Table 2.

Cation-π interaction energies between amino acid His and cations in gas phase

His (Aromatic motif)	Downside		Upside
	Energy a	Length b	Energy a	Length b
Na+	−16.457	2.461	−10.478	2.420
K+	−10.066	2.957	−2.358	2.925
Ca++	−54.331	2.493	−45.771	2.483
Zn2+	−147.406	2.137	−144.355	2.820
Lys(CH3NH3+)	−8.193	3.107 c	−0.198	3.069 c
Arg(CHN2NH2+)	−9.268	3.911 c	−2.918	3.883 c
His+	Downside		Upside
(Organic cation)	Energy a	Length d	Energy a	Length d
Phe	−7.809	3.269	−3.613	4.809
Tyr	−7.887	3.256	−3.655	4.799
Trp	−13.642	3.166	−12.057	3.276

^a Energies are in kcal/mol, calculated using B3LYP/6-31+G(d,p) method.

^b Angstrom (Å).

^c Distance from N of CH₃NH₃⁺ (or CHNH₂NH₂⁺) to the center of imidazole ring.

^d Distance from N of imidazole to the aromatic center of amino acids (Phe, Tyr, and Trp).