Characterization of a SARS-CoV-2 spike protein reference material

Development of diagnostic testing capability has advanced with unprecedented pace in response to the COVID-19 pandemic. An undesirable effect of such speed is a lack of standardization, often leading to unreliable test results. To assist the research community surmount this challenge, the National Research Council Canada has prepared a SARS-CoV-2 spike protein reference material, SMT1-1, as a buffered solution. Value assignment was achieved by amino acid analysis (AAA) by double isotope dilution liquid chromatography-tandem mass spectrometry (LC-ID-MS/MS) following acid hydrolysis of the protein, in combination with ultraviolet-visible spectrophotometry (UV-Vis) based on tryptophan and tyrosine absorbance at 280 nm. Homogeneity of the material was established through spectrophotometric absorbance readings at 280 nm. Transportation and long-term storage stabilities were assessed by monitoring relative changes in oligomeric state by size-exclusion liquid chromatography (LC-SEC) with UV detection. The molar concentration of the spike protein in SMT1-1 was 5.68 +/- 0.22 mu mol L-1 (k = 2, 95% CI), with the native trimeric form accounting for similar to 94% of the relative abundance. Reference mass concentration and mass fraction values were calculated using the protein molecular weight and density of the SMT1-1 solution. The spike protein is highly glycosylated which leads to analyte ambiguity when reporting the more commonly used mass concentration. After glycoprotein molar mass determination by LC-SEC with multi-angle light scattering detection, we thus reported mass concentration values for both the protein-only portion and intact glycoprotein as 0.813 +/- 0.030 and 1.050 +/- 0.068 mg mL(-1) (k = 2), respectively.

Automated summary

The development of diagnostic testing capability in response to COVID-19 has made significant progress, but the lack of standardization has resulted in unreliable test results. To address this challenge, the National Research Council Canada has prepared a SARS-CoV-2 spike protein reference material to assist the research community. The material was characterized using various analytical techniques, including amino acid analysis and spectrophotometry, and its stability and concentration were determined.