Phosphorescence and optically detected magnetic resonance (ODMR) measurements of tryptophan (W) and 5-methyltryptophan (MeW) triplet states are reported in stereoisomeric peptide complexes with polynucleotides. The peptide diastereomers KWK, K(MeW)K, KGWK, and KGWGK in which the W or MeW are either D Or L and the lysine (K) is L have been prepared and their complexes with poly(deoxythymidylic) acid (poly dT), poly(inosinic) acid (poly 1), and poly(uridylic) acid (poly U) have been studied. Complex formation with polynucleotide results in a phosphorescence redshift and reduction of the zero field splitting D value of W or MeW that varies with polynucleotide, the length and sequence of the peptide, and with the diastereomer. In addition, the kinetic parameters of the triplet state show considerable variation between complexes. These results are discussed in terms of the stereochemistry of peptide bonding to the single stranded polynucleotide and differing types of aromatic amino acid-base interaction: intercalation, aromatic stacking, and edge-to-face contact. Our results are consistent with binding of the peptide to helical regions of the polynucleotide in a single-stranded helical beta-sheet structure in which alternating side chains of an all L oliogopeptide face inward toward the bases and outward from the helix. The orientation is determined by the N-terminal lysine that binds in a stereospecific manner to DNA phosphates through its alpha- and epsilon-amino groups. This model resembles one originally proposed by Gabbay et al. [Gabbay, E. J.; Adawadkar, P. D.; Wilson, W. D. Biochemistry 1976, 15, 146; Gabbay, E. J.; Adawadkar, P. D.; Kapicak, L.; Pearce, S.; Wilson, W. D. Biochemistry 1976, 15, 152] for peptide binding to duplex DNA.