A structural approach to discovery

Xtal BioStructures specializes in X-ray crystallography, protein production, and biophysical characterization to enable Structural Biology insights.

We produce high quality proteins, custom designed to meet the needs of our clients, for use in applications including X-ray crystallography, cryo-EM, biophysical characterization, high-throughput screening, assay development, and pilot studies.

Construct Design and Expression

We offer construct design and synthesis with codon optimization using bacterial, baculovirus/insect cell, and mammalian expression systems.

Protein Purification

We custom design purification methods to reach rigorous standards of protein purity, employing affinity-based techniques, classical chromatography, or refolding methods. We are experienced with challenging proteins, the assembly of higher order complexes, and the purification of proteins from natural sources.


Our quality control analysis of protein purity includes SDS-PAGE, Western blot verification, mass spectrometry, size-exclusion chromatography, and thermal stability profiling. Our rigorous standards include purity at the level of sequence, quaternary structure, and post-translational modification.

Using thermal shift assay (TSA) analysis (also known as differential scanning fluorimetry, or DSF), we offer mapping of protein stability as a function of buffer, pH, salt concentration, and additives.

XTAL Protein List (PDF)

We have approximately 150 high purity proteins available for X-ray crystallography, biophysical characterization, screening, or assay development applications.

Biophysical methods such as TSA, SPR, or ITC are orthogonal to commonly used high-throughput screening methods and can provide confirmation of the molecular interaction of HTS hits. Each method provides different data to guide discovery and development.

Thermal Stability Analysis (TSA) or Differential Scanning Fluorimetry (DSF)

  • Measures the temperature at which a protein unfolds (Tm)
  • Measures ligand-induced changes to thermal stability
  • Ranks ligands by affinity; is powerful method for fragment-based screening
  • Maps protein stability in various buffer conditions
  • Has minimal protein requirements
  • Can screen 1000’s of compound per week

Surface Plasmon Resonance (SPR)

  • Measures association and dissociation rates (Ka and Kd); enables calculation of the equilibrium dissociation constant (KD)
  • Elucidates the interaction kinetics between a protein and its ligand
  • Requires attachment of the protein or the ligand to a biosensor chip

Isothermal Titration Calormitery (ITC)

  • Provides a complete thermodynamic profile of protein:ligand interactions
  • Protein and ligand remain  free of modification, tags, or coupling
  • Quantitatively determines equilibrium binding affinity (Ka), binding stoichiometry, and enthalpy of binding (ΔH)
  • Can determine entropy (ΔS), free energy of binding (ΔG), and dissociation constant (Kd) by extension

Our crystallographers have determined the structures of hundreds of proteins and protein complexes, guiding lead optimization for small molecule and biologic therapeutics. We use high intensity synchrotron X-ray sources, maximizing resolution of the final structure. Our expertise encompasses protein construct design, which can be a critical factor in the success of structure determination as well as in the relevance of the data.

Structure-guided Drug Discovery

The structure of a protein:ligand complex reveals the molecular interactions governing ligand binding, and can show corresponding conformational changes in the protein. A co-structure provides experimental validation of the ligand’s mode of binding, and provides insights to guide optimization of the therapeutic lead.

Antibody: Antigen Structures

An antibody:antigen structure is a definitive epitope map. Understanding the molecular basis of epitope binding is key for optimizing antibody affinity and specificity, which is of increasing interest with the rise of antibodies as therapeutics.

Fragment Based Screenings (FBS)

FBS can identify low-molecular-weight ligands that bind to a target protein. These fragments can be grown, or multiple fragments can be joined together, to produce drug candidates in a rational way. Thermal shift analysis (TSA) or surface plasmon resonance (SPR) assays can be employed to identify fragment ligands, while X-ray structure determination of the target:ligand complex can guide the development of the fragments into drug candidates.

Epigenetics, the heritable modification of gene expression caused by mechanisms other than changes in DNA sequence, is increasingly recognized as a key factor in disease. While many diseases may have an epigenetic component, the primary focus of epigenetics research is in the field of oncology.

Bromodomain Proteins

XTAL has many bromodomain proteins expressed, purified, characterized, and immediately available; many have also been crystallized. In addition to single domains, we have several tandem domain proteins available.

Specificity Panel

XTAL has proteins from most of the major branches of the bromodomain family in stock, ready for use in specificity testing.

Histone Deacetylase (HDAC) Proteins and the Nucleosome Core Particle (NCP)

XTAL has experience in the purification of HDACs and the NCP (four proteins + DNA), and has several HDAC proteins immediately available. Modified NCP sequences or histones may be custom produced.

XTAL Epigenetic Protein List (PDF)

XTAL has many purified epigenetic proteins immediately available for use in X-ray crystallography, HTS, assay development, pilot studies, and other applications.

In addition to the more specific services offered by XTAL, our scientists have a broad multi-disciplinary expertise that encompasses many different commonly used assay methodologies. Some techniques, such as Thermal Shift Screening are much more versatile than commonly understood.

Assay Development

Both cell based and enzymatic assays, where the readout is absorbance, fluorescence or luminescence. Western blots, protein and antibody microarray work are also routinely done in our facility. If you have difficulty in locating a laboratory to enable your unique assay requirements, contact us to see if we can facilitate development at XTAL or recommend on of our trusted partners.

HTS Thermal Shift Screening

To measure protein stability, undertake preformulation studies or rapidly screen for molecular interactions between a target protein and its ligand. We have the capacity to perform compound screening services, up to the level of 10,000 per week.

Computational Studies

In silico screening and homology building services can also be provided as part of the structure-guided drug discovery program.

Cryo-EM Services

  • Protein construct design, production, and purification for cryo-EM utilization.
  • AKTA Pure Micro FPLC system, to enable small sample volume and micropreparative columns.
  • TEM negative stain characterization.
  • Cryo-EM screening and high-resolution data collection (Thermo Scientific Glacios & Krios).
  • Image analysis and structure determination