Biology Reference
In-Depth Information
the one in the screenshot below. One simply chooses the calculation type at the top,
enter the temperature, pH, and ionic strength (line 2) and the two known
Ca-EGTA concentrations. Not only are the traditional find free Ca 2 þ and find
total Ca 2 þ calculations performed, but also the occasionally useful find total
EGTA given the free and total (or free and bound) Ca 2 þ levels can be performed.
There are also the slightly more complex versions for Ca-Mg-ATP-EGTA equili-
bria. Finally, there is the more comprehensive version (Web MaxC) that allows any
combination of cations (Al 3 þ ,Ba 2 þ ,Ca 2 þ ,Cd 2 þ ,Cu 2 þ ,Fe 2 þ ,Mg 2 þ ,Sr 2 þ ,Zn 2 þ )
and 12 di
erent chelators (including EGTA, BAPTA, Br 2 BAPTA, EDTA, ATP,
ADP, and citrate), but the simplicity and functionality are the same as the simple Ca-
EGTA version above. There are also versions posted that use the Schoenmakers
constants and conditions and other versions will be posted as they are written. These
programs can thus be helpful in designing solutions with particular free ion con-
centrations, but should be used with understanding of the limitations.
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V. Discussion and Summary
ered [Ca 2 þ ], and often
these solutions are complicated by multiple equilibria, and theoretical and practi-
cal limitations. Here we have discussed some of the basic principles that are
involved, several key factors that complicate the process and provide some practi-
cal tools and advice to increase the probability that one can make the desired
solution. However, neither the calculations nor the solution preparation nor
measurement are foolproof. One must be alert to some of the potential caveats,
and make independent measurements when possible.
Often it is useful to make a very careful set of calibration standards at a selected
ionic strength, temperature, and pH, using simpler solutions (e.g., containing
simple Ca-EGTA bu
It is important to be able to prepare solutions with bu
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ers) for standardization of either a Ca 2 þ electrode or fluo-
rescent Ca 2 þ indicator (as in Fig. 6 ). Note also that there are [Ca 2 þ ] solution sets
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