Organic Chemistry

042 CHAPTER 27 Nucleosides, Nucleotides, and Nucleic Acids

2.More solvation in the products. Negatively charged ions are stabilized in an

aqueous solution by solvation. Because the reactant has 3.3 negative charges,

while the sum of the negative charges on the products is 3.9 there is

more solvation in the products than in the reactant.

3.Greater resonance stabilization in the products. Delocalization of a lone pair

on the oxygen joining the two phosphorus atoms is not very effective because it

puts a positive charge on an oxygen that is next to a partially positively charged

phosphorus atom. When the phosphoanhydride bond breaks, one additional lone

pair can be effectively delocalized.

Similar factors explain the large negative when ATP is hydrolyzed to AMP

and pyrophosphate, and when pyrophosphate is hydrolyzed to two equivalents of

phosphate.

PROBLEM 7 SOLVED

ATP has values of 0.9, 1.5, 2.3, and 7.7; ADP has values of 0.9, 2.8, and 6.8; and

phosphoric acid has values of 1.9, 6.7, and 12.4.

Do the calculation showing that at pH 7.3:

a. the charge on ATP is

b. the charge on ADP is

c. the charge on phosphate is

SOLUTION TO 7a Because pH 7.3 is much more basic than the values of the first

three ionizations of ATP, we know that these three groups will be entirely in their basic

forms at that pH, giving ATP three negative charges. We need to determine what fraction of

the group with 7.7 will be in its basic form at pH 7.3.

Because this equation has two unknowns, one of the unknowns must be expressed in terms

of the other unknown. Using the definition of the acid dissociation constant we can

define in terms of and

3 A- 4

3 A- 4 + 3 HA 4

=

3 A- 4

3 A- 4 +

3 A- 43 H+ 4

Ka

=

Ka

Ka+ 3 H+ 4

3 HA 4 =

3 A- 43 H+ 4

Ka

Ka=

3 A- 43 H+ 4

3 HA 4

3 HA 4 3 A- 4 ,Ka, 3 H+ 4.

(Ka),

3 HA 4 =concentration of the acidic form

3 A- 4 =concentration of the basic form

concentration in the basic form

total concentration

=

3 A- 4

3 A- 4 + 3 HA 4

pKa

pKa

• 1.1


• 2.8


• 3.3

pKa

pKa pKa

¢G°¿

O −O −OH −O

O O

O

P

−

−O

+

O

P

O

O

P

−

O

O

P

−

three lone pairs

can be effectively

delocalized

four lone pairs

can be effectively

delocalized

1 2.8+1.1 2 ,