Abstract

Near infrared (NIR) spectroscopy was investigated as a non-destructive and rapid method to characterise the secondary structures of proteins in solid state. The absorption spectra of 11 reference proteins (0-46% α-helix, 6-73% β-sheet) were obtained using an Antaris NIR Analyser and quantitatively analysed using a chemometric software program to correlate the NIR spectral data to the secondary structure content obtained from X-ray crystallography and Raman spectroscopy. Simple linear regression analyses of the normalised second derivative NIR spectra indicated that 6289 cm⁻¹ (R² = 0.99, RMSEC = 2.6) and 4602 cm⁻¹ (R² = 0.96, RMSEC = 5.6) were highly sensitive wave number regions for β-sheet structure and the calibration models were highly predictive for three additional proteins which served as external validation standards (average error 2% and 3%, respectively). The second derivative NIR spectra at 4602 cm⁻¹ was also found to be sensitive to α-helical content (R² = 0.93, RMSEC = 5.8) and predictive of the validation standards (average error 6%). The results demonstrate the potential of NIR spectroscopy as a rapid and non-invasive tool for quantification of secondary structure content of proteins in solid state.

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