Biacore T100 Software Handbook BR-1006-48 Edition AE
169
Kinetics and affinity analysis 9
T-value corresponds to a low standard error. As a general guideline, parameters
with a T-value greater than about 10 should be regarded as significant.
The choice of whether to display parameter significance as standard error or T-
value is made on the
Fit
tab of the
Tools:Preferences
dialog.
The significance of a parameter is a measure of how much a change in the
parameter value affects the closeness of fit. A parameter with low significance
can have a wide range of values without affecting the fit. Typically (but not
always), parameters with a low significance have unreasonable values: for
example typical values for the mass transfer constant for proteins are around
10
8
RU·M
-1
s
-1
, but evaluation of data with no mass transfer limitation might
return a value of 10
12
or higher. Similarly, rate constants that lack significance
are often assigned values outside the reasonable range for biomolecular
interactions, or outside the range that can be measured with Biacore.
Notes:
The standard error and the
Check Kinetic Data
tool assess parameter
significance in different ways, even if the results of the assessment may
sometimes be related.
Check Kinetic Data
tests the contribution of a
group of parameters (rate constants for the interaction and mass
transport processes) to the closeness of fit by examining the results of
correlated changes, whereas the standard error is a mathematical
assessment of the significance of a single parameter. If the
Check Kinetic
Data
tool indicates that the rate constants are not significant, the
standard error for the constants may be expected to be high. However,
the converse is not always true (a high standard error will not always be
reflected in the behavior of the
Check Kinetic Data
tool).
Even if parameters with low significance can have a wide range of values
without affecting the fit, repeated evaluation of the same data set will
always return the same value for all parameters. Consistency of a value
between repeated evaluations is
not
a test of significance.
U-value
The U-value is an estimate of the
uniqueness
of the calculated values for rate
constants and R
max
. If parameters are correlated, the fitting procedure can
determine their relative magnitudes but not absolute values (for example,
knowing the affinity gives the ratio but not the values for rate constants). The U-
value is determined by testing the dependence of the fit on correlated variations
in pairs of parameters, and is reported on the
Report
tab as a single value for
the whole fitting. U-values above about 25 indicate that absolute values for two
or more of the parameters (rate constants and R
max
) are correlated and cannot
be determined. If the U-value is below about 15 the parameter values are not
significantly correlated.
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