Determination of the phase transition temperatures of phosphocholine and phosphoethanolamine liposomes with FT-IR spectroscopy and differential scanning calorimetry

Prijelaz lipida u lipidnim dvoslojima iz faze gela u fluidnu fazu javlja se pri temperaturi mekšanja (Tm) koja ovisi o van der Waalsovim interakcijama između ugljikovodičnih lanaca molekula lipida i nekovalentnim interakcijama između polarnih skupina molekula lipida i molekula vode. Standardna tehnika za određivanje Tm je diferencijalna pretražna kalorimetrija (DSC), dok se FT-IR spektroskopijom u temperaturno ovisnom načinu snimanja mogu istodobno odrediti Tm i strukturne promjene u slaganju lipida. U ovom diplomskom radu pripremljeni su lipidni dvosloji od 1,2-dipalmitoil-sn-glicero-3-fosfokolina (DPPC) i 1,2-dipalmitoil-sn-glicero-3-fosfoetanolamina (DPPE) u vodenoj otopini NaCl (100 mmol/L). Vrijednosti Tm su za oba uzorka određene FT-IR spektroskopijom i DSC metodom: na temelju FT-IR spektara analizom vrpci koje potječu od simetričnog istezanja metilenskih skupina sCH2 i strižne vibracije metilenskih skupina CH2 te iz pojedinih parametara DSC krivulje. Za dvosloj DPPC lipida Tm određena spomenutim metodama je unutar granica nesigurnosti, dok je za DPPE lipid opaženo odstupanje od ~2 °C. Opažena odstupanja pripisana su promjenama nekovaletnih interakcija između nepolarnih i polarnih skupina kod DPPE lipida u lipidinom dvosloju.Transition of lipids in lipid bilayers from gel to fluid phase occurs at melting temperature (Tm), which depends on van der Waals interactions between hydrocarbon chains and noncovalent interactions created by lipid polar groups with water molecules. Standard technique for determination of Tm is differential scanning calorimetry (DSC), whereas FT-IR spectroscopy applied in temperature-dependent regime provides simultaneously Tm and lipid structural order changes. In this diploma work lipid bilayers are made of 1,2-dipalmitoyl-sn-glycero-3-phoshocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) dissolved in NaCl water solution (100 mmol/L). For both sample sets Tm values were determined using FT-IR spectroscopy and DSC: in FT-IR spectra the bands originated from symmetric methylene group stretching νsCH2 and methylene group scissoring γCH2 were analyzed, and from particular parameters from a DSC curve. Tm determined by these two methods coincides for bilayers made of DPPC lipids, whereas a discrepancy of ~2 °C was observed for DPPE lipid. Observed deviations were attributed to unsynchronized changes in noncovalent interactions between nonpolar and polar groups of DPPE lipids in the lipid bilayer