The promise of macromolecular crystallization in microfluidic chips

Polymer-based microfluidic device for on-chip counter-diffusive crystallization and in situ X-ray crystallography at room temperature

Proteins are long chains of amino acid residues that perform a myriad of functions in living organisms, including enzymatic reactions, signalling, and maintaining structural integrity. Protein function is determined directly by the protein structure. X-ray crystallography is the primary technique for determining the 3D structure of proteins, and facilitates understanding the effects of protein structure on function. The first step towards structure determination is crystallizing the protein of interest. We have developed a centrifugally-actuated microfluidic device that incorporates the fluid handling and metering necessary for protein crystallization. Liquid handling takes advantage of surface forces to control fluid flow and enable metering, without the need for any fluidic or pump connections. Our approach requires only the simple steps of pipetting the crystallization reagents into the device followed by either spinning or shaking to set up counter-diffusive protein crystallization trials. The use of thin, UV-curable polymers with a high level of X-ray transparency allows for in situ X-ray crystallography, eliminating the manual handling of fragile protein crystals and streamlining the process of protein structure analysis. We demonstrate the utility of our device using hen egg white lysozyme as a model system, followed by the crystallization and in situ, room temperature structural analysis of the hub domain of calcium-calmodulin dependent kinase II (CaMKIIβ).

Merits and advances of microfluidics in the pharmaceutical field: design technologies and future prospects

Microfluidics is used to manipulate fluid flow in micro-channels to fabricate drug delivery vesicles in a uniform tunable size. Thanks to their designs, microfluidic technology provides an alternative and versatile platform over traditional formulation methods of nanoparticles. Understanding the factors that affect the formulation of nanoparticles can guide the proper selection of microfluidic design and the operating parameters aiming at producing nanoparticles with reproducible properties. This review introduces the microfluidic systems' continuous flow (single-phase) and segmented flow (multiphase) and their different mixing parameters and mechanisms. Furthermore, microfluidic approaches for efficient production of nanoparticles as surface modification, anti-fouling, and post-microfluidic treatment are summarized. The review sheds light on the used microfluidic systems and operation parameters applied to prepare and fine-tune nanoparticles like lipid, poly(lactic-co-glycolic acid) (PLGA)-based nanoparticles as well as cross-linked nanoparticles. The approaches for scale-up production using microfluidics for clinical or industrial use are also highlighted. Furthermore, the use of microfluidics in preparing novel micro/nanofluidic drug delivery systems is presented. In conclusion, the characteristic vital features of microfluidics offer the ability to develop precise and efficient drug delivery nanoparticles.

Attaining atomic resolution from in situ data collection at room temperature using counter-diffusion-based low-cost microchips

Sample handling and manipulation for cryoprotection currently remain critical factors in X-ray structural determination. While several microchips for macromolecular crystallization have been proposed during the last two decades to partially overcome crystal-manipulation issues, increased background noise originating from the scattering of chip-fabrication materials has so far limited the attainable resolution of diffraction data. Here, the conception and use of low-cost, X-ray-transparent microchips for in situ crystallization and direct data collection, and structure determination at atomic resolution close to 1.0 Å, is presented. The chips are fabricated by a combination of either OSTEMER and Kapton or OSTEMER and Mylar materials for the implementation of counter-diffusion crystallization experiments. Both materials produce a sufficiently low scattering background to permit atomic resolution diffraction data collection at room temperature and the generation of 3D structural models of the tested model proteins lysozyme, thaumatin and glucose isomerase. Although the high symmetry of the three model protein crystals produced almost complete data sets at high resolution, the potential of in-line data merging and scaling of the multiple crystals grown along the microfluidic channels is also presented and discussed.

