A similar ethidium: RNA neighbor-exclusion binding model has been calculated, using the beta-structural (RNA) element as the asymmetric unit. Saturating concentrations of ethidium are known to give rise to neighbor-exclusion binding in synthetic poly r (A-U) and poly r (G-C), and to approach neighbor-exclusion binding levels in naturally occurring double-stranded RNA. Although double-stranded RNA exists solely in the A- form (the B- form is excluded by the C2' hydroxyl group for steric reasons), it is possible that beta-RNA arises within premeltons in A-RNA. This would explain how ethidium and other simple intercalators bind to double-stranded RNA.

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