Steady-state and lifetime fluorescence studies of pyrene probes in model membranes: trailblazing analysis of two-dimensional quenching kinetics with nitroxide probes and excimer formation

Manuel, Miguel Alexandre da Silva

http://hdl.handle.net/10400.1/7808

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Authors

Manuel, Miguel Alexandre da Silva

Advisor(s)

Martins, Jorge

Melo, Eurico

Abstract(s)

Many fundamental biochemical processes occur in biological membranes, a great number of which depend on lateral diffusion, such as the mitochondrial respiratory chain, the photosynthetic electron transport system and sensory and regulatory processes. Lateral diffusion in membranes has been exhaustedly studied but seldom has a theoretical formalism adequate to 2D reactions been used. In his work, the kinetic formalism introduced by Razi Naqvi in 1974 was used to analyze fluorescence quenching and excimer of several pyrene probes. We found that the fluorescence quenching of py10-PC by the spin probe 10-DOXYL-PC is, within the experimental error, in complete accordance with the theoretical prediction. However, for the system involving py6-PC quenched by 5-DOXYL-PC, large deviations are perceived, with the quenching efficiency appearing to be higher than the theoretical prediction, but increasing the collisional distance allows for a very good approximation with the experimental values. We discuss several possible influences that may explain this situation, which analogously occurred in the quenching of free pyrene by the same spin-probe. We show that the excimer formation of py10-PC and py6-PC is in complete accordance with the theoretical prediction, until probe molar proportion of 2 and 4 mol %, respectively. Above these proportions, the experiments display a quenching efficiency lower than expected. Molecular dynamics have shown increased ordering and more tight packing in the py10-PC vicinity and interdigitation of pyrene group into the apposed leaflet, accounting for the diminishing diffusion of both probes. We also found that high cholesterol content severely hinders the diffusion dynamics of both probes, this decrease seems to result from the organization introduced by the cholesterol presence. The results overall presented are proof that it is possible to gain valuable knowledge of the lipid membrane by studying the kinetics of reactions in 2D with the adequate formalism, while mainly using steady-state fluorescence.