Microfluidics for pharmaceutical nanoparticle fabrication: The truth and the myth

Using micro-sized channels to manipulate fluids is the essence of microfluidics which has wide applications from analytical chemistry to material science and cell biology research. Recently, using microfluidic-based devices for pharmaceutical research, in particular for the fabrication of micro- and nano-particles, has emerged as a new area of interest. The particles that can be prepared by microfluidic devices can range from micron size droplet-based emulsions to nano-sized drug loaded polymeric particles. Microfluidic technology poses unique advantages in terms of the high precision of the mixing regimes and control of fluids involved in formulation preparation. As a result of this, monodispersity of the particles prepared by microfluidics is often recognised as being a particularly advantageous feature in comparison to those prepared by conventional large-scale mixing methods. However, there is a range of practical drawbacks and challenges of using microfluidics as a direct micron- and nano-particle manufacturing method. Technological advances are still required before this type of processing can be translated for application by the pharmaceutical industry. This review focuses specifically on the application of microfluidics for pharmaceutical solid nanoparticle preparation and discusses the theoretical foundation of using the nanoprecipitation principle to generate particles and how this is translated into microfluidic design and operation.

A simple and versatile microfluidic device for efficient biomacromolecule crystallization and structural analysis by serial crystallography

Determining optimal conditions for the production of well diffracting crystals is a key step in every biocrystallography project. Here, a microfluidic device is described that enables the production of crystals by counter-diffusion and their direct on-chip analysis by serial crystallography at room temperature. Nine 'non-model' and diverse biomacromolecules, including seven soluble proteins, a membrane protein and an RNA duplex, were crystallized and treated on-chip with a variety of standard techniques including micro-seeding, crystal soaking with ligands and crystal detection by fluorescence. Furthermore, the crystal structures of four proteins and an RNA were determined based on serial data collected on four synchrotron beamlines, demonstrating the general applicability of this multipurpose chip concept.

Fabrication and characterization of dexibuprofen nanocrystals using microchannel fluidic rector

Purpose

Dexibuprofen is an enantiomer of ibuprofen with low bioavailability which results from its hydrophobic nature. Nanosuspensions have developed a podium to solve the in vitro dissolution problem that frequently occurs in current research.

Materials and methods

The drug and polymer solutions were mixed in a microchannel fluid reactor and the successive embryonic nanosuspension was decanted into a vial having the polymer solution. The impact of different process and formulation parameters including inlet angle, antisolvent and solvent flow rate(s), mixing time, drug concentration, polymer type and concentration was evaluated.

Results and discussion

Stable dexibuprofen nanocrystals with a particle size of 45±3.0 nm and polydispersity index of 0.19±0.06 were obtained. Differential scanning calorimetry and powder X-ray diffraction confirmed the crystallinity. The key parameters observed were inlet angle 10°, antisolvent to solvent volume of 2.0/0.5 mL/min, 60 minutes mixing with 5 minutes sonication, Poloxamer-407 with a concentration of 0.5% w/v and drug concentration (5 mg/mm). The 60-day stability studies revealed that the nanocrystals were stable at 4°C and 25°C. The scanning electron microscopy and transmission electron microscopy images showed crystalline morphology with a homogeneous distribution.

Conclusion

Stable dexibuprofen nanocrystals with retentive distinctive characteristics and having marked dissolution rate compared to raw and marketed formulations were efficiently fabricated. In future perspectives, these nanocrystals could be converted to solid dosage form and the process can be industrialized by chemical engineering approach.

Crystallization via tubing microfluidics permits both in situ and ex situ X-ray diffraction

A microfluidic platform was used to address the problems of obtaining diffraction-quality crystals and crystal handling during transfer to the X-ray diffractometer. Crystallization conditions of a protein of pharmaceutical interest were optimized and X-ray data were collected both in situ and ex situ.

Advanced Methods of Protein Crystallization

This chapter provides a review of different advanced methods that help to increase the success rate of a crystallization project, by producing larger and higher quality single crystals for determination of macromolecular structures by crystallographic methods. For this purpose, the chapter is divided into three parts. The first part deals with the fundamentals for understanding the crystallization process through different strategies based on physical and chemical approaches. The second part presents new approaches involved in more sophisticated methods not only for growing protein crystals but also for controlling the size and orientation of crystals through utilization of electromagnetic fields and other advanced techniques. The last section deals with three different aspects: the importance of microgravity, the use of ligands to stabilize proteins, and the use of microfluidics to obtain protein crystals. All these advanced methods will allow the readers to obtain suitable crystalline samples for high-resolution X-ray and neutron crystallography.

