1 (622)

Prévia do material em texto

Solutions for Carbohydrates and Nucleic Acids
50(b) continued
N
N
O
N
H
H
N N
CH3
O
H
O
N
N H
O
N
H
H
N
N
ribose
ribose riboseN
N
NN
N
H
H
Nucleosides are less rapidly hydrolyzed in aqueous acid because the site of protonation (the N in 
adenosine, and in cytidine, the oxygen shown with the negative charge in the second resonance form) is 
much less basic than the aliphatic amine in an aminoglycoside as shown in part (a). Nucleosides require 
stronger acid, or longer time and higher temperature, to be hydrolyzed.
 This is important in living systems, as it would cause genetic damage or even death of an organism if 
its DNA or RNA were too easily decomposed. Organisms go to great lengths and expend considerable 
energy to maintain the structural integrity of their DNA.
51
ribose
guanine
cytosine
adenine thymine
The polar resonance forms show how the hydrogen bonds are particularly strong. Each oxygen has 
significant negative charge, and in each pair, one H—N is polarized more strongly because the N has 
positive charge.
O OH
H
H
OH
OH
H
H
HO
H
CH2OH
OH
HO
OH
OH
HO O
H
H
H
H
H
OHH
OHH
CHO
HHO
OHH
CH2OH
H
H
H
H
O
OH
HO
OH
OH
HO
H OH
H
OH
H
HO O
H
H
OH
H
OH
H
H
OH
H
O
H
H
OH
OH
HO
OH HN
HO
OH
OH
HO O
H
H
H
H
H
COCH3
52
(a) (b) (c)
53
(a) (b)
(c) (d)
54
(a) D-(−)-ribose (b) D-(+)-altrose (c) L-(+)-erythrose (d) L-(−)-galactose (e) L-(+)-idose
2 3
4
617