BIOINORGANIC CHEMISTRY A Short Course Second Edition

NUCLEIC ACIDS 51

averaged) structures obtained from X - ray crystallography or NMR. When
changing the pucker phase angle from 0 ° to 360 ° , one steps through all possible
conformations of the ribose ring. This pseudorotation cycle can be depicted as
a conformational wheel such as that found in Figure 2.8 of reference 14. Figure
9.3 of reference 14 shows a view of C 2 ′ - endo and C 3 ′ - endo conformations
observed in A - and B - type polynucleotides.
The distance between two subsequent base pairs along the helical axis is
called helical rise (h). The pitch (P) is the length of the helix axis for one
complete helix turn. The turn angle per nucleotide or twist angle ( t ) is given
by 360 ° /number of nucleotides per turn. Data describing some properties of

Figure 2.15 Complimentary antiparallel double - stranded (ds) DNA.

O

H

H H

H

H 2 C

H

O

P

O-

• OO

A

O

H

H

H H

H 2 C

H

O

P

O

• O O
  C

O

H

H

H

H

H 2 C

H

O

P

O

-O O

G

O

H

O

H

H H

H 2 C

H

O

P

O

• O O

T

O

H

H

H H

CH 2

H

O

P

• O O-

O

A

O

H

H

H H

CH 2

H

O

P

O

O-

O

C

O

H

H

H H

CH 2

H

O

P

O

O-

O

G

O

H

O

H

H H

CH 2

H

O

P

O

O-

O

T

5'-end

3'-end

5'-end

3'-end