OCD的結果顯示，在PG與鈣離子都存在的情況下，daptomycin的OCD光譜在232(nm)附近會有明顯翻轉，其光譜翻轉情形與daptomycin和鈣離子濃度有關。藉由分析其光譜翻轉，發現與細胞膜結合的daptomycin超過臨界濃度，daptomycin會改變其與細胞膜的結合狀態；且確認了daptomycin與鈣離子的化學劑量比例約為1到2之間。LXD的結果顯示，與daptomycin作用會使細胞膜變薄，而與鈣離子作用會使細胞膜變厚。在daptomycin、PG與鈣離子並存的情況下，細胞膜則會變厚，我們提出不同模型進行計算來解釋細胞膜厚度的變化，進一步討論daptomycin、鈣離子與細胞膜的作用。 ;Antimicrobial peptides are widely used by animals and plants in their innate immune systems to eliminate invading pathogens or microbes via directly targeting to their membranes. The composition of the membrane is difficult to be changed by gene mutation so that it is rare to exhibit antibiotic resistance. Therefore, studies on the antibacterial mechanism of antibacterial peptides is an key issue to solve the serious problem of antibiotic resistance. Daptomycin, a cyclic lipopeptide, represents a new structural class of the FDA approved antibiotics. It interacts with the cytoplasmic membranes of Gram-positive pathogens causing membrane permeabilization to kill cells. The antibiotic activity is calcium ion dependent and correlates with the targeted membrane’s content of phosphatidylglycerol (PG), otherwise its underlying molecular mechanism is so far unknown in despite of clinical usages and researches for many years. Here we used oriented circular dichroism (OCD) to probe the change of second structure to determine the binding states of daptomycin in membranes. Lamellar X-ray diffraction (LXD) was used to monitor the thickness change of membrane induced by daptomycin binding. In the coexistence of daptomycin, PG and calcium ions, the result shows that OCD spectra of daptomycin will be significantly reversed around 232 (nm) and the reversed spectra is correlated to the concentrations of daptomycin as well as calcium ions. Consequently, it indicates that the concentration of binding daptomycin excess a threshold, daptomycin will change its state. And the stoichiometric ratio of daptomycin to calcium was confirmed between 1 and 2. The LXD result shows that daptomycin induced membrane thinning and calcium ions induced membrane thickening, respectively. In the coexistence of daptomycin, PG and calcium ions, membrane thickening was observed. In this study, we attempt to propose different models to fit experimental data to clarify the change of membrane thickness in different concentrations of daptomycin and calcium ions. Finally, the daptomycin/calcium ion/membrane interaction will be discussed.
