Structural and energetic effects in the molecular recognition of amino acids by 18-crown-6

Absolute 18-crown-6 (18C6) affinities of five amino acids (AAs) are determined using guided ion beam tandem mass spectrometry techniques. The AAs examined in this work include glycine (Gly), alanine (Ala), lysine (Lys), histidine (His), and arginine (Arg). Theoretical electronic structure calculations are performed to determine stable geometries and energetics for neutral and protonated 18C6 and the AAs as well as the proton bound complexes comprised of these species, (AA)H(+)(18C6). The proton affinities (PAs) of Gly and Ala are lower than the PA of 18C6, whereas the PAs of Lys, His, and Arg exceed that of 18C6. Therefore, the collision-induced dissociation (CID) behavior of the (AA)H(+)(18C6) complexes differs markedly across these systems. CID of the complexes to Gly and Ala produces H(+)(18C6) as the dominant and lowest energy pathway. At elevated energies, H(+)(AA) was produced in competition with H(+)(18C6) as a result of the relatively favorable entropy change in the formation of H(+)(AA). In contrast, CID of the complexes to the protonated basic AAs results in the formation of H(+)(AA) as the only direct CID product. H(+)(18C6) was not observed, even at elevated energies, as a result of unfavorable enthalpy and entropy change associated with its formation. Excellent agreement between the measured and calculated (AA)H(+)-18C6 bond dissociation energies (BDEs) is found with M06 theory for all complexes except (His)H(+)(18C6), where theory overestimates the strength of binding. In contrast, B3LYP theory significantly underestimates the (AA)H(+)-18C6 BDEs in all cases. Among the basic AAs, Lys exhibits the highest binding affinity for 18C6, suggesting that the side chains of Lys residues are the preferred binding site for 18C6 complexation in peptides and proteins. Gly and Ala exhibit greater 18C6 binding affinities than Lys, suggesting that the N-terminal amino group provides another favorable binding site for 18C6. Trends in the 18C6 binding affinities among the five AAs examined here exhibit an inverse correlation with the polarizability and proton affinity of the AA. Therefore, the ability of the N-terminal amino group to compete for 18C6 complexation is best for Gly and should become increasing less favorable as the size of the side chain substituent increases.