Chemistry Reference
In-Depth Information
visible light to aromatic compounds (e.g., naphathalene and anthracene) and
conjugated alkenes (e.g., dye molecules and porphyrins) yielded excited states
that went through rapid and efficient energy transfer to O
2
to generate
1
O
2
[150,
151]. generally, energy transfer to produce
1
O
2
(
k
= 1-3 × 10
9
/M/s) competes
with electron transfer (
k
≤ 1 × 10
7
/M/s), and thus, many photosensitizers form
both
1
O
2
and
O
•−
[152]. The overall yields of
1
O
2
depend on the reaction condi-
tions, excitation wavelength, and type of sensitizer [136, 152, 153]. The pH,
solvent, and temperature can significantly affect the quantum yields [135, 154,
155]. The quantum yields of
1
O
2
obtained from a number of biological mole-
cules have been summarized [150]. Examples include quantitative character-
ization of
1
O
2
in UVA excitation of 6-thioguanines in aqueous solution [156].
Recently, the generation of
1
O
2
through the irradiation of air-saturated
solutions of Phe, Tyr, and Trp in their zwitterionic forms and their methyl esters
as well as of a few test proteins and immunoglobulins, in D
2
O or MecN using
266-nm pulse from a frequency quadrupled continuum Nd:yAg laser (8 ns,
3 mJ), was analyzed quantitatively [132]. This study reported quantum yields
for generation of
1
O
2
by biological macromolecules. The lifetime of
1
O
2
in D
2
O
was longer than in H
2
O, which was related to the role of high-frequency O-H
vibrational modes in nonradiative electronic-to-vibrational energy transfer
processes which lead to deactivation of
1
O
2
[13]. The H
2
O/D
2
O test is thus a
versatile tool to study
1
O
2
in biological systems [157]. In most of the solvents,
τ
Δ
varied from 1 to 100 µs. The τ
Δ
in water was determined to be 2-4 µs at
neutral pH [158]. The following reactions (e.g., Phe) explain the formation of
1
O
2
(Eqs. 4.34-4.37):
Phe
+ →
h
ν
1
Phe
(4.34)
1
Phe
→
3
Phe
(4.35)
3
Phe O
+
3
→
Phe O
+
1
(4.36)
2
2
1
O
→
1
O
+
h
ν (
1270
nm
).
(4.37)
2
2
The quantum yields of
1
O
2
were obtained by measuring the yields of its
near-infrared (IR) emission (Eq. 4.37). A strong peak at ∼1270 nm is the phos-
phorescence characteristic of a singlet oxygen. The kinetic traces in the inset
of Figure 4.8 indicate a lifetime (τ
Δ
) of 48 µs, similar to an earlier reported
value of 53 ± 5 µs [158]. Table 4.5 reports the quantum yields of
1
O
2
emission.
The protonation state of the α-amino group in the aromatic amino acids had
the influence on the quantum yields. For example, the quantum yields of
N-acetylated amino acids were slightly greater than those of their free amino
acids (Table 4.5). An equimolar mixture of free aromatic amino acids in solu-
tion had a weighted average
1
O
2
quantum yield of 0.08, double that of the
quantum yields for proteins and immunoglobulins (Table 4.5). An increase in
viscosity of the matrix of protein could hinder and also limit the generation
of
1
O
2
.
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