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.