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Title: Competition between π and Non-π Cation-Binding Sites in Aromatic Amino Acids: A Theoretical Study of Alkali Metal Cation (Li+, Na +, K+)-Phenylalanine Complexes
Authors: Siu, FM
Ma, NL
Tsang, CW
Issue Date: 2004
Source: Chemistry - a European journal, 2004, v. 10, no. 8, p. 1966-1976
Abstract: To understand the cation-π interaction in aromatic amino acids and peptides, the binding of M+ (where M+ = Li+, Na+, and K+) to phenylalanine (Phe) is studied at the best level of density functional theory reported so far. The different modes of M+ binding show the same order of binding affinity (Li+ > Na+ > K+), in the approximate ratio of 2.2:1.5:1.0. The most stable binding mode is one in which the M+ is stabilized by a tridentate interaction between the cation and the carbonyl oxygen (O=C), amino nitrogen (-NH2), and aromatic π ring; the absolute Li+, Na+, and K+ affinities are estimated theoretically to be 275, 201, and 141 kJ mol-1, respectively. Factors affecting the relative stabilities of various M +-Phe binding modes and conformers have been identified, with ion-dipole interaction playing an important role. We found that the trend of π and non-π cation bonding distances (Na+-π > Na +-N > Na+-O and K+ -π > K +-N > K+-O) in our theoretical Na+/K +-Phe structures are in agreement with the reported X-ray crystal structures of model synthetic receptors (sodium and potassium bound lariat ether complexes), even though the average alkali metal cation-π distance found in the crystal structures is longer. This difference between the solid and the gas-phase structures can be reconciled by taking the higher coordination number of the cations in the lariat ether complexes into account.
Keywords: Alkali metals
Binding affinities
Cation-pi interactions
Molecular modeling
Publisher: Wiley-VCH
Journal: Chemistry - a European journal 
ISSN: 0947-6539
EISSN: 1521-3765
DOI: 10.1002/chem.200305519
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