Modeling of vibrational spectra of L-tryptophan in condensed states

Background and Objectives: This work is devoted to the interpretation of IR and Raman spectra of Trp in the condensed states. For this purpose, we calculated the complexes in the zwitterionic form of Trp with the water molecules. Obtained results allow us to determine the influence of hydrogen bonds on the vibrational spectra of Trp in the aqueous solution and solid state. Materials and Methods: The calculation of the normal modes and intensities of IR and Raman spectra of Trp was performed using Gaussian 09 software package based on the DFT method with the use of the B3LYP/6-311++G(d,p) functional. We used the reaction field model SCRF (the dielectric constant $\varepsilon$=78.39). As the structural models, we considered the complexes of Trp with one and four water molecules. Results: The calculation results and comparison with experiment showed that for the simulation of the vibrational spectra of Trp in the zwitterionic form in the aqueous solution the most appropriate for application was the complex of Trp with the single water molecule positioned between bipolar groups, and for the simulation of the spectra in the solid state – the complex of Trp with four water molecules. Conclusion: It is shown that the lengths of the hydrogen bonds in the complex of Trp with one water molecule for N$^{+}$H… O$_{w}$ and O$_{w}$ H…O are equal to 2.82 and 2.68 Å correspondingly, and the energy of the hydrogen bond – 5.65 kcal/mol; the length of the hydrogen bonds in the complex of Trp with four water molecules vary in the range from 2.77 to 2.92 Å. The forming of the hydrogen bonds between the ionic groups of Trp in the zwitterion form and water molecules leads to an increase in the frequency of the valence vibration of N$^{+}$H bond at 30–200 cm$^{-1}$.