Experiments based on Förster resonance energy transfer (FRET) are widely used to obtain information on
conformational dynamics of biomolecular systems. To reliably measure FRET, accurate knowledge of photophysical
properties of the used fluorophores is indispensable. In high FRET constructs donor (D) and acceptor (A) fluorophores
can approach each other close enough that electronic interactions might occur. When separated by distances on the order
of van der Waals radii, photophysical properties can be changed reversibly, opening new non-radiative relaxation
pathways, or irreversibly, chemically altering the fluorophores. Even transient contacts can thus compromise accurate
FRET measurements. To study FRET and competing D-A contact-induced processes we labeled the amino acid cystein
(Cys) with two fluorophores. A donor (D; TMR or Cy3B) was attached to the thiol group and an acceptor (A; Atto647N)
to the amino group of Cys. Absorption spectroscopy, steady-state fluorescence spectroscopy, and time-correlated single-photon
counting (TCSPC) were used to characterize the different A-Cys-D complexes at the ensemble level. In addition,
we performed single-molecule FRET experiments using alternating-laser excitation to study the heterogeneity of the
FRET-systems. We identified competing quenching processes severely changing D and A quantum yields upon
fluorophore contact. The results are applicable for quantitative analysis of FRET in dynamic molecular systems that
allow transient contact between D and A fluorophores.
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