A Stripe-to-Droplet Transition Driven by Conformational Transitions in a Binary Lipid鈭扡ipopolymer Mixture at the Air鈭扺ater Interface
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- 作者：Rita J. El-Khouri ; Shelli L. Frey ; Alan W. Szmodis ; Emily Hall ; Karlina J. Kauffman ; Timothy E. Patten ; Ka Yee C. Lee ; Atul N. Parikh
- 刊名：Langmuir
- 出版年：2011
- 出版时间：March 1, 2011
- 年：2011
- 卷：27
- 期：5
- 页码：1900-1906
- 全文大小：930K
- 年卷期：v.27,no.5(March 1, 2011)
- ISSN：1520-5827
文摘
We report the observation of an unusual stripe鈭抎roplet transition in precompressed Langmuir monolayers consisting of mixtures of poly(ethylene) glycol (PEG) amphiphiles and phospholipids. This highly reproducible and fully reversible transition occurs at approximately zero surface pressure during expansion (or compression) of the monolayer following initial compression into a two-dimensional solid phase. It is characterized by spontaneous emergence of an extended, disordered stripe-like morphology from an optically homogeneous phase during gradual expansion. These stripe patterns appear as a transient feature and continuously progress, involving gradual coarsening and ultimate transformation into a droplet morphology upon further expansion. Furthermore, varying relative concentrations of the two amphiphiles and utilizing amphiphiles with considerably longer ethylene glycol headgroups reveal that this pattern evolution occurs in narrow concentration regimes, values of which depend on ethylene oxide headgroup size. These morphological transitions are reminiscent of those seen during a passage through a critical point by variations in thermodynamic parameters (e.g., temperature or pressure) as well as those involving spinodal decomposition. While the precise mechanism cannot be ascertained using present experiments alone, our observations can be reconciled in terms of modulations in competing interactions prompted by the pancake鈭抦ushroom鈭抌rush conformational transitions of the ethylene glycol headgroup. This in turn suggests that the conformational degree of freedom represents an independent order parameter, or a switch, which can induce large-scale structural reorganization in amphiphilic monolayers. Because molecular conformational changes are pervasive in biological membranes, we speculate that such conformational transition-induced pattern evolution might provide a physical mechanism by which membrane processes are amplified.