二次谐波光谱和布鲁斯特角显微镜研究二棕榈酰磷脂酰胆碱和维生素B2的相互作用

二棕榈酰磷脂酰胆碱（DPPC）分子在气液界面上形成的Langmuir膜是一种重要的生物膜模拟体系，其手性结构及其与外来物质的相互作用一直是相关学科研究的前沿问题。维生素B₂（VB₂）是人体中一种重要的营养物质，它在代谢障碍引起的脂质沉积性类疾病中有大量的实例应用，经常在一些特殊的临床症状中有出乎意料的治疗奇效。目前，VB₂如何参与到膜上生物事件的过程和细胞乃至生命的作用过程中的研究报道较少，特别是VB₂分子与磷脂分子靶标的立体相互作用，其可能发生的手性分子识别现象会在许多生物事件中起着关键作用。综合二次谐波-线二色光谱（SHG-LD）、Langmuir膜天平和布鲁斯特角显微镜(BAM)技术初步研究了VB₂和DPPC分子在气液界面上的相互作用，分别从气液界面上介观水平和宏观水平上互补表征脂质分子在气液界面上的分子骨架自组装的结构。压缩等温线发现纯水界面L-DPPC和D-DPPC液态扩展相/液态凝聚相(LE/LC)共存阶段的膜压几乎不变，race-DPPC的共存相膜压区域稍微缩短，VB₂水溶液界面上race-DPPC的LE/LC共存相消失。此外，弹性模量研究表明VB₂分子可以提高L-DPPC单分子层膜的弹性模量，但降低D-DPPC和race-DPPC单层膜的弹性模量。结合SHG-LD研究发现，在膜压13 mN·m-1下，L-DPPC在纯水和VB₂水溶液界面上表面手性过量值（DCE）保持不变。与纯水界面相比较，D-DPPC在VB₂水溶液上DCE值出现反转，而race-DPPC的DCE值则不随亚相改变而变化。相同膜压下，BAM观察到单一手性相互作用使得L-DPPC和D-DPPC在纯水界面上各自组装成不同枝臂弯曲方向的手性三叶草微畴(microdomain)。VB₂诱导D-DPPC微畴，使其直径增大1～2倍。同时，VB₂也诱导了race-DPPC单层膜上近似圆形状的微畴伸展，并长出了三条有曲率的枝臂。对此可以解释为VB₂降低了非单一手性相互作用的能量，使得race-DPPC出现手性相分离。与此同时，VB₂也诱导了race-DPPC单层膜微畴的手性结构发生变化。该研究有助于理解VB₂调节磷脂膜横向组织结构的分子机理，在细胞膜界面发生的过程中，脂层单层的二维特性和生物分子之间的相互作用可能决定了生物分子的亲和力。

Interaction Between Dipalmityl Phosphatidylcholine and Vitamin B2 Studied by Second Harmonic Spectroscopy and Brewster Angle Microscopy

Langmuir monolayers formed by dipalmityl phosphatidylcholine (DPPC) molecules at the air/liquid interface are an important model system for studying the biomembranes, and their chiral structure and interaction with foreign substances have been the frontiers of related fields. Vitamin B₂ (VB₂) is an important medicine for treating lipid deposition diseases caused by metabolic disorders. The molecular mechanism of the interaction between VB₂ and phospholipids is the key to understanding the function of VB₂. In this report, the interaction between VB₂ and DPPC at the air/water interface was studied using second-harmonic generation linear dichroism (SHG-LD), Langmuir membrane balance and Brewster Angle Microscopy (BAM). The compression isotherm shows that the surface pressure in the liquid expansion phase/liquid condensed phase (LE/LC) of L-DPPC and D-DPPC at the water interface is almost unchanged, while the film pressure region in the co-existing phase of race-DPPC is slightly shortened. The LE /LC coexistence phase of race-DPPC disappears at the interface of the VB₂ aqueous solution. In addition, the elastic modulus studies show that the VB₂ molecule can increase the elastic modulus of L-DPPC monolayers, but reduce the elastic modulus of D-DPPC and race-DPPC monolayers. SHG-LD results showed that the Degree of Chiral Excess (DCE) of L-DPPC at the interface between VB₂ and water was unchanged at the surface pressure of 13 mN·m-1, and the DCE of D-DPPC at the interface between VB₂ and water was reversed. The DCE value of race-DPPC does not change with the change of subphase. Under the same surface pressure, BAM observed that the interaction of L-DPPC and D-DPPC to assemble into homochiral microdomains with different arm curvature directions at the pure water interface, and VB₂ could induced the increase of D-DPPC microdomains by 1 to 2 times. Meanwhile, VB₂ induced the extension of approximately circular microdomains on race-DPPC monolayer and the growth of three curvature arms, which could be explained as VB₂ reducing the energy of non-monolayer chiral interactions and resulting in chiral phase separation. VB₂ also induced changes in the chiral structure of race-DPPC monolayers. This study is of great significance for understanding the regulation of VB₂ on the macrostructure and biological function of the phospholipid membrane.