Direct spectroscopic evidence for intramolecular Ï€-Ï€ interactions in solutions of Λ-β 1-[Co(R,R-picchxn)(aa)] n+ [picchxn=N,N"²-di(2-picolyl)-1,2-diaminocyclohexane; aa=α-methyl-S- tryptophanato(1-), R-aminobenzylmalonato(1-)]

High-resolution NMR studies of Λ-β 1-[Co(R,R-picchxn) (S-α-Me-trp)](ClO 4) 2 (picchxn=N,N"²-di(2- picolyl)-1,2-diaminocyclohexane, α-Me-trpH=α-methyltryptophane), and Λ-β 1-[Co(R,R-picchxn)(R-ABMA)] ClO 4·1.5H 2O (ABMAH 2=2-amino-2-benzylpropandioic acid) dissolved in DMSO-d 6 and D 2O prove that significant intramolecular Ï€-Ï€ interactions between a pyridyl ring of the tetradentate and pendant aromatic residues of the aminoacidates persist in solution. These non-classical bonding effects are accompanied by NH-Ï€ bonds involving an amine H atom of the coordinated aminoacidate, which lies in a hydrophobic region between the Ï€-stacked aromatic rings. The intramolecular Ï€-Ï€ interactions are stronger in D 2O, in line with expectations associated with the hydrophobic effect, and significant charge transfer is evident between the indole and pyridyl rings in the methyltryptophane complex. The potential importance of such interactions in biochemical systems is highlighted.