Crystallization of amino acids on self-assembled monolayers of rigid thiols on gold

Self-assembled monolayers (SAMs) of rigid biphenyl thiols are employed as heterogeneous nucleants for the crystallization of L-alanine and DL-valine. Powder X-ray diffraction and interfacial angle measurements reveal that the L-alanine crystallographic planes corresponding to nucleation are {200}, {020}, and {011} on SAMs of 4'-hydroxy-(4-mercaptobiphenyl), 4'-methyl-(4-mercaptobiphenyl), and 4,(4-mercaptophenyl)pyridine on gold (111) surfaces, respectively. In the case of DL-valine, monolayer surfaces that act as hydrogen bond acceptors (e.g., 4'-hydroxy-(4-mercaptobiphenyl) and 4-(4-mercaptophenyl)-pyridine) induce the racemic crystal to nucleate from the {020} plane whereas the nucleating plane for the 4'-methyl-(4-mercaptobiphenyl) surface is the fast-growing {100} face. The observation of crystal nucleation and orientation can be attributed to the strong interfacial interactions, in particular, hydrogen bonding, between the surface functionalities of the monolayer film and the individual molecules of the crystallizing phase. Molecular modeling studies are also undertaken to examine the molecular recognition process across the interface between the surfactant monolayer and the crystallographic planes. Similar to binding studies of solvents and impurities on crystal habit surfaces, binding energies between SAMs and particular amino acid crystal faces are calculated and the results are in good agreement with the observed nucleation planes of the amino acids. In addition to L-alanine and DL-valine, the interaction of SAMs and mixed SAMs of rigid thiols on the morphology of α-glycine is examined (Kang, J. F.; Zaccaro, J.; Ulman, A.; Myerson, A. Langmuir 2000, 16, 3791), and similarly the calculations are in good agreement. These results suggest that binding energy calculations can be a valid method to screen self-assembled monolayers as potential templates for nucleation and growth of organic and inorganic crystals.