Gaseous- versus solution-phase recognition of some aromatic amino esters by 2,8,14,20-tetrakis(L-valinamido)[4]resorcinarene

The effects of the physical environment on the molecular recognition of some aromatic amino esters (A = 3-(3,4)dihydroxyphenyl)alanine methyl ester (DOPAOMe), 3-(3,4)dihydroxyphenyl)alanine ethyl ester (DOPAOEt) and tryptophan ethyl ester (trpOEt)) by the flattened-cone 2,8,14,20-tetrakis(L-valinamido)[4]resorcinarene (1L) have been investigated in both the gas-phase by ESI-MS spectrometry and in CDCl3 solutions by 1H and 13C NMR spectroscopy. It is found that the non-covalent [1L·H·DOPAOMe]+ and [1L·H·DOPAOEt]+ complexes are stable in the gas-phase. The last one is stable in CDCl3 solutions as well, while [1L·H·trpOEt]+ is not. The formation of the stable [1L·H·DOPAOEt]+ complex in CDCl3 is not affected by the presence of traces of additives, like D2O, DCl or ethyl acetate, or by absorption on silica. APT-13C NMR analysis of [1L·H·DOPAOEt]+ suggests that the amino ester is mainly located inside the resorcinarene cavity, in conformity with the most stable structure arising from MC/MD simulations.