UC San Diego

Lipids are amphiphilic molecules that naturally form membrane assemblies in

aqueous conditions. Along with proteins, they are a major component in all biological

membranes found in archaeal, prokaryotic, and eukaryotic cells. Lipid molecules play

critical roles in controlling specific protein functions such as cell signaling and gating in

the membrane. The molecular composition of the lipids, with a hydrophilic headgroup

and hydrophobic acyl tail, allow membranes to continuously and responsively change its

configuration to maintain these functions. In particular, the intermolecular forces and

electrostatic forces among the headgroup and tail regulate the biophysical properties of the membrane such as fluidity, compression, curvature, deformation energy, membrane packing. Studying the biophysical properties of lipid membranes is not only critical for

understanding their fundamental biological functions in nature but also for developing

promising biomaterials. The first two chapters of this dissertation present the study of

synthetic lipids and natural extracted lipids to understand the molecular and biophysical

factors that influence membrane properties. Synthetic nature-inspired lipids based on

archaea membranes, with a tethered acyl chain in their hydrophobic tail part connecting

two headgroups, were evaluated using a temperature-dependent leakage assay to understand

the effect of tethering the tail groups of two individual lipid molecules on improving

membrane stability. The effect of the tethered chain on the membrane stability according to

temperature change was analyzed by calculating the entropy of activation in transition state

theory. Next, the biophysical properties of lipid membranes were studied to understand the

effect of headgroup and tail structure on the membrane, both with synthetic phospholipids

and with natural lipid extracts. Specifically, three biophysical characteristics, i.e. lateral

diffusion of membranes, ion channel lifetimes on membranes, and effective elastic modulus

of the membranes, were measured by fluorescence recovery after photobleaching, black lipid

membrane (BLM), and atomic force microscopy (AFM). In the chapter 4, the pore formation

and function of the Alzheimers disease (AD) associated channel-forming protein, beta-amyloid

(Abeta), was examined in Brain total lipid extract (BTLE) membranes and model membranes

The structure and ion conducting properties of Abeta were studied using BLM and AFM The

increase in anionic lipid content in a membrane alters the formation and ion conducting

behavior of Abeta pores. The last chapter of this dissertation describes a semi-synthetic ion

channel platform capable of detecting small molecule analytes using a gramicidin A (gA).

The sensor system utilizes a monoclonal antibody and its fab fragments to sequester the channel activity of a C-terminal modified gA derivative initially. By introducing a small molecule into the system, the channel activity of gA derivative was restored by competitive

binding to the antibody. The sensitivity of the system was examined by two methods: total

transported charge from gA derivatives and channel event frequencies of gA-derivatives.

With a picomolar detection threshold, this sensing method has potential applications in

both targeting biological warfare agents such as dipicolinic acid and in designing portable

detection devices with leakage-proof membranes. This dissertation presents the biophysical

studies of synthetic and natural archaeal lipids with/without ion channel forming proteins,

and the development of a picomolar sensor platform based on the response to external

stimuli with a chemically modified gramicidin A ion channel.