An exhaustive umbrella sampling simulation of the alanine dipeptide in solution is reported The presence of stable Y conformations in solution is assessed by both quantum calculations and experimental data from X-ray and NMR protein-solved structures available in the protein coil library. The agreement between experimental and simulation Ramachandran plots of the dipeptide in solution is excellent. A suitable explanation of the stabilization of the Y conformation mediated by water for the alanine dipeptide is discussed on the basis of Car-Parrinello MD calculations of the dipeptide in water.