Effects of hydrogen bonding and side‐chain conformation on the Raman bands of tryptophan‐2,4,5,6,7‐d5

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Abstract
Tryptophan‐2,4,5,6,7‐d5 (dTrp), in which all the carbon atoms of the indole ring are deuterated, is sometimes used for isotopic labelling of Trp residues in proteins and peptides. The vibrational wavenumbers of the deuterated indole ring were examined in the Raman spectra of eight crystals of dTrp derivatives and compared with the crystal structure and the indole N1‐H stretching wavenumber, a direct indicator of hydrogen bonding strength. The comparison has shown that the Wd4 mode around 1445 cm−1 and the Wd5 mode around 1410 cm−1 of dTrp increase in wavenumber with increase in the strength of hydrogen bonding at the indole N1H site. Another in‐plane vibration of the deuterated indole ring, Wd3, around 1520 cm−1 has been found to change in wavenumber as a function of the absolute value of the torsional angle |χ2, 1| about the Cα—Cβ—C3—C2 linkage. These marker bands of hydrogen bonding and conformation are expected to be useful in the structural analysis of proteins and peptides that are labelled with dTrp.

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