Tryptophan Raman bands sensitive to hydrogen bonding and side‐chain conformation

Abstract

Abstract Raman Spectra of seven tryptophan derivatives in the crystalline state were examined to find Raman bands whose frequencies reflect the strength of hydrogen bonding at the N 1 H site of the indole ring or the conformation of the indole ring relative to the amino acid backbone. Two indole ring vibrations, W4 around 1490 cm −1 and W6 around 1430 cm −1 , showed a correlation between their Raman frequencies and the infrared frequency of the N‐1‐H stretching mode, an indicator of hydrogen bond strength. W4 and W6 increase in frequency with increase in hydrogen bond strength and the frequency variation is particularly large for W6. On the other hand, another indole ring vibration, W3, observed around 1550 cm −1 , changes in frequency as a function of the torsional angle, χ 2,1 , of the C‐2C‐3C‐βC‐α linkage. As the absolute value of χ 2,1 becomes larger and the C‐α atom moves away from the C‐2 atom, the W3 frequency increases. In the Raman spectra of proteins excited with visible radiation, the W3 band is usually strong and can be used as a conformational marker, whereas the W4 band is very weak and the W6 band is overlapped by strog scattering due to C–H bending vibrations of aliphatic side‐chains. In UV resonance Raman spectra, however, all these Raman bands are enhanced and may provide key information on the hydrogen bonding and conformation of tryptophan side‐chains.