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 P2(1)2(1)2(1), Z = 4 with the unit-cell dimensions a = 6.0073(5) Angstrom/6.0095(5) Angstrom, b = 12.3030(7) Angstrom/12.3388(7) Angstrom, c = 5.7732(4) Angstrom/5.7904(3) Angstrom, alpha = beta = gamma = 90degrees and L-valine crystallizes in the monoclinic space group P2(1), Z = 4 with lattice constants a = 9.6737(5) Angstrom, b = 5.2664(3) Angstrom, c = 12.0196(6) Angstrom, alpha = gamma = 90degrees, beta = 90.722(3)degrees 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. (C) 2002 Elsevier Science B.V. All rights reserved.