Peptides have a dominant role in biology; yet the study of their physical properties is at best sporadic. Peptide quantitative structure-activity relationship (QSAR) lags far behind the QSAR analysis of drug-like organic small molecules. Traditionally, QSAR has focussed on experimentally determined partition coefficients as the main descriptor of hydrophobicity. A partition coefficient ( ) is the ratio between the concentrations of an uncharged chemical substance in two immiscible phases: most typically water and an organic solvent, usually 1-octanol. A distribution coefficient ( ) is the equivalent ratio for charged molecules. We report here a compilation of partition and distribution coefficients for linear peptides compiled from literature reports, suitable for the development and benchmarking of peptide and prediction algorithms. 1. Introduction Peptides abound in nature, functioning as hormones, including bradykinins, insulin, gastrins, oxytocins, and various growth factors; as neuropeptides [1], such as encephalins and endorphins; as MHC-bound epitopes, the principal recognition element in cellular immunology [2]; as intermediates in the degradation of proteins [3]; as bacteriocins [4], such as microcins; as antimicrobial host defence peptides [5], such as dermcidins and defensins; and as venom peptides [6], such as α-, μ-, and ω-conotoxins, χ-conopeptides, conantokins, and contulakins; to name but a few of their many important and diverse roles and functions. However, peptides have historically been regarded by the pharmaceutical industry as poor drug candidates [7], not least as they are thought to lack desirable Lipinski-like qualities, such as possessing a low molecular weight [8]. Naturally occurring peptides also often have a limited half-life. If administered orally, they are rapidly broken down by endo- and exopeptidases within the gut, reducing oral bioavailability. For this reason, therapeutic peptides are often delivered parenterally, which can be both impractical and expensive. The use of peptides also has many potential advantages, if these practical problems can be overcome. Peptides can be highly specific, thus reducing unnecessary side effects; whilst naturally occurring peptides are likely to exhibit low toxicity. Thus, many peptides have been licensed as drugs [7], including captopril, nesiritide, ceruletide, and exenatide. Peptide vaccines are, likewise, an area of active research in both clinical and preclinical environments [9]. So-called cell-penetrating peptides form another avenue being vigorously investigated, this time as
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