Phase transitions and assembly phenomena: applications to water and membrane biophysics

Uncovering general principles governing organization of chemical and biophysical systems remains a central question in statistical mechanics. Forces associated with phase transformation phenomena can in certain instances, provide a generic framework to understand the physics of self-assembly. For instance, the physics of air-water interfaces plays a central role in theories of interfacial solvation and in modern theories of the hydrophobic effect. However, the importance of accurately addressing fluctuations in the shapes of these soft interfaces is often overlooked.

In the first part of the seminar, I describe the construction of a coarse-grained model of water that resolves fluctuations on large length scales using a discrete lattice model and couples these to fluctuations on finer scales. Drawing from studies of surface roughness in Ising models, care is taken to ensure that the lattice degrees of freedom optimally describe long wavelength liquid density fluctuations such as those at liquid-vapor interfaces. This coarse-grained model only requires as input the surface tension and the pair correlation function of water and predicts the solvation behavior of hydrophobic objects with various shapes and sizes with surprising accuracy. This success establishes a minimally complicated coarse-grained model of water which is uniquely well suited for exploring hydrophobic and interfacial phenomena that involve disparate length scales, e.g., association of hydrophobic solutes with interfaces, solvation of large hydrophobic objects with microscopic features, and the growth of hydrophobic assemblies beyond the nanometer scale.

In the second part of the talk I will discuss role played by phase transformation forces in modulating the dynamic arrangement of trans-membrane proteins. Specifically, I will show that the insertion of a protein in a lipid membrane under physiological conditions can induce a local phase transformation and promote the formation of soft interfaces. This result implies a driving force for assembly distinct from those predicted by elastic theories. I will discuss the length and energy scales associated with this new self-assembly force. These results might provide a framework to explain the dynamic arrangements of proteins and other species in lipid membranes.