Beta-azidoalanine as an IR probe: application to amyloid Abeta(16-22) aggregation

Beta-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 beta-azidoalanine as an IR probe of the local electrostatic environment in proteins, azidopeptide 4 was prepared by its incorporation into Abeta(16-22) peptide of the Alzheimer's disease amyloid beta-protein at position Ala21. The amide I IR spectrum of 4 in D2O suggests that the azidopeptide thus modified forms in-register beta-sheets in aggregates as observed for normal Abeta(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 beta-azidoalanine will be used as an effective IR probe for providing site-specific information about the local electrostatic environments of proteins.