Vibrational spectra of human tissues: Experimental data and density functional theory calculations comparison

Biophotonics techniques enabling molecular sensing of tissues and biological samples of had been widely applied to biomedical problems. Vibrational spectroscopy technologies like Fourier-Transform Infrared (FTIR) spec-troscopy is a well-reported example. However, the correct interpretation of measured vibrational spectra of macromolecules and tissues emerging from the complexity of structures is a challenge. Computer simulations can be used as a strategic method to understand interactions between specific parts of a macromolecule. Here we present two implementations of vibrational calculations based on diagonalization of Hessian matrix obtained via Density Functional Theory calculations using Car-Parrinello Molecular Dynamics (CPMD package), Born -Oppenheimer Molecular Dynamics quickstep GPW (CP2K package), and a molecular dynamics calculations based on tight binding (CP2K-TB) was also employed to obtain FTIR spectra via electric dipole correlation function. These calculations were proposed to capture vibrational pieces of information related to FTIR of the human oral mucosa. Eleven models of peptide which represents collagen type I macromolecule including confined water molecules (ranging from 0 to 8 waters), were tested. CPMD and CP2K methods presented an excellent agreement of vibrational bands positions and maximum intensity to experimental data (goodness-of-fit R-2 similar to 0.90). On the other hand, this agreement was very poor for the CP2K-TB case in spite of the correct overall relative spectral intensity with respect to experimental data. We found that the peptide model enclosing eight waters (labeled C8) in CPMD calculations presented the highest value of R-2 = 0.99. The CPMD method, despite some limitations, outperformed the CP2K calculation in our case. Here we only used a collagen cutout and confined water, but it is possible to add other molecules such as proteoglycans to investigate and characterize a tissue. The short collagen mimetic peptide has implications for the conformational behavior of native collagen. The method itself can be used for other macromolecules. Furthermore, we assignment of vibrational modes in the high-wavenumber region ( > 2, 000.cm(-1)) is also presented.