付里叶变换红外光谱研究短杆菌肽A及其在脂双层中加Na~＋前后的构象变化

SODIUM CATION INDUCED CONFORMATIONAL CHANGES OF GRAMICIDIN A IN PHOSPHOLIPID BILAYER: A FOURIER TRANSFORM INFRARED SPECTROSCOPIC STUDY

用ＦＴＩＲ光谱研究了短杆菌肽Ａ（ＧＡ）在固态，组装入ＤＰＰＣ脂持体，以及Ｎａ＋结合状态的构象变化。固态时酰胺Ⅰ带峰位于１６３ｃｍ－１左右，且在１６８５ｃｍ－１处有一肩峰；酰胺Ⅱ带位于１５３０ｃｍ１左右，整个分子为反平行的β螺旋结构，为非通道构象。当ＧＡ组装入脂质体后，酰胺Ⅰ带仍位于１６３３ｃｍ－１，但１６８５ｃｍ－１的肩峰消失，酰胺Ⅱ带移至１５５０ｃｍ－１，分子间氢键转变为分子内氢键，二聚体从原来的双股螺旋变为头－头相连的单股螺旋，是ＧＡ的膜内通道构象。Ｎａ＋的结合并不改变ＧＡ的骨架结构，但通过对酰胺Ⅰ带的拟合分析发现：加Ｎａ＋后位于１６４４ｃｍ－１的无规结构含量减少１０％，而有序结构增加，提示部分无视结构可能有一种“门”的作用：当无离子通过时为“关闭”状态，当与离子结合后即“打开”，这样通过构象的改变来实现其通道功能。

FTIR was used to examine the seconda ry structural changes of GA in solid state,phospholipid bilayer and in the presence of sodium cation. In solid state a strong amide Iband Was observed at-1630cm-1 accompanied by a shoulder at-1685cm-1. Thestrongest amide Ⅱ band was at-1530cm-1. The predominant structure of GA was doub le-stranded anti-panalle helix. This is the non-channal conformation of GA. Thereconstitution of GA in DPPC did not change its 1633cm-1 band but the shoulder at1685cm-1 disappeared. The amide Ⅱ band also shifted to 1550cm-1. These data suggestedthat GA had undertaken a conformational change: from the double-stranded β helix to asingle-stranded β helix consisting of the head-to-head dimer. This is the channel.conformation of GA. Binding of Na+ to the GA channel did not change the main βbelical conformtion in the hydrophobic area. But a ed-quantitative analysis indicated thatthe unordered structure of the peptide decreased from 22% to 11% in the presence of Na+.The result suggested that the sodium binding caused a partial change in the unordered structure, which probably acts as a " door" of the GA channel. The "door" is "closed"when there is no ion binding but can be "opened" by adding of Na+.