Toward Methodology for Detection of Cancer-Promoting S100A4 Protein Conformations in Subnanomolar Concentrations Using Raman and SERS

The cancer-promoting S100A4 protein has been studied using Raman and surface-enhanced Raman spectroscopy, RS and SERS, respectively. Two different forms of this protein, dimeric (MW ≈ 26 kDa) and oligomeric (MW > 50 kDa), were purified and then individually studied in high concentration (ca. millimolar) by RS and in low concentration (ca. nanomolar) by SERS, to distinguish the characteristic vibrational modes of each species. The results have shown that a clear difference between these species was detectable, both by RS and by SERS. The nanomolar concentration used in this study is well representative of the average concentration of this protein in the plasma of healthy individuals. For SERS measurement, Ag-colloids were mixed with the protein solution, and a laser line of 532 nm was employed for all vibrational measurements. This is the first time that this cancer progression-promoting protein is studied by Raman and SERS spectroscopy and in as low concentrations as below nanomolar. Here, the novel results of the spectral analysis are presented and discussed.