5  α-Amino Acids In Water: A Review of VCD and ROA Spectra  
135
In 2008, Jacob et al. discussed a RoA signature yielded by tryptophan in the peptide backbone based on the BP86/tZvP calculated spectra of six different conformers of N-acetyl-(S)-tryptophan-N’-methylamide [220]. they confirmed the earlier
experimental findings [221] that the RoA intensity of the tryptophan band at ca.
1540 cm
−1
reflected the conformation of the tryptophan side chain. In 2009, Luber
and Reiher described a method for the quantum-chemical determination of modes
with maximum Raman and RoA intensity (intensity-tracking calculations) applied
for several chiral molecules including L-tryptophan [222]. the authors suggested a
method to reveal especially high potential for large molecules, for which ordinary
calculations are very time-consuming. the procedure, applied to L-tryptophan, selected the band at ca. 1540 cm
−1
as the most appropriate for tracking the molecule.
Neither vCd nor RoA spectra have been registered for L-tyrosine in aqueous solution because of its extremely low solubility even in the film state [38]. the
only computational study done for this molecule was that performed by tanaka et al.
in 2006 mimicking the spectra of aromatic amino acid residues in a protein [165].
5.6 Summary and Perspectives
5.6.1 Not-yet-measured Amino Acids
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