Infrared Spectra of Beauvericin-Alkaline Earth Metal Ion Complexes─Ion Preference to Physiological Ions.

Beauvericin (Bv) is a naturally occurring ionophore that selectively transports ions through cell membranes. However, the intrinsic ion selectivity of Bv for alkaline earth metal ions (M2+) is yet to be established due to inconsistent results from condensed phase experiments. Based on fluorescence quenching rates, Ca2+ appears to be preferred while extraction experiments favor Mg2+. In this study, we apply cold ion trap─infrared spectroscopy to Bv-M2+ coupled with electrospray ionization mass spectrometry. The mass spectrum shows that Bv favors binding to physiologically active ions Mg2+ and Ca2+ although it can form complexes with all four alkaline earth metal ions. Infrared spectroscopy, as measured by the H2 tag technique, reveals that Bv binds Mg2+ and Ca2+ ions by six carbonyl oxygens in the center of its cavity. This observation is supported by theoretical calculations. Other alkaline earth metal ions are bound by three carbonyl groups at the amide face. This difference in configuration is consistent with the binding preferences for the alkaline earth metal ions.

Ion Recognition beyond Size Matching: Cooperative Hydration Effect on the K+ Selectivity of Valinomycin over Na+ Revealed by Cryogenic Double Ion Trap Infrared Spectroscopy.

The naturally occurring ionophore valinomycin (VM) selectively transports K+ across the biological membrane, which makes VM a plausible antivirus and antibacterial candidate. The K+ selectivity of VM was rationalized based on a size-matching model despite structural inconsistency between experiments and computations. In this study, we investigated the conformations of the Na+VM complex with 1-10 water molecules using cryogenic ion trap infrared spectroscopy with computational calculations. It shows that the water molecule penetrates the cavity of VM deeply enough to distort the C3-symmetric structure of gas-phase Na+VM, in stark contrast to hydrated clusters of K+VM with C3-symmetric structure, where H2O is located outside the cavity. The high affinity to K+ would be ascribed to minimal hydration-induced structural deformation of K+VM compared to Na+VM. This study highlights a novel cooperative hydration effect on the K+ selectivity and will provide an updated understanding of its ionophoric properties beyond the traditional size-matching model.

Can Ag+ Permeate through a Potassium Ion Channel? A Bottom-Up Approach by Infrared Spectroscopy of the Ag+ Complex with the Partial Peptide of a Selectivity Filter.

Silver and silver ions have a long history of antimicrobial activity and medical applications. Nevertheless, the activity of Ag+ against bacteria, how it enters a cell, has not yet been established. The K+ channel, a membrane protein, is a possible route. The addition of a channel inhibitor (4-aminopyridine) to modulate the Ag+ uptake could support this view. However, the inhibitor enhances the uptake of Ag+, the opposite result. We have applied cold ion trap infrared laser spectroscopy to complexes of Ag+ and Ac-Tyr-NHMe (a model for GYG) which is a portion of the selectivity filter in the K+ channel to consider the question of permeation. With support from quantum chemical calculations, we have determined the stable conformations of the complex. The conformations strongly suggest that Ag+ would not readily permeate the K+ channel. The mechanism of the unexpected enhancement by the inhibitor is discussed.

Cation-responsive cavity expansion of valinomycin revealed by cryogenic ion trap infrared spectroscopy.

Valinomycin (VM) is a natural K+-selective ionophore that transports K+ through the cell membrane. VM captures K+ in its central cavity with a C3-symmetric ß-turn-like backbone. Although the binding affinity is drastically decreased for the VM-sodium (Na+VM) complex with respect to K+VM, VM holds relatively high affinity to Rb+ and Cs+. The high affinity for larger ions irrespective of ionic size seems to conflict with the expected optimal size matching model and raises questions on what factors determine ion selectivity. A combination of infrared spectroscopy with supporting computational calculations reveals that VM can accommodate larger Rb+ and Cs+ by flexibly changing its cavity size with the elongation of its folded ß-turn-like backbone. The high affinity to Rb+ and Cs+ can be ascribed to a size-dependent cavity expansion. These findings provide a new perspective on molecular recognition and selectivity beyond the conventional size matching model.

Na+ Selective Binding by Beauvericin and Its Mechanism Studied by Mass-Coupled Cold Ion Trap Infrared Spectroscopy.

Beauvericin (Bv) is a cyclic hexadepsipeptide mycotoxin that selectively transports ions across cell membranes. Characterization of its intrinsic ion affinity has been complicated by different previous results in condensed phases and biological membranes. We report the marked specificity between alkali metal ions by Bv using experimental and computational methods. Mass spectrometry shows Bv readily binds all five alkali ions; however, the complex with Na+ is the most abundant species, indicating a strong binding preference. Gas phase infrared spectroscopy and theoretical calculations show that Li+, K+, Rb+, and Cs+ are coordinated by three amide carbonyl oxygens on the N-methylamino-l-phenylalanyl face. Selectivity for Na+ is achieved as Bv sequesters Na+ in the center of its cavity formed by three amide carbonyl and three ester carbonyl groups, a configuration unique among alkali metal ions. This finding provides insight into the correlation between selectivity and conformation of Bv, essential for development of this mycotoxin.

