Direct observation of surface structure of d-alanine and d-/l-valine crystals by atomic force microscopy and comparison with X-ray diffraction analysis

A direct observation of surface morphology characterization of the crystals of d-alanine, d-valine and l-valine at molecular scale was presented by atomic force microscopy (AFM). The crystal structures of d-alanine, l-alanine and l-valine have been determined by X-ray diffraction methods. d- and l-alanine crystallize in the orthorhombic space group P212121, Z=4 with the unit-cell dimensions a=6.0073(5)Å/6.0095(5)Å, b=12.3030(7)Å/12.3388(7)Å, c=5.7732(4)Å/5.7904(3)Å, α=β=γ=90° and l-valine crystallizes in the monoclinic space group P21, Z=4 with lattice constants a=9.6737(5) Å, b=5.2664(3) Å, c=12.0196(6) Å, α=γ=90°, β=90.722(3)° at 270 K. The l-valine molecule was found to take two different conformations—gauche I and trans form. AFM images clearly show an ordered molecular morphology of orthorhombic alanine and monoclinic valine unit cells, which are in fair agreement with the X-ray diffraction data.