorowska e,f , Ewa Kaznowska b and J ´ ozef Cebulski g a Institute of Nuclear Physics Polish Academy of Sciences, Krakow, Poland b Centre for Innovative Research in Medical and Natural Sciences, Faculty of Medicine, University of Rzeszow, Poland c Institution of Experimental and Clinical Medicine, Faculty of Medicine, University of Rzeszow, Poland d Clinical Department of Neurology Rzeszow State Hospital, Rzeszow, Poland e Department of Computer Engineering, Faculty of Mathematics and Natural Sciences, University of Rzeszow, Poland f Laboratory of Bioelectromagnetism, Institute of Biotechnology, University of Rzeszow, Poland g Faculty of Mathematics and Natural Sciences, Centre for Innovation and Transfer of Natural Sciences and Engineering Knowledge, University of Rzeszow, Poland Accepted 21 December 2018 Abstract. Alzheimer’s disease (AD) is a disease of advanced civilization and a common form of dementia in people over 65 years of age. We used Fourier transform infrared (FTIR) spectroscopy combined with principal component analysis (PCA) to determine changes in the quantity and quality of the cerebrospinal fluid from AD patients at three different stages of the disease (ADI, ADII, and ADIII), as well as from patients with mild cognitive impairment (MCI). Moreover, based on the FTIR spectra, we calculated the ratio of -helix and -sheet secondary protein structures as well as the lipid-protein balance as potential AD markers. The FTIR spectra of cerebrospinal fluid obtained from MCI, ADI, ADII, and ADIII patients showed that peaks corresponding to protein and deoxyribonucleic acid (DNA), and phospholipid and lipid vibrations were shifted in comparison with those of control subjects. Furthermore, the levels of these chemical compounds were lower in the patients than in the control subjects. The -sheet secondary protein structure levels were increased in the MCI and AD patients compared with the control subjects. In addition, significant changes in the lipid-protein balance were observed. Interestingly, as the disease progressed, the lipid-protein balance became further disrupted, that is, the lipid amount decreased with disease progression. PCA analysis of lipid-protein FTIR regions revealed that the spectra could be used to distinguish between controls and patients with MCI, ADI, ADII, and ADIII. Keywords: Alzheimer’s disease, cerebrospinal fluid, Fourier transform infrared spectroscopy, lipid-protein balance, PCA- LDA ∗ Correspondence to: Izabela Zawlik, Laboratory of Molecular Biology, Center for Innovative Research in Medical and Natural Sciences, Faculty of Medicine, University of Rzeszow, Warzywna 1A, 35-959 Rzeszow, Poland. Tel.: +48 17 851 68 10; E-mail: izazawlik@yahoo.com. INTRODUCTION Due to the phenomenon of aging societies worldwide, the percentage of elderly persons has sig- nificantly increased, and as a result, the number of ISSN 1387-2877/19/$35.00 © 2019 – IOS Press and the authors. All rights reserved Corrected Proof