Use of nonradiative decays of extrinsic fluorophores as structural and dynamical probes in protein environments: Fluorescence quenching

Oxygen-Insensitive Aggregates of Pt(II) Complexes as Phosphorescent Labels of Proteins with Luminescence Lifetime-Based Readouts

The synthesis and photophysical properties of a tailored Pt(II) complex are presented. The phosphorescence of its monomeric species in homogeneous solutions is quenched by interaction with the solvent and therefore absent even upon deoxygenation. However, aggregation-induced shielding from the environment and suppression of rotovibrational degrees of freedom trigger a phosphorescence turn-on that is not suppressed by molecular oxygen, despite possessing an excited-state lifetime ranging in the microsecond scale. Thus, the photoinduced production of reactive oxygen species is avoided by the suppression of diffusion-controlled Dexter-type energy transfer to triplet molecular oxygen. These aggregates emit with the characteristic green luminescence profile of monomeric complexes, indicating that Pt-Pt or excimeric interactions are negligible. Herein, we show that these aggregates can be used to label a model biomolecule (bovine serum albumin) with a microsecond-range luminescence. The protein stabilizes the aggregates, acting as a carrier in aqueous environments. Despite spectral overlaps, the green phosphorescence can be separated by time-gated detection from the dominant autofluorescence of the protein arising from a covalently bound green fluorophore that emits in the nanosecond range. Interestingly, the aggregates also acted as energy donors able to sensitize the emission of a fraction of the fluorophores bound to the protein. This resulted in a microsecond-range luminescence of the fluorescent acceptors and a shortening of the excited-state lifetime of the phosphorescent aggregates. The process that can be traced by a 1000-fold increase in the acceptor's lifetime mirrors the donor's triplet character. The implications for phosphorescence lifetime imaging are discussed.

Multiple binding sites of fluorescein isothiocyanate moieties on myoglobin: photophysical heterogeneity as revealed by ground- and excited-state spectroscopy

Fluorescein isothiocyanate (FITC)-myoglobin conjugates were synthesized with a binding stoichiometry of one to three fluorophores per protein. FITC binding sites were determined by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS). Five lysine residues and the N-terminal amino group were identified as preferential binding sites. The ground and excited-state absorption spectra and the fluorescence decay of the conjugates in the native and denatured state of the carrier protein were analyzed. For comparison, unbound FITC and FITC covalently bound to a polysaccharide (dextran) were studied. For FITC, FITC-dextran and the FITC-myoglobin conjugates, only one FITC absorption peak was obtained in the ground state spectrum. Similarly, the excited state absorption (ESA) spectra of unbound FITC and of FITC-dextran showed only one single maximum whereas two maxima were detected for the native FITC-myoglobin conjugates. One of these sub-bands disappeared following urea treatment of the conjugate. We conclude that ESA measurements of extrinsic fluorophores on proteins can be used to monitor different micro-environments of the fluorophore and to distinguish between different conformational states of the labeled protein. This method can be a useful tool for analysing coexisting protein conformations.