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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.