Microfluidic Fabrication of Hydrocortisone Nanocrystals Coated with Polymeric Stabilisers

Hydrocortisone (HC) nanocrystals intended for parenteral administration of HC were produced by anti-solvent crystallisation within coaxial assemblies of pulled borosilicate glass capillaries using either co-current flow of aqueous and organic phases or counter-current flow focusing. The organic phase was composed of 7 mg/mL of HC in a 60:40 (v/v) mixture of ethanol and water and the anti-solvent was milli-Q water. The microfluidic mixers were fabricated with an orifice diameter of the inner capillary ranging from 50 µm to 400 µm and operated at the aqueous to organic phase flow rate ratio ranging from 5 to 25. The size of the nanocrystals decreased with increasing aqueous to organic flow rate ratio. The counter-current flow microfluidic mixers provided smaller nanocrystals than the co-current flow devices under the same conditions and for the same geometry, due to smaller diameter of the organic phase stream in the mixing zone. The Z-average particle size of the drug nanocrystals increased from 210–280 nm to 320–400 nm after coating the nanocrystals with 0.2 wt % aqueous solution of hydroxypropyl methylcellulose (HPMC) in a stirred vial. The differential scanning calorimetry (DSC) and X-ray powder diffraction (XRPD) analyses carried out on the dried nanocrystals stabilized with HPMC, polyvinyl pyrrolidone (PVP), and sodium lauryl sulfate (SLS) were investigated and reported. The degree of crystallinity for the processed sample was lowest for the sample stabilised with HPMC and the highest for the raw HC powder.

McCLEC, a robust and stable enzymatic based microreactor platform

A microfluidic chip for cross-linked enzyme crystals (McCLEC) is presented and demonstrated to be a stable, reusable and robust biocatalyst-based device with very promising biotechnological applications. The cost-effective microfluidic platform allows in situ crystallization, cross-linking and enzymatic reaction assays on a single device. A large number of enzymatic reuses of the McCLEC platform were achieved and a comparative analysis is shown illustrating the efficiency of the process and its storage stability for more than one year.

Stable low-fouling plasma polymer coatings on polydimethylsiloxane

Polydimethylsiloxane (DMS) is a popular material for microfluidics, but it is hydrophobic and is prone to non-specific protein adsorption. In this study, we explore methods for producing stable, protein resistant, tetraglyme plasma polymer coatings on PDMS by combining extended baking processes with multiple plasma polymer coating steps. We demonstrate that by using this approach, it is possible to produce a plasma polymer coatings that resist protein adsorption (<10 ng/cm(2)) and are stable to storage over at least 100 days. This methodology can translate to any plasma polymer system, enabling the introduction of a wide range of surface functionalities on PDMS surfaces.

Lessons from high-throughput protein crystallization screening: 10 years of practical experience

INTRODUCTION

X-ray crystallography provides the majority of our structural biological knowledge at a molecular level and, in terms of pharmaceutical design, is a valuable tool to accelerate discovery. It is the premier technique in the field, but its usefulness is significantly limited by the need to grow well-diffracting crystals. It is for this reason that high-throughput crystallization has become a key technology that has matured over the past 10 years through the field of structural genomics. Areas covered : The authors describe their experiences in high-throughput crystallization screening in the context of structural genomics and the general biomedical community. They focus on the lessons learnt from the operation of a high-throughput crystallization-screening laboratory, which to date has screened over 12,500 biological macromolecules. They also describe the approaches taken to maximize the success while minimizing the effort. Through this, the authors hope that the reader will gain an insight into the efficient design of a laboratory and protocols to accomplish high-throughput crystallization on a single-, multiuser laboratory or industrial scale. Expert opinion : High-throughput crystallization screening is readily available but, despite the power of the crystallographic technique, getting crystals is still not a solved problem. High-throughput approaches can help when used skillfully; however, they still require human input in the detailed analysis and interpretation of results to be more successful.

