Modeling Biologically Important NH…π Interactions Using peri-Disubstituted Naphthalenes.

For the first time, systematic studies of 8-aryl and 8-pyrrolyl derivatives of 1-aminonaphthalene as simple, synthetically available, and nicely preorganized models were conducted for a better understanding the properties of NH center dot center dot center dot pi interactions involved in the stabilization of the secondary and tertiary protein structures as well as the recognition of guest molecules by biological receptors. It was shown that the NH center dot center dot center dot pi binding is especially effective when the NH-donor is a positively charged group, for example, Me2NH+, and the pi-donor is an electron-rich aromatic substituent, in particular, the 1-pyrrolyl or the 4-hydroxyphenyl group. Using protonated tetrafluoroborate salts, a strong counterion effect was demonstrated by means of theoretical calculations. Through several mechanisms, including short CH center dot center dot center dot F contacts, bifurcation, and long-range dispersion, the counterion promotes considerable structural changes and weakens the NH center dot center dot center dot pi interactions from 12-15 kcal mol(-1) in "naked" cations to 5-9 kcal mol(-1) in the salts. To this end, 8-(2,5-dimethylpyrrol-1-yl)-N,N-dimethylnaphthalene-1-ammonium tetrafluoroborate, with the record linearity and shortness (2.07 angstrom) of the NH center dot center dot center dot pi-centroid bond, was recognized as the most appropriate model with the strongest NH center dot center dot center dot pi interaction ever described.