The ribose sugar geometry is defined by the following five endocyclic torsion angles:

The magnotudes of such angles are all interrelated and, therefore, the geometry of the ribose ring can be defined from two parameters: the pseudorotation phase angle (P) and the pucker amplitude. The ribose ring is not aplanar and usually presents C2'-endo (South) or C3'-endo (North) conformation.

For the sugar pucker conformation, homonuclear ³J_H-H coupling constant serve as the most direct determinant ( 01JB19-19). These constants can be measured in a qualitative way from 2D ¹H-¹H TOCSY, 2D ¹H-¹³C TOCSY-HMQC or 2D ¹H-¹³C TOCSY-HSQC experiments:

• Very weak J_H1'-H2' and strong J_H3'-H4' cross-peaks correspond to pure N-type conformation (preferred conformation in RNA).
• Strong J_H1'-H2' and weak J_H3'-H4' cross-peaks correspond to pure S-type conformation.
• J_H2'-H3' is similar in both states.
• Intermediate intensities indicate an equilibrium between N and S states.

 

Other coupling constants in the sugar ring:

• ¹J_CH coupling constants:
• ¹J_C1'-H1' in the range 166.0-170.0 Hz
• All other pairs in the range 145-153 Hz
• ¹J_CC  coupling constants:
• ¹J_C1'-C2'  and ¹J_C4'-C5' in the range 42.0-43.5 Hz
• ¹J_C2'-C3'  and ¹J_C3'-C4' in the range 37.5-38.5 Hz

Sugar puckering can also be studied from:

CSA-DD and DD-DD cross-correlation data.

CT-NOESY experiments ( 01JMR262-153)

MULDER (Mostly Universal Dihedral angle Extractor). A tool for extracting torsion angles from NMR data ( 02JB339-24)