Formation of Chiral Self-Assembled Structures of Amino Acids on Transition-Metal Surfaces: Alanine on Pd(111)

The structure and self-assembly of alanine on Pd(111) is explored using X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), reflection–absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM), and supplemented by density functional theory (DFT) calculations to explore the stability of the proposed surface structures formed by adsorbing alanine on Pd(111) and to simulate the STM images. Both zwitterionic and anionic species are detected using RAIRS and XPS, while DFT calculations indicate that isolated anionic alanine is significantly more stable than the zwitterion. This observation is rationalized by observing dimeric species when alanine is dosed at ∼270 K and then cooled to trap metastable surface structures. The dimers form due to an interaction between the carboxylate group of anionic alanine with the NH₃⁺ group of the zwitterion. Adsorbing alanine at 290 K results in the formation of dimer rows and tetramers resulting in only short-range order, consistent with the lack of additional diffraction spots in LEED. The stability of various structures is explored using DFT, and the simulated STM images are compared with experiment. This enables the dimer rows to be assigned to the assembly of anionic-zwitterionic dimers and the tetramer to the assembly of two dimers in which three of the alanine molecules undergo a concerted rotation by 30°.