β-Azidoalanine as an IR probe: Application to amyloid Aβ(16-22) aggregation

β-Azidoalanine dipeptide 1 was synthesized, and its azido stretching vibration in H₂O and dimethyl sulfoxide (DMSO) was studied by using Fourier transform (FT) IR spectroscopy. The dipole strength of the azido stretch mode is found to be about 19 and 5 times larger than those of the CN and SCN stretch modes, respectively, which have been used as local environmental IR sensors. The azido stretch band in H₂O is blue-shifted by about 14 cm^-1 in comparison to that in DMSO, indicative of its sensitivity to the electrostatic environment. To test the utility of β-azidoalanine as an IR probe of the local electrostatic environment in proteins, azidopeptide 4 was prepared by its incorporation into Aβ(16-22) peptide of the Alzheimer's disease amyloid β-protein at position Ala21. The amide I IR spectrum of 4 in D₂O suggests that the azidopeptide thus modified forms in-register β-sheets in aggregates as observed for normal Aβ(16-22). The azido peak frequency of 4 in aggregates is almost identical to that in DMSO, indicating that the azido group is not exposed to water but to the hydrophobic environment. We believe that β-azidoalanine will be used as an effective IR probe for providing site-specific information about the local electrostatic environments of proteins.