Oligonucleotide structural parameters that influence binding of 5'-O-triphosphoadenylyl-(2' .fwdarw. 5')-adenylyl-(2' .fwdarw. 5')-adenosine to the 5'-O-triphosphoadenylyl-(2' .fwdarw. 5')-adenylyl-(2' .fwdarw. 5')-adenosine dependent endoribonuclease: chain length, phosphorylation state, and heterocyclic base