The TCR from the alloreactive clone 2C recognizes p2C (LSPFPFDL)/Ld and QL9 (QLSPFPFDL)/Ld complexes with affinities of 2 x 10(6) and 10(7) M(-1). Recently, it was proposed that the Phe at position 4 of p2C is critical for recognition by the 2C TCR. To further characterize the role of this peptide position in binding to Ld and in recognition by the 2C TCR, we changed the corresponding peptide position in QL9 (position 5) to other amino acids. Binding affinities of these peptides for Ld and of the peptide/Ld complexes for a soluble single chain TCR were determined. Unexpectedly, it was shown that this peptide position has a significant effect on Ld binding (100-fold), with positively charged and hydrophobic residues having a beneficial effect and negatively charged residues having a detrimental effect. Measurements of the binding affinities of these peptide/Ld complexes for the 2C TCR showed that at 4 degrees C only a Tyr substitution at this position retained high affinity for the TCR. However, significant differences in TCR binding were observed among QL9 peptide variants at 4 degrees C compared with that at 37 degrees C. The influence of this peptide position on both Ld binding and TCR binding may suggest that the 2C TCR recognizes an Ld conformational determinant that is altered by interactions with the residue at position 5 of QL9. A strong correlation was also observed between peptide-Ld affinity and the ability of peptides to sensitize Ld target cells for lysis by CTL 2C. The results are considered in view of recent models on the relationship between T cell activity and TCR-peptide-MHC binding properties.