The alpha(v) beta(3) integrin is implicated in human tumor metastasis and in angiogenesis. The design of low-molecular-mass alpha(v) beta(3) antagonists by "spatial screening" led to the highly active peptides c() and c(). Here the influence of the amino acids in positions 4 and 5 flanking the RGD-sequence on the inhibition of vitronectin and fibrinogen binding to the isolated alpha(v) beta(3) and alpha(IIb)beta(3) receptors was investigated. The influence of the side chain and the backbone conformation on activity and selectivity was studied. The compounds were divided into conformational classes. For each class at least one representative peptide was subjected to detailed structure determination in solution. The peptides of classes 1, 2, and 3 show a beta lI’/gamma-turn arrangement with the D-amino acid in the i + 1 position of the beta II’-turn. By contrast, the peptides of class 4 reveal a modified beta Il’/gamma-turn pattern with glycine in the i + 1 position of the beta II’-turn and the D-amino acid in the i + 1 position of the gamma-turn. Class 1 is divided into two subclasses : besides the beta II’/gamma-turn arrangement a gamma/gamma-turn motif is found for two members of this class. Structure-activity relationship (SAR) investigations show that the amino acid in position 4 and the proton of the amide bond between residues 3 and 4 are essential for high biological activities toward alpha(v) beta(3) BY contrast, the amino acid in position 5 has no influence on the activity. A bent conformation of the RGD-sequence, as observed for the peptides of classes 1 and 2, fits the alpha(v) beta(3) better than the alpha(IIb)beta(3) receptor and so increases the selectivity of these peptides.