Oxygen is an efficient quencher of fluorescence because it is a triplet in its electronic ground state. The unpaired spins of O2 can induce the excited state of the fluorescent molecule to undergo intersystem crossing from the singlet state to the triplet state.
(a) How would you verify the system experimentally?
(b) Assume that the quenching rate constant (the rate constant for the collision between O2 and fluorescent molecules is 1.0 x 1010 M-1 s -1 . How many collisions per second on average does each fluorescent molecule in solution experience if the [O2] = 3.4 x 10-4 M and [F] = 0.5 M, where F is the fluorescent molecule.
(c) The fluorescence lifetime of pyrene, a molecule that is often used to probe biological systems, is 500 ns, while that of tryptophan is about 5 ns. Explain why under normal atmospheric conditions O2 can interfere only with the fluorescence of pyrene but not that of tryptophan.
(d) A quantitative relationship of fluorescence quenching is the Stern-Volmer equation Io/I = 1 + kqto[Q] Where Io and I are the fluorescence intensities in the absence and presence of the quencher resepctively, kq is the quenching rate constant, to is the lifetime of the fluorescent state in the absence 2 of the quencher, and [Q] is the concentration of quencher. Use this equation to support your relationship in (c).
Oxygen is an efficient quencher of fluorescence because it is a triplet in its electronic ground state. The unpaired spins of O2 can induce the excited state of the fluorescent molecule to undergo intersystem crossing from the singlet state to the triplet state.
(a) How would you verify the system experimentally?
(b) Assume that the quenching rate constant (the rate constant for the collision between O2 and fluorescent molecules is 1.0 x 1010 M-1 s -1 . How many collisions per second on average does each fluorescent molecule in solution experience if the [O2] = 3.4 x 10-4 M and [F] = 0.5 M, where F is the fluorescent molecule.
(c) The fluorescence lifetime of pyrene, a molecule that is often used to probe biological systems, is 500 ns, while that of tryptophan is about 5 ns. Explain why under normal atmospheric conditions O2 can interfere only with the fluorescence of pyrene but not that of tryptophan.
(d) A quantitative relationship of fluorescence quenching is the Stern-Volmer equation Io/I = 1 + kqto[Q] Where Io and I are the fluorescence intensities in the absence and presence of the quencher resepctively, kq is the quenching rate constant, to is the lifetime of the fluorescent state in the absence 2 of the quencher, and [Q] is the concentration of quencher. Use this equation to support your relationship in (c).
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