The binding of peptides containing lysyl and aromatic residues to poly(A) in its single-stranded form at pH 7 leads to a change of its circular dichroism (CD) spectrum, which is mainly due to the stacking of the aromatic amino acid with the bases of poly(A). Comparison is made between the binding of peptides having different primary structures which gives indications on the way the peptides bind to poly(A). A method is described which allows the calculation of the binding parameters from CD data. The magnitude of the association constant depends on the size of the aromatic ring and decreases in the order tryptophan greater than tyrosine greater than phenylalanine. The CD amplitude decreases linearly with the concentration of bound molecules. These results are discussed with respect to the role played by aromatic amino acids in complex formation between nucleic acids and proteins.

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