The invention relates to a molecular complex comprising at least one polylysine conjugate (PLL), comprising a main PLL straight chain and at least one molecule F having an average molecular weight of between 50 daltons and 1000 daltons that is covalently bonded to said main chain, and at least one molecule M that is unstable in solution, the conjugate(s) and the molecule(s) M being bonded by means of a non-covalent bond. The invention also relates to a composition comprising a complex of this kind, to a method for obtaining said composition and use thereof, and to the use of one or more PLL-based conjugates for improving the hydrophilicity, the effectiveness, and the activity of a molecule that is unstable in solution, over a time period that is compatible with the use of said molecule. The invention also relates to a method for identifying a PLL-based conjugate or a combination of a plurality of PLL-based conjugates that makes it possible to improve the hydrophilicity, the effectiveness, and the activity of a molecule that is unstable in solution, and to a kit for implementing said method.
