Amino Acid Monolayers and Films on Copper

The monolayer structures for tryptophan are consistent with one and two molecules per unit cell, respectively, for Cu(100) and Cu(l 11). The structures observed for D- and L-tryptophan are related by mirror inversion which is consistent with the symmetry relationship between the two molecules. A mixture of the optical isomers, DL-tryptophan does not form an ordered monolayer, thus there is no segregation or cooperative interaction between the different isomers. 

In addition to forming ordered monolayer structures, ordered multilayer films of several hundred Angstrom thickness were also grown for tryptophan. Ordered multilayers could be grown for DL-tryptophan even though the DL-tryptophan monolayer was disordered. 

Electron beam damage effects followed the general rule that molecular groups in intimate contact with the metal substrate and aromatic groups appear relatively stable. Thus in the monolayer, alanine, with a methyl group likely sticking out from the surface, was the only molecule found to be unstable. In multilayer films, only tryptophan with the aromatic indole group to stabilize the molecule was found to yield multilayers stable under electron beam irradiation. 

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