Microfluidics aid membrane protein structure studies

How do scientists discover better and more efficient drugs? A good starting point is to obtain atomic level structural information about the drug molecules and the proteins they interact with. Getting this detailed structural information, however is not a simple task.

Most drugs work at the molecular level by interacting with, and influencing, the function of a specific protein. Proteins are the workhorse molecules in living systems that control the chemical reactions of life and many other cellular functions. An important class of proteins are embedded in the outer membrane of the cell. These membrane proteins allow the cell to receive signals from the surrounding environment. The majority of drug targets are these membrane proteins. Although the majority of all proteins are membrane proteins, structure databases are lacking information on the majority of them.

Membrane proteins are difficult to study because they are only stable in the membrane environment. Consequently, it is difficult to isolate them and grow the large crystals that are required to determine their atomic structures using traditional methods.

However, a revolutionary new technique is now being applied to this problem, serial femtosecond crystallography (SFX). SFX can use tiny microcrystals of proteins that are easier to grow than conventional crystals. The microcrystals are irradiated with a very high energy X-ray free electron laser (XFEL) that can reveal atomic scale structures using even the smallest crystals. A major problem in SFX, however, is figuring out how to deliver the crystals, and synchronizing their flow to the pulsing X-ray laser. Arizona State University researcher Alexandra Ros and her group are working on the design of special microfluidic devices to solve this problem.

Ros is an associate professor in the School of Molecular Sciences and a member of the Biodesign Center of Applied Structural Discovery (CASD). The work is a collaborative effort between Ros; her graduate students Austin Echelmeier, Daihyun Kim, Jorvani Cruz Villarreal; CASD Director Petra Fromme and her graduate student Jesse Coe; and other scientists who are part of the NSF-BioXFEL Science and Technology Center at ASU.

“While not immediately obvious, sample delivery is the bottleneck in crystallography with XFELs,” Ros said.

A 3D-printed liquid jet injector in the European XFEL sample chamber setup for serial femtosecond crystallography experiments. The sample jet containing protein crystals is hit by the pulsed X-ray laser XFEL to reveal atomic level structural details of proteins.

Microfluidic devices allow controlled mixing of small quantities of solutions. They can be customized and 3D-printed to suit a variety of applications. Ros's group takes advantage of the attraction and repulsion of different materials to oil and water to control the flow of liquid droplets containing protein microcrystals in the device. The goal is to controllably generate droplets that allow protein crystals to coincide with the laser pulses and thereby maximize the information obtained from the valuable protein crystals. An article describing this work was published recently in the Journal of Applied Crystallography.

The Ros group with the ASU BioXFEL community and other researchers at XFEL facilities around the world are designing, testing and applying such tailored sample injection techniques to facilitate crystallography with XFELs. This work will be important to the development of the the tabletop compact XFEL (CXFEL) currently being developed at ASU. The CXFEL will be the first machine of its kind that will place ASU scientists at the forefront of a potential revolution in solving protein structures, in visualizing of molecules as they undergo chemical reactions, in designing new drugs and in many other areas where atomic level medical imaging is critical.

Written by Zina Al-Sahouri, School of Molecular Sciences