Do Stereochemical Effects Overcome a Charge-Induced Perturbation in Isolated Protonated Cyclo(Tyr-Tyr)?

Two diastereomers of the protonated diketopiperazine (DKP) dipeptide cyclo(Tyr-Tyr), namely, cyclo(LTyr-LTyr)H⁺ and cyclo(LTyr-DTyr)H⁺, are studied in a cryogenic ion trap by means of IR photodissociation spectroscopy combined with quantum chemical calculations. The two diastereomers have similar structures in which one of the rings is folded over the DKP ring and the other one is extended in a trans geometry, allowing a strong OH⁺···π interaction to take place. This contrasts to the observation of a stacked geometry for neutral cyclo(LTyr-LTyr) only under supersonic expansion conditions that do not exist for cyclo(LTyr-DTyr). In the protonated form, the strength of the OH⁺···π interaction is different for the two diastereomers, resulting in a ∼110 cm^-1 difference in the ν(OH⁺) frequency and a smaller but clearly identifiable difference in the protonated amide ν(NH) frequency. Stereochemical effects are therefore still evidenced despite the strong perturbation due to the excess charge.