Structural heterogeneity in the collision complex between organic dyes and tryptophan in aqueous solution.

The heterogeneity on photoinduced electron transfer (PET) kinetics between a labeled fluorophore and an amino acid residue has been extensively studied in biopolymers. However in aqueous solutions, the heterogeneity on PET kinetics between a fluorophore and a quencher has rarely been reported. Herein, we selected four commonly used fluorophores, such as tetramethylrhodamine (TMR), Rhodamine B (RhB), Alexa fluor 546 (Alexa546), and Atto655, and studied their respective PET kinetics in 50 mM tryptophan solutions with femtosecond transient absorption spectroscopy to explore the structural heterogeneity in their corresponding collision complexes. We measured the decay of the first excited electronic state of respective fluorophore with and without 50 mM tryptophan in aqueous solutions, and derived the charge separation rate in their corresponding collision complexes. We found that the PET process of all selected fluorophores in 50 mM tryptophan solutions has two charge separation rates, which indicates that the relevant states in the collision complex between respective fluorophore and tryptophan have strong structural heterogeneity. These femtosecond PET measurements are in agreement with Vaiana's molecular dynamics simulation (J. Am. Chem. Soc.2003, 125, 14564). In addition, with the obtained PET kinetic parameters, we derived the relative brightness of the collision complex between respective fluorophore and tryptophan, which are important parameters for the PET based fluorescence correlation spectroscopy study involving these fluorophores in biopolymers.