Fourier Transform Infrared Spectroscopy Studies of the Secondary Structure in mt-PA6 and mt-PA6 Treated with L-Arginine, L-Asparagine, and (+)-Citrulline in Aqueous Solution:

FT-IR studies of the secondary structure of mt-PA6 and mt-PA6 treated with L-arginine, (+)-citrulline, and L-asparagine were carried out in aqueous buffer solution. Spectral subtraction was used to remove the interfering water bands in the amide absorption region, and the amide I absorptions were monitored to investigate changes in the secondary structure of the mt-PA6 induced by the binding of these amino acids. The spectral data showed a narrowing of the amide I absorption of mt-PA6 on treatment with L-arginine, (+)-citrulline, and L-asparagine. Deconvolution of the amide I absorption revealed vibrational bands characteristic of β-sheet (1632 cm-1), disordered or bound water (1644 cm-1), and turns (1660 cm-1) secondary structures for the mt-PA6. Curve-fitting methods were also used to examine the changes in spectra and secondary structure of the mt-PA6 resulting from amino acid treatment. Analysis of the spectral data shows that loss in intensity of the bands near 1644 and 1660 cm-1 is responsible for the amino acid-induced narrowing of the mt-PA6 amide I absorption. In addition, the spectral data could suggest different binding interactions for L-arginine to mt-PA6 compared to the L-asparagine and (+)-citrulline.