Redshift of Excitation Wavelength Caused by the Concentration of L-Tryptophan in Water: A Theoretical and Experimental Study

A redshift in the wavelength of excitation spectra is experimentally measured as a function of the concentration parameter for tryptophan solutions in water. To understand the microscopic causes of this behavior, theoretical calculations obtained from four model clusters are carried out: (Trp)1 - (H2O)9, (Trp)2 - (H2O)18, (Trp)3 - (H2O)27 and (Trp)4 - (H2O)36, where there are interactions among 1, 2, 3 and 4 molecules of tryptophan. According to the literature, each interaction occurred with nine molecules of water to stabilize its expected zwitterionic form. In these models, the molecules of tryptophan appear at an adjacent distance among them to generate an analogous behavior when there is an experimental increase in the concentration. It is evident that the distance between adjacent molecules of tryptophan decreases as their concentration increases. The optical properties of these clusters are obtained by studying the corresponding excited states and the molecular orbitals involved, showing charge transfers by using time-dependent density functional theory (TD-DFT) methods. The experimental spectroscopic data are obtained by using the clusters proposed, and good agreement is found by drawing a comparison with the theoretical data.