Structures of benzenesulfonylamino-3-(4-benzenesulfonyloxy-phenyl)-propionic acid and 2-(toluene-4-sulfonylamino)-3-[4-(toluene-4-sulponyl-oxy)-phenyl]-propionic acid: Variations in L-tyrosine backbone conformation, intramolecular aromatic π-π stacking and short C-H⋯O interactions