No abstract

The effect of basis set superposition error ͑BSSE͒ on Gaussian-2 and Gaussian-3 calculated alkali metal cation-ligand affinities has been studied. For these systems, we found that the standard Boys-Bernadi full counterpoise ͑CP͒ method often leads to correction terms that are physically incorrect. This problem may be rectified by using the geometry corrected counterpoise ͑GCP͒ method. The relationship between CP, GCP corrections, and deformation energy is discussed. In order to yield good agreement with existing experimental Li ϩ and Na ϩ ligand affinities, we recommend the adoption of either the G3 ͑with GCP correction͒ or the G2͑MP2,SVP͒-FC ͑without GCP correction͒ protocols. In the case of K ϩ , the GCP correction is of negligible magnitude, and hence GCP corrections may be omitted in the G2͑MP2,SVP͒-ASC affinity calculations for these complexes.

To understand the cation-pi 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[double bond]C), amino nitrogen (--NH(2)), and aromatic pi 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 pi and non-pi cation bonding distances (Na(+)-pi>Na(+)-N>Na(+)-O and K(+)-pi>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-pi 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.