Amino acid capture by aqueous interfaces. Implications for biological uptake.

The interactions of natural amino acids with water-hydrophobic interfaces are central to the control of key biological processes, such as passive transport, and to the overall structure and stability of membrane proteins. We still have a very poor knowledge of these interactions, and our aim in this work is to investigate the thermochemistry and dynamics properties of simple aliphatic amino acids (glycine and valine) across a water-organic interface. The study has been carried out by means of Born-Oppenheimer molecular dynamics simulations focusing on the role that the hydrophobicity of the side chain has on the phase transfer mechanism of the amino acid. Data for the energetics of the uptake processes have been reported, and it is expected that the reported results will be helpful in the design of future experiments with systems of biological relevance. We have shown that neutral tautomers exhibit a noticeable affinity for the interface that increases with increasing hydrophobicity of the side chain. Moreover, the zwitterionic form of valine (but not that of glycine) does also exhibit a significant affinity for the interface. An important finding is that the neutral and zwitterionic tautomers are roughly isoergonic in the organic layer close to the interface. This result suggests a two-step mechanism for the water-to-organic phase transfer that involves neutralization of a partially hydrated zwitterion in the organic layer prior to uptake into the bulk. Though the mechanisms for glycine and valine are similar, the predicted energetics and dynamics for the first step display noteworthy differences that should be measurable and may have important biological implications.