A bottom-up approach to the ion recognition mechanism of K+ channels from laser spectroscopy of hydrated partial peptide-alkali metal ion complexes.

K+ channels allow selective permeation of K+, but not physiologically abundant Na+, at almost diffusion limit rates. The conduction mechanism of K+ channels is still controversial, with experimental and computation studies supporting two distinct conduction mechanisms: either with or without water inside the channel. Here, we employ a bottom-up approach on hydrated alkali metal complexes of a model peptide of K+ channels, Ac-Tyr-NHMe, to characterize metal-peptide, metal-water, and water-peptide interactions that govern the selectivity of K+ channels at a molecular level. Both the extension to the series of alkali metal ions and to temperature-dependent studies (approaching physiological values) have revealed the clear difference between permeable and non-permeable ions in the spectral features of the ion complexes. Furthermore, the impact of hydration is discussed in relation to the K+ channels by comparisons of the non-hydrated and hydrated complexes.

Double Ion Trap Laser Spectroscopy of Alkali Metal Ion Complexes with a Partial Peptide of the Selectivity Filter in K+ Channels─Temperature Effect and Barrier for Conformational Conversions.

Potassium ion channels selectively permeate K+, as well as Rb+ and Cs+ to some degree, while excluding Na+ and Li+. Conformations of alkali metal complexes of Ac-Tyr-NHMe, a model peptide of the selectivity filter in a K+ channel, were previously found to correlate with the permeability of alkali metal ions to a K+ channel by cold ion trap infrared spectroscopy. With an additional temperature-controlled ion trap, we examined the conformations of the alkali metal complexes, allowing the ions to collide with a He buffer gas at different temperatures, prior to spectroscopic investigation. The conformational distribution of the K+-peptide complex shows the most significant variation with temperature, which suggests that this complex has more flexibility when complexed with K+ and suggests lower barrier heights than other metal-peptide complexes. The variability of the conformational distribution with temperature for the ions follows the same order of ion permeability of a K+ channel. This work demonstrates that the additional temperature-controlled ion trap is a powerful tool to explore the conformational landscape of flexible molecular systems.

Potassium and sodium ion complexes with a partial peptide of the selectivity filter in K+ channels studied by cold ion trap infrared spectroscopy: the effect of hydration.

Potassium channels allow K+ to rapidly diffuse, while the selectivity filter (SF) actively blocks Na+. The presence of water in the SF during ion translocation remains under debate due to the experimental and computational challenges in characterizing the interactions between water, ions, and the SF. Our bottom-up approach has been applied to a system composed of a partial peptide of the SF (Ac-tyrosine-NHMe) with a metal ion and a single water molecule to probe these interactions. The IR photodissociation spectra of M+Ac-tyrosine-NHMe(H2O) (M = Na, K) combined with quantum chemical calculations revealed that the water molecule binding sites are ion-dependent. In addition, the ion-peptide distances are elongated significantly for the K+ complex in comparison to the Na+ complex by the addition of a single water molecule. This striking structural difference with the water molecule is discussed in relation to ion selectivity and translocation within the K+ channel.

Rethinking Ion Transport by Ionophores: Experimental and Computational Investigation of Single Water Hydration in Valinomycin-K+ Complexes.

Valinomycin is a macrocyclic ionophore that transports K+ across hydrophobic membranes. Its function depends on selectivity, capture, transport, and release of the ion. While thermodynamics clearly indicate that valinomycin binds K+ preferentially over all other alkali ions, characterizing the capture/transport/release of K+ by valinomycin at the molecular level remains a challenge. The bracelet-like structure of valinomycin-K+ (K+VM) has the ion completely enveloped, facilitating transport through the cell membrane. We report that hydration by a single water molecule, (K+VM)(H2O), produces three different conformers, identified by infrared spectroscopy and supporting computational studies. For two minor conformers, the water prevents the ionophore from closing, a conformation that would inhibit diffusion through the membrane. However, the dominant conformer encloses both the ion and the water, replicating the bracelet-like K+VM and arguably enhancing diffusion through the membrane. This potential for active participation of water in transport through the hydrophobic cellular membrane has never been previously considered.

Alkali and Alkaline Earth Metal Ions Complexes with a Partial Peptide of the Selectivity Filter in K+ Channels Studied by a Cold Ion Trap Infrared Spectroscopy.

The front cover artwork is provided by Takashi Tsujino (Science Graphics Co., Ltd.) . The image shows the efficacy of a bottom-up approach to ion selectivity of K+ channels. The GYG-K+ complex, which replicates the local portion of K+ channels, has three conformations with an equivalent distribution. Read the full text of the Article at 10.1002/cphc.202000033.