Ab initio study of hydrogen-bond formation between cyclic ethers and selected amino acid side chains

Binding energies for hydrogen-bonded complexes of six cyclic ethers with five hydrogen-bond donor molecules that mimic selected amino acid side chains have been calculated at the MP2/6-31G*, MP2/6-31+G*, MP2/6-311++G**(single point), and MP2/aug-cc-pvtz levels, using geometries obtained with or without counterpoise corrections throughout the geometry optimization. The calculated basis set superposition error (BSSE) amounts to 10-20% and 5-10% of the uncorrected binding energies for the neutral and ionic species, respectively, at the MP2/aug-cc-pvtz level. The authors conclude that the O...H distances in the hydrogen bonds and binding energies for the studied systems may be determined with uncertainties of up to 0.08 A and 1-2 kcal/mol, respectively, in comparison with the MP2/aug-cc-pvtz values at a reasonable computational cost by performing standard geometry optimization at the MP2/6-31+G* level. Hydrogen-bond formation energies are more negative for cyclic ethers compared to their counterparts with a C=C double bond in the ring next to the oxygen atom. The less negative hydrogen-bonding energy and the increased O...H separation have been attributed to the reduced basicity of the ether oxygen when the lone pairs can enter conjugation with the pi-electrons of the Calpha=Cbeta double bond. The present study is the first step toward the development of an affordable computational level for estimating the binding energies of natural product, fused ring ether systems to the human estrogen receptor.