Relationships between conformational and vibrational features of tryptophan characteristic Raman markers

Area of Science:

• Biophysical Chemistry
• Spectroscopy
• Protein Science

Background:

• Tryptophan (Trp) residues in proteins exhibit characteristic Raman spectral markers.
• Previous studies linked Trp Raman marker variations to environmental changes, including Fermi resonance and water accessibility.
• Ambiguities persist in interpreting these spectral features, necessitating further investigation.

Purpose of the Study:

• To clarify the structural and vibrational relationships of Trp Raman markers.
• To re-evaluate existing assignments of specific Trp Raman markers (W3, W4, W5, W7, W8).
• To investigate indole-containing compounds with varying structural complexity.

Main Methods:

• Joint experimental and theoretical analysis.
• Multiconformational approach.
• Raman spectroscopy of skatole, tryptophan, and Gly-Trp-Gly.

Main Results:

• The study challenges the Fermi-resonance origin assignment for the W4-W5 doublet (∼1360-1340 cm⁻¹).
• It suggests that the W7 and W8 markers (∼880 cm⁻¹ and ∼760 cm⁻¹) are not solely dependent on environmental factors.
• Reconsideration of existing assignments for several Trp Raman markers is indicated.

Conclusions:

• Existing assignments for certain Trp Raman markers need re-evaluation.
• The interpretation of the W4-W5 doublet and the environmental dependence of W7 and W8 markers require revision.
• This work provides a foundation for more accurate interpretation of Trp Raman spectra in biological systems.