What are the differences between A-form and B-form nucleic acid?

Basis of differentiation 

A-form nucleic acid

B-form nucleic acid

Shape of helix

Ribbon-like, open cylindrical core

Compact 

Sugar conformation

C3′ endo

C2′ endo

Number of base pairs per turn

About 11 bp

About 10 bp

Base-pair placement

Displaced away from central axis, closer to major groove

Centered over helical axis

Helix diameter 

Approximately 2.3 nm

Approximately 9 nm

Helix pitch 

Approximately 28 degrees

Approximately 34 degrees

Twist angle

Approximately 33 degrees

Approximately 36 degrees

Ideal environmental conditions

Low humidity

High humidity