Division for Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Bijenička 54, 10000 Zagreb, Croatia.
Division of Analytical Chemistry, Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia.
Spectrochim Acta A Mol Biomol Spectrosc. 2024 Dec 5;322:124773. doi: 10.1016/j.saa.2024.124773. Epub 2024 Jul 9.
The transformation of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipid bilayers from the gel (L) to the fluid (L) phase involves an intermediate ripple (P) phase forming a few degrees below the main transition temperature (T). While the exact cause of bilayer rippling is still debated, the presence of amphiphilic molecules, pH, and lipid bilayer architecture are all known to influence (pre)transition behavior. In particular, fatty acid chains interact with hydrophobic lipid tails, while the carboxylic groups simultaneously participate in proton transfer with interfacial water in the polar lipid region which is controlled by the pH of the surrounding aqueous medium. The molecular-level variations in the DPPC ripple phase in the presence of 2% palmitic acid (PA) were studied at pH levels 4.0, 7.3, and 9.1, where PA is fully protonated, partially protonated, or fully deprotonated. Bilayer thermotropic behavior was investigated by differential scanning calorimetry (DSC) and Fourier-transform infrared (FTIR) spectroscopy which agreed in their characterization of (pre)transition at pH of 9.1, but not at pH 4.0 and especially not at 7.3. Owing to the different insertion depths of protonated and deprotonated PA, along with the ability of protonated PA to undergo flip-flop in the bilayer, these two forms of PA show a different hydration pattern in the interfacial water layer. Finally, these results demonstrated the hitherto undiscovered potential of FTIR spectroscopy in the detection of the events occurring at the surface of lipid bilayers that obscure the low-cooperativity phase transition explored in this work.
1,2-二棕榈酰-sn-甘油-3-磷酸胆碱（DPPC）脂质双层从凝胶（L）相转变为液体（L）相涉及在主相变温度（T）以下几度形成中间纹波（P）相。虽然双层波纹形成的确切原因仍存在争议，但众所周知，两亲分子、pH 值和脂质双层结构都影响（预）相变行为。特别是脂肪酸链与疏水性脂质尾部相互作用，而羧酸基团同时参与极性脂质区域与界面水的质子转移，这由周围水相介质的 pH 值控制。在 pH 值为 4.0、7.3 和 9.1 时，研究了存在 2%棕榈酸（PA）时 DPPC 纹波相的分子水平变化，其中 PA 完全质子化、部分质子化或完全去质子化。通过差示扫描量热法（DSC）和傅里叶变换红外（FTIR）光谱研究了双层热致行为，它们在 pH 值为 9.1 时对（预）相变的特征一致，但在 pH 值为 4.0 时不一致，尤其是在 pH 值为 7.3 时不一致。由于质子化和去质子化 PA 的不同插入深度，以及质子化 PA 在双层中翻转的能力，这两种形式的 PA 在界面水层中表现出不同的水合模式。最后，这些结果表明，FTIR 光谱在检测脂质双层表面发生的事件方面具有尚未发现的潜力，这些事件掩盖了本工作中探索的低协同相变。
