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.