14.7 For a sample in a 1-cm cuvette, if the extinction coefficient of a molecule is 1 × 104 M−^1 cm−^1
at 500 nm, what is the absorbance of a sample at 500 nm if the concentration is (a) 1μM
and (b) 0.2 mM?
14.8 A protein sample is measured to have absorbance values of 0.717 and 0.239 at 240 and
280 nm, respectively. Assume that the spectrum is due to the presence of the aromatic
amino acid residues tryptophan and tyrosine and determine the concentrations of these
residues using the following extinction coefficients.
Wavelength (nm) Extinction coefficient (M−−^1 cm−−^1 )Tyrosine Tryptophan240 11,300 1960
280 1500 538014.9 For a given electron how is the rate of stimulated emission related to the light energy
density?
14.10 What is the physical basis for the restriction of Δl=±1 for an electronic transition?
14.11 Describe the difference between a stimulated and spontaneous process.
14.12 Why can a stimulated emission process be used to make a laser and not spontaneous
emission?
14.13For a given molecule, how is the fluorescence spectrum related to the absorption spectrum?
14.14 What factors give rise to the linewidth in biological samples?
14.15What is the FRET efficiency if the value of the parameter rois 10 Å and the distance between
the energy donor and acceptor is (a) 1 Å, (b) 10 Å, and (c) 20 Å?
14.16 If the observed lifetime of an excited state is 1.0 ns−^1 and the natural radiative rate con-
stant is 0.1 ns−^1 , what is the nonradiative rate constant?
14.17 For bacterial reaction centers (Chapter 20), the quantum yield of fluorescence is 10−^4.
The natural radiative decay rate is 10^8 s−^1. Assuming that there are no other nonradiative
processes, what is the rate constant for the initial transfer of an electron from the excited
state?
14.18 Why do some photosynthetic organisms have chlorosomes attached to the cell membrane?
14.19 What is the interpretation of an off-diagonal peak in a two-dimensional optical spectrum?
14.20 What can be measured in a single-molecule experiment that can only be inferred in a
bulk experiment?
14.21 Why does the measured switching time between conformations vary in a single-molecule
experiment?
14.22 Explain the signals shown for a FRET measurement of a Holliday junction; be sure to
relate the signals to the three equilibrium states of the Holliday junction. Cy3 is the donor
and Cy5 is the acceptor.