Selective binding of amino add residues to tRNA^Phe

The interaction of amino acid amides with tRNA^Phe was studied by measurements of the Wye base fluorescence. Binding of phenylalanine-, tyrosine- and tryptophan-amides leads to considerable quenching, whereas the asides of e.g. glycine and lencine do not induce quenching nnder the sane conditions. Binding constants at 0,13 M salt - 100 M^-1 for Phe-, 110 M^-1 for Tyr- and 300 M^-1 for Trp-amide - are about a factor of 6 higher than those evaluated from independent measurements for binding to simple single-stranded polynucleotides; the corresponding factor is 10 for double-stranded polynucleotides. Since the apparent enthalpy changes derived from measurements at different temperatures remains relatively low (-9 to -20 kJ/mol), the increased affinity appears to be mainly due to an increase of the entropy changes. Titration experiments performed in the presence of Mg²⁺ indicate cooperative interactions of the aromatio residues with the anticodon loop that are consistent with preferential binding to one of two loop conformations. Measurements of binding constants at different pH-values indicate the protonation of a tENA residue in the tryptophanamido-tRNA^Phe oomplex characterised by a pK value of about 7.0.