Chemistry Reference
In-Depth Information
independent, it is safe to acquire multiple (
2)
R
2
dispersion data sets at different
B
0
field strength. When two sets of
R
2
dispersion data recorded at two magnetic field
strengths are analyzed for each residue, the unknown parameters for optimization are
p
a
,
k
ex
,
>
,and
R
2
at one
B
0
field, and
R
2
at another
B
0
field strength. Third, even
when
R
2
data recorded at two magnetic field strengths are available, the parameters
may not be well optimized because the experimentally varied
do
/2
p
n
CP
range may not be
sufficiently wide. Thus, it is beneficial if
R
2
value(s) are determined by independent
experiments so that the number of unknown parameters is decreased. Several reports of
R
2
determinations have beenmade [
14
,
74
,
122
,
135
]. However, such determination of
R
2
by other methods may not be necessarily performed because of the added measure-
ment time and when the group fit will be performed subsequently.
The group fit is useful to extract
p
a
and
k
ex
of the group of residues [
116
,
136
-
138
].
As described above,
p
a
and
k
ex
may not be accurately determined by the individual fit.
Fig. 5 Ribbon presentation of HIV-1 protease and the overview of the regions CPMG R
2
dispersion profiles for (a,
red
) the terminal
b
-sheet region, (b,
gray
) the core of the protein, and
(c,
blue
) the flap region. The terminal
-sheet residues exhibited significant chemical exchange
(
p
a
~ 0.94 and
k
ex
~ 650 s
1
) by the CPMG R
2
dispersion experiments [
137
]. In contrast, the flap
region exhibited too large R
2
values in the CPMG R
2
dispersion experiments to be analyzed [
137
].
However, the flap region had been found to undergo conformational exchange by the model-free
analysis previously [
141
]. To prevent misinterpretation of data, since CPMG R
2
dispersion
experiments can detect exchange in a limited time scale, inspection of R
2
is important as well as
evaluation of the optimized parameters
b
