Photoactive Yellow Protein Adsorption at Hydrated Polyethyleneimine and Poly-L-Glutamic Acid Interfaces

Chiral and achiral vibrational sum-frequency generation (VSFG) spectroscopy was performed in the 1400-1700 and 2800-3800 cm(-1) range to study the interfacial structure of photoactive yellow protein (PYP) adsorbed on polyethyleneimine (PEI) and poly-L-glutamic acid (PGA) surfaces. Nanometer-thick polyelectrolyte layers served as the substrate for PYP adsorption, with 6.5-pair layers providing the most homogeneous surfaces. When the topmost material was PGA, it acquired a random coil structure with a small number of beta(2)-fibrils. Upon adsorption on oppositely charged surfaces, PYP yielded similar achiral spectra. However, the VSFG signal intensity increased for PGA surfaces with a concomitant redshift of the chiral C-alpha-H and N-H stretching bands, suggesting increased adsorption for PGA compared to PEI. At low wavenumbers, both the backbone and the side chains of PYP induced drastic changes to all measured chiral and achiral VSFG spectra. Decreasing ambient humidity led to the loss of tertiary structure with a re-orientation of ff-helixes, evidenced by a strongly blue-shifted chiral amide I band of the fi-sheet structure with a shoulder at 1654 cm(-1). Our observations indicate that chiral VSFG spectroscopy is not only capable of determining the main type of secondary structure of PYP, i.e., fi-scaffold, but is also sensitive to tertiary protein structure.

Automated summary

Chiral and achiral vibrational sum-frequency generation (VSFG) spectroscopy was conducted to investigate the interfacial structure of PYP adsorbed on PEI and PGA surfaces. The results showed that PGA surfaces exhibited a random coil structure with a small number of β(2)-fibrils. Adsorption on surfaces with opposite charges showed similar achiral spectra for PYP, but the VSFG signal intensity increased for PGA compared to PEI, indicating enhanced adsorption for PGA. Both the backbone and side chains of PYP induced significant changes to chiral and achiral VSFG spectra. Decreasing ambient humidity led to a loss of tertiary structure and a re-orientation of FF-helices, as evidenced by a strongly blue-shifted chiral amide I band and a shoulder at 1654 cm(-1) for the fi-sheet structure.