ATP has p Ka values of 0.9,1.5,2.3, and 7.7 ; ADP has p Ka...

ATP has $p K_{a}$ values of $0.9,1.5,2.3,$ and $7.7 ;$ ADP has $p K_{a}$ values of $0.9,2.8,$ and $6.8 ;$ and the alkyl phosphate has $\mathrm{p} K_{\mathrm{a}}$ values of $1.9$ and $6.7 .$ Do the calculation showing that at $\mathrm{pH} 7.4$ the charge on ATP is $-3.3.$

Key Concepts

Net Charge Calculation

To compute the overall charge of a polyprotic molecule at a specific pH, one must combine the contributions from each ionizable group. This involves determining the fraction of proton loss for each group using the Henderson-Hasselbalch equation, and then summing the resulting charges. The total charge is often non-integral because it reflects a statistical average over a mixture of ionization states.

Fractional Ionization

Each acidic group in a polyprotic acid is not necessarily 100% deprotonated or protonated at a given pH. Fractional ionization quantifies the degree to which an individual proton is lost, based on the pH relative to its pKa. This concept is central to calculating intermediate charges, such as the -3.3 charge on ATP at pH 7.4.

Polyprotic Acids

Polyprotic acids have more than one proton that can dissociate, with each dissociation event characterized by its own acid dissociation constant (pKa). In molecules like ATP, different groups lose protons at different pH values, resulting in a mixture of protonated and deprotonated forms. Understanding polyprotic behavior is essential for predicting how the molecule’s charge varies with pH.

Acid Dissociation Constant (pKa)

The pKa value represents the pH at which half of a particular acidic group is deprotonated. It is a key parameter in determining the ionization state of acidic groups in a molecule. By comparing the pH to each pKa of the molecule, one can predict the degree to which each proton is dissociated.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates pH, pKa, and the ratio of the concentrations of the deprotonated and protonated forms of an acid. It is given by the formula pH = pKa + log([A?]/[HA]), and is used to calculate the fractional ionization of each group in polyprotic molecules like ATP.

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