Biology Reference
In-Depth Information
nitr-2 is no longer used. For the three remaining nitr compounds, photolysis is
most e
cient at the absorbance maximum for the nitrobenzhydrol group, about
360 nm, although light between 330 and 380 nm is nearly as e
Y
V
ective. The quantum
ciency of the Ca
2
þ
-bound form is about 1/25 (nitr-5, 0.035 and nitr-7, 0.042) and
is somewhat less in the Ca
2
þ
-free form (0.012 and 0.011). The absorbance at this
wavelength is 5500 M
1
cm
1
(decadic molar extinction coe
Y
e
cient) for nitr-5 and
nitr-7, and 11,000 M
1
cm
1
for nitr-8. The structures of the nitr series of com-
pounds are given in
Fig. 1
; and the photochemical reaction of the most popular
member of this group, nitr-5, is shown in
Fig. 2
. The physico-chemical properties
of these and other photosensitive chelators are summarized in
Table I
.
These chelators share the advantages of the parent BAPTA chelator: high
specificity for Ca
2
þ
over H
þ
and Mg
2
þ
(Mg
2
þ
a
Y
Y
nities, 5-8 mM),
lack of
dependence of Ca
2
þ
a
Y
nity on pH near pH 7, and fast bu
V
ering kinetics. One
limitation is that the drop in a
nity in the nitr compounds after photolysis is
relatively modest, about 40-fold for nitr-5 and nitr-7. The Ca
2
þ
a
Y
nity of nitr-5
drops from 0.15 to 6
m
M at 120-mM ionic strength after complete photolysis.
These a
Y
nities must be reduced at higher ionic strength, roughly in proportion to
the tonicity (
Tsien and Zucker, 1986
). By incorporating a cis-cyclopentane ring
into the bridge between the chelating ether oxygens of BAPTA, nitr-7 was created
Y
COO
−
COO
−
COO
−
COO
−
COO
−
COO
−
COO
−
COO
−
N
N
N
N
O
O
O
O
Me
OH
OH
O
Me
O
H
H
nitr-5
nitr-7
O
O
NO
2
NO
2
COO
−
COO
−
COO
−
COO
−
COO
−
COO
−
N
N
N
OMe
OO
OH
HO
OH
O
O
O
H
H
H
nitr-9
nitr-8
O
O
O
NO
2
NO
2
O
2
N
Fig. 1
Structures of the nitr series of photolabile chelators, which release calcium on exposure to light.