Artificial chaperone-assisted refolding in a microchannel

Protein refolding using a simple dilution method in a microchannel often led to the formation of protein aggregates, which bound to the microchannel wall, resulting in low refolding yields. To inhibit aggregation and improve refolding yields, an artificial chaperone-assisted (ACA) refolding, which employed detergents and beta-cyclodextrin was used. Model proteins, hen egg white lysozyme and yeast alpha-glucosidase, were successfully refolded in a microchannel. The microscopic observation showed that the ACA method suppressed protein aggregation and facilitated the refolding of lysozyme, whereas significant aggregation was observed when a simple dilution method was employed. The ACA method increased the lysozyme refolding yield by 40% over the simple dilution approach. Similarly, for a-glucosidase, the refolding yield using the ACA method (ca. 50%) was approximately three times compared with the simple dilution method. The ACA refolding method is a suitable approach to use in the refolding of proteins using a microfluidic system.

Preparation of hydrocortisone nanosuspension through a bottom-up nanoprecipitation technique using microfluidic reactors

In this work, the possibility of bottom-up creation of a relatively stable aqueous hydrocortisone nanosuspension using microfluidic reactors was examined. The first part of the work involved a study of the parameters of the microfluidic precipitation process that affect the size of generated drug particles. These parameters included flow rates of drug solution and antisolvent, microfluidic channel diameters, microreactors inlet angles and drug concentrations. The experimental results revealed that hydrocortisone nano-sized dispersions in the range of 80-450 nm were obtained and the mean particle size could be changed by modifying the experimental parameters and design of microreactors. The second part of the work studied the possibility of preparing a hydrocortisone nanosuspension using microfluidic reactors. The nano-sized particles generated from a microreactor were rapidly introduced into an aqueous solution of stabilizers stirred at high speed with a propeller mixer. A tangential flow filtration system was then used to concentrate the prepared nanosuspension. The nanosuspension produced was then characterized using photon correlation spectroscopy (PCS), Zeta potential measurement, transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray analysis. Results showed that a narrow sized nanosuspension composed of amorphous spherical particles with a mean particle size of 500+/-64 nm, a polydispersity index of 0.21+/-0.026 and a zeta potential of -18+/-2.84 mV was obtained. Physical stability studies showed that the hydrocortisone nanosuspension remained homogeneous with slight increase in mean particle size and polydispersity index over a 3-month period.

Autoinduction of protein expression

This unit contains protocols for the use of lactose-derived autoinduction in Escherichia coli. The protocols allow for reproducible expression trials to be undertaken with minimal user intervention. A basic protocol covers production of unlabeled proteins for functional studies. Alternate protocols for selenomethionine labeling for X-ray structural studies, and multi-well plate growth for screening and optimization are also included.

Microfluidic crystallization

Microfluidics offers a wide range of new tools that permit one to revisit the formation of crystals in solution and yield insights into crystallization processes. We review such recent microfluidic devices and particularly emphasize lab-on-chips dedicated to the high-throughput screening of crystallization conditions of proteins with nanolitre consumption. We also thoroughly discuss the possibilities offered by the microfluidic tools to acquire thermodynamic and kinetic data that may improve industrial processes and shed a new light on nucleation and growth mechanisms.

Enhanced bacterial protein expression during auto-induction obtained by alteration of lac repressor dosage and medium composition

