We have used Fourier transform infrared spectroscopy to analyze protein structure at nanomolar concentrations and compared its sensitivity with other commonly used spectral techniques such as circular dichroism and fluorescence. Less than 10 nM concentration of protein (immunoglobin G) was required in order to obtain IR spectra with good signal-to-noise ratio that could be utilized for curve-fitting analysis to obtain individual band areas assigned to specific secondary structural features. No signals were observable on circular dichroism, and the fluorescence signals were within the noise level for the same concentration of the protein. The results suggest that FT-IR in combination with the ATR technique has high potential for protein structural analysis, and less than 15 picomole protein is sufficient for the structural analysis.
Bal Ram Singh and Michael P. Fuller, "FT-IR in Combination with the Attenuated Total Reflectance Technique: A Very Sensitive Method for the Structural Analysis of Polypeptides," Appl. Spectrosc. 45, 1017-1021 (1991)