This report describes a new set of amino acid side chain descriptors, the isotropic surface area (ISA), and the electronic charge index (ECI) relevant to peptide quantitative structure--activity relationship (QSAR) studies. These features are derived from optimized three-dimensional structures of the natural and unnatural amino acids. Since the descriptors are derived considering side chain three-dimensional structure, 3D-QSARs result. Using the method of partial least squares, 3D-QSARs of peptide sets were developed. A comparison of the results to those obtained with the principal properties or z-scales shows that the ISA and ECI are comparable for parameterizing the structural variability of the peptide series and represent an interesting alternative.