In the process of protein production, controls such as SDS-gels already indicate towards the purity of a protein sample. Additionally, during standard concentration determination via UV/VIS, contamination by DNA or RNA can be assessed. In the following section, typical methods for assessment of protein quality are described with the focus on crystallography-grade protein quality. 

After protein production and before crystallization it is essential to check for protein quality. Initial assessment via native Mass-Spec can confirm the correct size of the purified protein with more precision than a SDS gel and can also give first indication regarding oligomerization states or very tightly bound cofactors as well as the presence of posttranslational modifications. Even if the purified protein has the correct molecular weight, it can still be unclear whether the protein is correctly folded.  

Protein folding is a key process in biology since it is ultimately responsible for their biological function. Proteins that are misfolded or not folded at all may lead to unsatisfactory results in subsequent experiments or significant loss of protein material due to aggregation over time. Ultraviolet circular dichroism (CD) is the method of choice to monitor changes of protein structure in solution providing information of  secondary protein structure, hence, the correct folding. 

Protein samples need not only to be correctly folded but also stable and monodisperse. Thermal shift assay (TSA) measures the melting temperature of a protein (Tm), which is an indication of protein stability. Several factors such as pH, salt, cofactor, or buffer composition influence protein stability. Ideally, the protein is measured and stored in a buffer where it is more stable.