Role of helical constraints of the EBS1–IBS1 duplex of a group II intron on demarcation of the 5′ splice site

Base-H1′ region of a 2D NOESY spectrum of nonexchangeable ¹Hs of the ID3 stem–loop. Sequential aromatic–anomeric (H8/6-H1′) connectivities are marked by lines. Sequential NOE connectivities were interrupted at residues A13/A14 and at 14 H1′-U15 H6. NOEs involving H2 resonances of A12, A13, and A14, which were broadened in the uncomplexed ID3, are boxed (from left to right): A14 H2-A14 H1′, A12 H2-A13 H1′, and A12 H2-A12 H1′. The spectrum was acquired with 72 scans and 576 increments and a mixing time of 350 msec.