Silicon Analogs of L-Amino Acids: Properties of Building Blocks of an Alien Biosphere

The PM3/PM7 semi-empirical quantum-chemical methods were for the first time used to calculate and analyze the geometric configuration parameters of silicon analogs of the 20 proteinogenic L-amino acids. Enthalpies of formation and dipole moments were calculated along with conformational parameters. Bond lengths of the silicon analogs were shown to significantly exceed the bond lengths in the carbon amino acids. Intramolecular hydrogen bonding was found to be possible in the silicon analog of aspartate, like in carbon aspartate. The lowest thermodynamic stability was observed for aromatic and heterocyclic amino acid analogs. The role that an aromatic analog may play in silicon compounds differs from the respective role in carbon compounds as a result of longer interatomic distances and weaker π conjugation. A polyalanine chain model was used to demonstrate that the α-helical conformation corresponds to the global minimum of the heat of formation in both carbon and silicon peptides.