What is the sugar pucker conformation of B-DNA?

The characteristic sugar-pucker conformation of B-DNA is C2′-endo.

DNA’s deoxyribose sugar is a five-membered ring that is not perfectly flat. It flexes into puckered shapes, and the label C2′-endo identifies which carbon is displaced from the approximate plane of the ring. This small geometric change affects the spacing and orientation of the phosphate backbone and helps produce the familiar B-form double helix.

B-DNA is the common right-handed form of duplex DNA under many physiological conditions. It has roughly 10.5 base pairs per turn, a relatively narrow and deep minor groove, and bases stacked near the helix axis. By comparison, A-DNA is usually associated with a C3′-endo sugar pucker and has a shorter, wider helix.

The textbook rule is useful but describes a preference rather than an unbreakable law. Real DNA is flexible: individual sugars can occupy nearby conformations, and sequence, hydration, bound proteins, and unusual structures can shift the pucker. A common mix-up is to assign C3′-endo to B-DNA; that conformation is the classic association for A-DNA and RNA helices.