Comparison of the Time-resolved Absorption and Phosphorescence from the Tryptophan Triplet State in Proteins in Solution

Role of the Triplet State and Protein Dynamics in the Formation and Stability of the Tryptophan Radical in an Apoazurin Mutant

The protein, azurin, has enabled the study of the tryptophan radical. Upon UV excitation of tyrosine-deficient apoazurin and in the presence of a Co(III) electron acceptor, the neutral radical (W48•) is formed. The lifetime of W48• in apoazurin is 41 s, which is shorter than the lifetime of several hours in Zn-substituted azurin. Molecular dynamics simulations revealed enhanced fluctuations of apoazurin which likely destabilize W48•. The photophysics of W48 was investigated to probe the precursor state for ET. The phosphorescence intensity was eliminated in the presence of an electron acceptor while the fluorescence was unchanged; this quenching of the phosphorescence is attributed to ET. The kinetics associated with W48• were examined with a model that incorporates intersystem crossing, ET, deprotonation, and decay of the cation radical. The estimated rate constants for ET (6 × 10⁶ s^–1) and deprotonation (3 × 10⁵ s^–1) are in agreement with a photoinduced mechanism where W48• is derived from the triplet state. The triplet as the precursor state for ET was supported by photolysis of apoazurin with 280 nm in the absence and presence of triplet-absorbing 405 nm light. Absorption bands from the neutral radical were observed only in the presence of blue light.

Fluorescence-based characterization of non-fluorescent transient states of tryptophan - prospects for protein conformation and interaction studies

Tryptophan fluorescence is extensively used for label-free protein characterization. Here, we show that by analyzing how the average tryptophan fluorescence intensity varies with excitation modulation, kinetics of tryptophan dark transient states can be determined in a simple, robust and reliable manner. Thereby, highly environment-, protein conformation- and interaction-sensitive information can be recorded, inaccessible via traditional protein fluorescence readouts. For verification, tryptophan transient state kinetics were determined under different environmental conditions, and compared to literature data. Conformational changes in a spider silk protein were monitored via the triplet state kinetics of its tryptophan residues, reflecting their exposure to an air-saturated aqueous solution. Moreover, tryptophan fluorescence anti-bunching was discovered, reflecting local pH and buffer conditions, previously observed only by ultrasensitive measurements in highly fluorescent photo-acids. Taken together, the presented approach, broadly applicable under biologically relevant conditions, has the potential to become a standard biophysical approach for protein conformation, interaction and microenvironment studies.

Enantioselective Quenching of Room-Temperature Phosphorescence Lifetimes of Proteins: Bovine and Human Serum Albumins

Enantioselective quenching of the room-temperature phosphorescence (RTP) lifetime of proteins was demonstrated due to the effects of various external chiral quenching agents. In the absence of quenchers, the RTP lifetimes for bovine serum albumin (BSA) and human serum albumin (HSA) were found to be 5.0 +/- 0.2 and 4.0 +/- 0.1 ms, respectively. The addition of various chiral quenchers (three pairs of binaphthols and two pairs of beta-blockers) into the deoxygenated sample solutions containing BSA and HSA reduced their RTP lifetimes significantly, i.e., from ca. 4-5 ms (in the absence) to an average lifetime of ca. 1-2 ms (in the presence) of the chiral quenchers. For the R and S enantiomers examined, marked differences in RTP lifetimes were observed, i.e., ranging from ca. 20-29% for the binaphthols to ca.14-16% for the beta-blockers. Such findings could lead to a better understanding of the relationship between chirality, dynamics/conformational changes, and biological functions of proteins.

Competition between ultrafast relaxation and photoionization in excited prefluorescent states of tryptophan and indole

The quantum yield of photoionization of TrpH and IndH from the nonrelaxed prefluorescent state S* increases with the temperature decrease. This effect is attributed to the competition between temperature independent ionization and ultrafast thermal relaxation S* --> S1. The rate constant of the relaxation does not depend on the solvent and on the presence of the amino acid side chain: the temperature dependences of photoionization quantum yield, obtained for TrpH and IndH in different solvents, practically coincide. The activation energy for the relaxation rate constant Er approximately 4.5 kJ/mol probably corresponds to intramolecular process or to the formation of the vibronically excited transient complex between photoexcited molecule and solvent molecules.

Environmental effects on the photochemistry of A2-E, a component of human retinal lipofuscin

Several retinal dystrophies are associated with the accumulation of lipofuscin, a pigment mixture, in the retinal pigment epithelium (RPE). One of the major fluorophores of this mixture has been identified as the bis-retinoid pyridinium compound, A2-E. Because this compound absorbs incident radiation that is transmitted by the anterior segment of the human eye, photophysical and photochemical studies were performed to determine if A2-E could photosensitize potentially damaging reactions. Steady-state fluorescence measurements indicate that the fluorescence emission maximum and quantum yield are very sensitive to the chemical environment and a correlation between these two parameters and the solvent dielectric constant is observed. Time-resolved absorption experiments of A2-E in pure organic solvents showed no formation of transient species on the timescale of our experiments. However, when these measurements were repeated for A2-E in Triton X-100 micelles, a short-lived (tau approximately 14 microseconds), weak absorption was observed. This species is quenched by oxygen (k = 2 x 10(9) M-1 s-1) and by the addition of the antioxidants, cysteine and N,N,N',N'-tetramethylphenylenediamine. Quenching of this species by 2,3,5-trimethylhydroquinone results in the formation of the 2,3,5-trimethylsemiquinone free radical and an increase in yield of the A2-E-derived species. Sensitization of the A2-E triplet excited state indicates that the species observed in micelles upon direct excitation is not consistent with the triplet excited state. Based on these data we tentatively assign this absorption to a free radical. In the RPE these initial processes can ultimately lead to damage to the tissue through the formation of peroxides and other oxidized species.

Measuring the rate of intramolecular contact formation in polypeptides

Formation of a specific contact between two residues of a polypeptide chain is an important elementary process in protein folding. Here we describe a method for studying contact formation between tryptophan and cysteine based on measurements of the lifetime of the tryptophan triplet state. With tryptophan at one end of a flexible peptide and cysteine at the other, the triplet decay rate is identical to the rate of quenching by cysteine. We show that this rate is also close to the diffusion-limited rate of contact formation. The length dependence of this end-to-end contact rate was studied in a series of Cys-(Ala-Gly-Gln)(k)-Trp peptides, with k varying from 1 to 6. The rate decreases from approximately 1/(40 ns) for k = 1 to approximately 1/(140 ns) for k = 6, approaching the length dependence expected for a random coil (n(-3/2)) for the longest peptides.