Is Tl⁺ an Analogue of K⁺? IR Spectroscopy of Tl⁺ and a Key Peptide of the Selectivity Filter in K⁺ Channels

Elemental substitution of K⁺ with Tl⁺ is a powerful method for enhancing the structural analysis of biomolecules because of significant enhancement of sensitivity. Its validity has been explained by their similar physical properties; however, the chemical analogy between K⁺ and Tl⁺ is still under debate. This is of prime importance in K⁺ channels because of the controversy on the translocation mechanism of K⁺ provoked by single-crystal X-ray diffraction using Tl⁺ as a replacement. Here, we have applied cryogenic ion trap infrared laser spectroscopy to a key part of the K⁺ channel to address the question of chemical similarity between Tl⁺ and K⁺ where the conformational distribution of the ion-peptide complexes correlates with ion permeability. The conformations of Tl⁺ with a partial peptide of the K⁺ channel selectivity filter were revealed by combining vibrational spectroscopy and quantum chemical calculations. Upon K⁺ → Tl⁺ substitution, there is a change in the conformer population distributions, significantly different from permeable K⁺, but very close to that of impermeable Na⁺. This suggests that substantial differences may exist for the interactions of Tl⁺ and K⁺ in the K⁺ channel selectivity filter. This difference is rationalized by the changes in the ion-peptide interaction, as directly revealed from the vibrational spectra. The fundamental differences between Tl⁺ and the peptide indicate that the chemical behavior of Tl⁺ does not necessarily replicate that of K⁺, highlighting the need for caution in utilizing elemental substitution of K⁺ by Tl⁺ in spectroscopic and crystallographic analyses.