Complete vibrational assignments of phosphatidylcholine combining FTIR and FT-Raman spectra with DFT and SQMFF calculations

Abstract

Complete assignments of phosphatidylcholine (PC) have been performed combining the FTIR and FT-Raman spectra with density functional theory (DFT) calculations and the scaled quantum mechanical force field (SQMFF) methodology. Here, the effects of water and of alkyl R and R’ chains on the main vibration modes of two anhydrous (C1 and C1-R) and two mono-hydrated (C2 and C2-R) species of PC have been analysed considering the phosphate groups with local C_2 V and C_3 V symmetries. Then, the main scaled force constants have been reported. Stabilities, reactivities and descriptors of species have been evaluated by using atomic charges, molecular electrostatic potentials (MEP), natural bond orbital (NBO), atoms in molecules (AIM) and frontier orbital studies. Thus, substitution of H atoms by R and R′ groups in both C1-R and C2-R species leads to increased molecular volume (V) and polarizability (α), while the dipole moment decreases. Different intra-molecular interactions predicted by NBO and AIM calculations could justify such changes. Besides, the presence of water molecules in C2-R species increases its α value. Mulliken charges and the MEP energy increase when the alkyl R and R’ chains increase while C2 shows lower MEP energy. AIM studies show multiple interactions in both hydrophilic and hydrophobic moieties that eventually increase the stabilities of species being C2 and C2-R more stable than C1 and C1-R, suggesting that the water molecules stabilize both mono-hydrated species. The MEP, NBO, and the ω, χ, and μ descriptors account for the higher reactivity of C1-R compared with C1, C2, and C2-R.