Simple models of nucleic acid interactions. 3. Aminoacyl derivatives of "bridged" nucleosides: chemical synthesis and substrate activity of 2'(3')-O-(N-acetyl)-L-leucyl-2''(3'')-O-L-phenylalanyl-(1,2-diadenosin-N⁶-yl)ethane, a spacer probe for ribosomal peptidyltransferase