On the quantitative molecular analysis of electronic energy transfer within donor-acceptor pairs

An extended Förster theory (EFT) on electronic energy transfer is presented for the quantitative analysis of time-resolved fluorescence lifetime and depolarisation experiments. The EFT, which was derived from the stochastic Liouville equation, yields microscopic information concerning the reorientation correlation times, the order parameters, as well as inter chromophoric distances. Weakly interacting donor and acceptor groups, which reorient and interact in a pair wise fashion, are considered, under isotropic and anisotropic conditions. For the analysis of experiments it is shown that not only do we need to consider the orientational distributions of the transition dipoles, but the internal reorienting molecular dynamics within the pair which is of even greater importance. The latter determines the shape as well as the rate of the observed donor fluorescence and depolarisation decays, which are most often not mono-exponential functions. It is shown that the commonly used Förster theory is a special case of the EFT. Strategies are presented for applying the EFT, which makes use of Brownian dynamics simulation.

Extended Förster theory for determining intraprotein distances: 2. An accurate analysis of fluorescence depolarisation experiments

The extended Förster theory (EFT) is for the first time applied to the quantitative determination of the intramolecular distances in proteins. It is shown how the EFT (J. Chem. Phys., 1996, 105, 10896) can be adapted to the analyses of fluorescence depolarisation experiments based on the time-correlated single photon counting technique (TCSPC). The protein system studied was the latent form of plasminogen activator inhibitor type I (PAI-1), which was mutated and labelled by the thiol reactive BODIPY(R) derivative {N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-yl)methyl iodoacetamide}. The energy migration occurs within pairs of photophysically identical donor groups that undergo reorientational motions on the timescales of energy migration and fluorescence relaxation. Unlike all models currently used for analysing fluorescence TCSPC data, the EFT explicitly accounts for the time-dependent reorientations that influence the rate of electronic energy transfer/migration in a complex manner. The complexity is related to the "kappa(2) problem", which has been discussed for years. The EFT brings the analyses of DDEM data to the same level of molecular description as in ESR and NMR spectroscopy, i.e. it yields microscopic information about the reorientation correlation times, the order parameters, as well as inter-chromophoric distances.

On the Quantitative Treatment of Donor–Donor Energy Migration in Regularly Aggregated Proteins

An algorithm is presented that quantitatively accounts for donor-donor energy migration (DDEM) among fluorophore-labeled proteins forming regular aggregates. The DDEM algorithm is based on Monte Carlo and Brownian dynamics simulations and applies to calculation of fluorescence depolarisation data, such as the fluorescence anisotropy. Thereby local orientations, as well as reorienting motions of the fluorescent group are considered in the absence and presence of DDEM and among, in principle, infinitely many proteins as they form regular aggregates. Here we apply the algorithm for calculating and illustrating the DDEM and the time-resolved fluorescence anisotropy under static as well as dynamic conditions within helical, linear and circular aggregate structures. A principal approach of the DDEM algorithm for analysing protein aggregates is also outlined.

Structural Bases of the Redox-dependent Conformational Switch in the Serpin PAI-2

Depending on the redox-status, the serpin plasminogen activator inhibitor type 2 (PAI-2) can exist in either a stable monomeric or polymerogenic form. The latter form, which spontaneously forms loop-sheet polymers, has an open beta-sheet A and is stabilized by a disulfide bond between C79 (in the CD-loop) and C161 (at the bottom of PAI-2). Reduction of this bond results in a closing of the beta-sheet A and converts PAI-2 to a stable monomeric form. Here we show that the stable monomeric and polymerogenic forms of PAI-2 are fully interconvertible, depending on redox-status of the environment. Our intramolecular distance measurements indicate that the CD-loop folds mainly on one side of the stable monomeric form of the inhibitor. However, the loop can translocate about 54A to the bottom of PAI-2 so that the C79-C161 disulfide bond can form under oxidizing conditions. We show also that the redox-active C79 can form a disulfide-link to the matrix protein vitronectin, suggesting that vitronectin can stabilize active PAI-2 in extracellular compartments. PAI-2 is therefore a rare example of a redox-sensitive protein for which the activity and polymerization ability are regulated by reversible disulfide bond formation leading to major translocation of a loop and significant conformational changes in the molecule.