(A) Arginine–phosphate and (B) Lysine–phosphate complexes stabilized by N-H⋯O-P H-bonds in POPC-POPG-bound CPP penetratin. The structural models were built based on experimentally measured site-specific 13C-31P distances (solid line) (Su et al, 2011), sidechain dihedral angles from the penultimate rotamer library (Lovell et al, 2000) and N-H⋯O-P geometric parameters in previous studies (Zhang et al, 1999; de Dios and Oldfield, 1994). Geometry parameters of the N-H⋯OP H-bonds: θ =120°, σ =168° and r = 2.8 Å. (C) Guanidinium-phosphate and guanidinium-water interactions of POPE-POPG-bound PG-1.