Table 1.

Comparison of the RNA sugar-phosphate backbone torsion angles (degrees) of the four idealized conformational categories found in dinucleotide platform structures with those in standard A-RNA steps. For the backbone torsion angles nomenclature, see Figure 5.

Conformational Classa	Labelb	Average sugar-phosphate torsion anglesd
		γ c	δ	ε	ζ	α+1	β+1	γ+1	δ+1
I	&a	56	82	−169	−95	−64	−178	51	82
II	#a	164	148	−168	146	−71	151	42	85
III	0a	53	149	−137	139	−75	158	48	84
IV	4g	48	148	−103	165	−155	165	49	83
A-RNA	1a	54	80	−150	−73	−65	173	54	80

^aConformational classes based on distinguishing torsional features of the sugar-phosphate-sugar unit: I - A-like homogeneous C3′ -endo puckered sugars, gauche⁻, gauche⁻ phosphodiester; II and III - mixed C2′-endo/C3′-endo puckering, trans, gauche⁻ phosphodiester; IV - mixed C2′-endo/C3′-endo puckering, trans, trans phosphodiester.

^bRespective RNA family labels according to the Richardson et al. nomenclature⁴⁴.

^cγ values for classes I-IV taken respectively from appropriate experimental examples (PDB ID/Nucleotide/Chain): 1hq1/163/B46; 1s72/137147, 1s72/26547, and 1hr2/226/A48. The idealized A-RNA structure is periodic and thus γ is identical to γ+1.

^dAverage backbone torsions found by Richardson et al. (categories I-IV) and Schneider et al. (A-RNA). For more details see Refs. 44 and 45.