The auto-induction method of protein expression in E. coli is based on diauxic growth resulting from dynamic function of lac operon regulatory elements (lacO and LacI) in mixtures of glucose, glycerol, and lactose. The results show that successful execution of auto-induction is strongly dependent on the plasmid promoter and repressor construction, on the oxygenation state of the culture, and on the composition of the auto-induction medium. Thus expression hosts expressing high levels of LacI during aerobic growth exhibit reduced ability to effectively complete the auto-induction process. Manipulation of the promoter to decrease the expression of LacI altered the preference for lactose consumption in a manner that led to increased protein expression and partially relieved the sensitivity of the auto-induction process to the oxygenation state of the culture. Factorial design methods were used to optimize the chemically defined growth medium used for expression of two model proteins, Photinus luciferase and enhanced green fluorescent protein, including variations for production of both unlabeled and selenomethionine-labeled samples. The optimization included studies of the expression from T7 and T7-lacI promoter plasmids and from T5 phage promoter plasmids expressing two levels of LacI. Upon the basis of the analysis of over 500 independent expression results, combinations of optimized expression media and expression plasmids that gave protein yields of greater than 1000 mug/mL of expression culture were identified.

Using nanoliter plugs in microfluidics to facilitate and understand protein crystallization

Protein crystallization is important for determining protein structures by X-ray diffraction. Nanoliter-sized plugs--aqueous droplets surrounded by a fluorinated carrier fluid--have been applied to the screening of protein crystallization conditions. Preformed arrays of plugs in capillary cartridges enable sparse matrix screening. Crystals grown in plugs inside a microcapillary may be analyzed by in situ X-ray diffraction. Screening using plugs, which are easily formed in PDMS microfluidic channels, is simple and economical, and minimizes consumption of the protein. This approach also has the potential to improve our understanding of the fundamentals of protein crystallization, such as the effect of mixing on the nucleation of crystals.

Life in the fast lane for protein crystallization and X-ray crystallography

The common goal for structural genomic centers and consortiums is to decipher as quickly as possible the three-dimensional structures for a multitude of recombinant proteins derived from known genomic sequences. Since X-ray crystallography is the foremost method to acquire atomic resolution for macromolecules, the limiting step is obtaining protein crystals that can be useful of structure determination. High-throughput methods have been developed in recent years to clone, express, purify, crystallize and determine the three-dimensional structure of a protein gene product rapidly using automated devices, commercialized kits and consolidated protocols. However, the average number of protein structures obtained for most structural genomic groups has been very low compared to the total number of proteins purified. As more entire genomic sequences are obtained for different organisms from the three kingdoms of life, only the proteins that can be crystallized and whose structures can be obtained easily are studied. Consequently, an astonishing number of genomic proteins remain unexamined. In the era of high-throughput processes, traditional methods in molecular biology, protein chemistry and crystallization are eclipsed by automation and pipeline practices. The necessity for high-rate production of protein crystals and structures has prevented the usage of more intellectual strategies and creative approaches in experimental executions. Fundamental principles and personal experiences in protein chemistry and crystallization are minimally exploited only to obtain "low-hanging fruit" protein structures. We review the practical aspects of today's high-throughput manipulations and discuss the challenges in fast pace protein crystallization and tools for crystallography. Structural genomic pipelines can be improved with information gained from low-throughput tactics that may help us reach the higher-bearing fruits. Examples of recent developments in this area are reported from the efforts of the Southeast Collaboratory for Structural Genomics (SECSG).

Structural genomics of membrane proteins

Improvements in membrane-protein molecular biology and biochemistry, technical advances in structural data collection and processing, and the availability of numerous sequenced genomes have paved the way for membrane-protein structural genomics efforts.

Improvements in the fields of membrane-protein molecular biology and biochemistry, technical advances in structural data collection and processing, and the availability of numerous sequenced genomes have paved the way for membrane-protein structural genomics efforts. There has been significant recent progress, but various issues essential for high-throughput membrane-protein structure determination remain to be resolved.

Screening of Protein Crystallization Conditions on a Microfluidic Chip Using Nanoliter-Size Droplets

Protein crystallization is a major bottleneck in determining tertiary protein structures from genomic sequence data. This paper describes a microfluidic system for screening hundreds of protein crystallization conditions using less than 4 nL of protein solution for each crystallization droplet. The droplets are formed by mixing protein, precipitant, and additive stock solutions in variable ratios in a flow of water-immiscible fluids inside microchannels. Each droplet represents a discrete trial testing different conditions. The system has been validated by crystallization of several water-soluble proteins.