Identifying and Modulating Accidental Fermi Resonance: 2D IR and DFT Study of 4-Azido-l-phenylalanine

Probing calmodulin-NO synthase interactions via site-specific infrared spectroscopy: an introductory investigation

Calmodulin (CaM) binds to a linker between the oxygenase and reductase domains of nitric oxide synthase (NOS) to regulate the functional conformational dynamics. Specific residues on the interdomain interface guide the domain-domain docking to facilitate the electron transfer in NOS. Notably, the docking interface between CaM and the heme-containing oxygenase domain of NOS is isoform specific, which is only beginning to be investigated. Toward advancing understanding of the distinct CaM-NOS docking interactions by infrared spectroscopy, we introduced a cyano-group as frequency-resolved vibrational probe into CaM individually and when associated with full-length and a bi-domain oxygenase/FMN construct of the inducible NOS isoform (iNOS). Site-specific, selective labeling with p-cyano-L-phenylalanine (CNF) by amber suppression of CaM bound to the iNOS has been accomplished by protein coexpression due to the instability of recombinant iNOS protein alone. We introduced CNF at residue 108, which is at the putative CaM-heme (NOS) docking interface. CNF was also introduced at residue 29, which is distant from the docking interface. FT IR data show that the 108 site is sensitive to CaM-NOS complex formation, while insensitivity to its association with the iNOS protein or peptide was observed for the 29 site. Moreover, narrowing of the IR bands at residue 108 suggests the C≡N probe experiences a more limited distribution of environments, indicating side chain restriction apparent for the complex with iNOS. This initial work sets the stage for residue-specific characterizations of structural dynamics of the docked states of NOS proteins.

Efficient and Inexpensive Synthesis of 15N-Labeled 2-Azido-1,3-dimethylimidazolinium Salts Using Na15NO2 Instead of Na15NNN

15N-Labeled azides are important probes for infrared and magnetic resonance spectroscopy and imaging. They can be synthesized by reaction of primary amines with a 15N-labeled diazo-transfer reagent. We present the synthesis of 15N-labeled 2-azido-1,3-dimethylimidazolinium salts 1 as a 15N-labeled diazo-transfer reagent. Nitrosation of 1,3-dimethylimidazolinium-2-yl hydrazine (2) with Na15NO2 under acidic conditions gave 1 as a 1:1 mixture of α- and γ-15N-labeled azides, α- and γ-1, rather than γ-1 alone. The isotopomeric mixture thus obtained was then subjected to the diazo-transfer reaction with primary amines 3 to afford azides 4 as a 1:1 mixture of β-15N-labeled azides β-4 and unlabeled ones 4'. The efficient and inexpensive synthesis of 1 as a 1:1 mixture of α- and γ-1 using Na15NO2 instead of Na15NNN facilitates their wide use as a 15N-labeled diazo-transfer reagent for preparing 15N-labeled azides as molecular probes.

Multidimensional Widefield Infrared-Encoded Spontaneous Emission Microscopy: Distinguishing Chromophores by Ultrashort Infrared Pulses

Photoluminescence (PL) imaging has broad applications in visualizing biological activities, detecting chemical species, and characterizing materials. However, the chemical information encoded in the PL images is often limited by the overlapping emission spectra of chromophores. Here, we report a PL microscopy based on the nonlinear interactions between mid-infrared and visible excitations on matters, which we termed MultiDimensional Widefield Infrared-encoded Spontaneous Emission (MD-WISE) microscopy. MD-WISE microscopy can distinguish chromophores that possess nearly identical emission spectra via conditions in a multidimensional space formed by three independent variables: the temporal delay between the infrared and the visible pulses (t), the wavelength of visible pulses (λvis), and the frequencies of the infrared pulses (ωIR). This method is enabled by two mechanisms: (1) modulating the optical absorption cross sections of molecular dyes by exciting specific vibrational functional groups and (2) reducing the PL quantum yield of semiconductor nanocrystals, which was achieved through strong field ionization of excitons. Importantly, MD-WISE microscopy operates under widefield imaging conditions with a field of view of tens of microns, other than the confocal configuration adopted by most nonlinear optical microscopies, which require focusing the optical beams tightly. By demonstrating the capacity of registering multidimensional information into PL images, MD-WISE microscopy has the potential of expanding the number of species and processes that can be simultaneously tracked in high-speed widefield imaging applications.

Probing the Hydrophobic Region of a Lipid Bilayer at Specific Depths Using Vibrational Spectroscopy

A novel spectroscopic approach for studying the flexibility and mobility in the hydrophobic interior of lipid bilayers at specific depths is proposed. A set of test compounds featuring an azido moiety and a cyano or carboxylic acid moiety, connected by an alkyl chain of different lengths, was synthesized. FTIR data and molecular dynamics calculations indicated that the test compounds in a bilayer are oriented so that the cyano or carboxylic acid moiety is located in the lipid head-group region, while the azido group stays inside the bilayer at the depth determined by its alkyl chain length. We found that the asymmetric stretching mode of the azido group (νN3) can serve as a reporter of the membrane interior dynamics. FTIR and two-dimensional infrared (2DIR) studies were performed at different temperatures, ranging from 22 to 45 °C, covering the Lβ-Lα phase transition temperature of dipalmitoylphosphatidylcholine (∼41 °C). The width of the νN3 peak was found to be very sensitive to the phase transition and to the temperature in general. We introduced an order parameter, SN3, which characterizes restrictions to motion inside the bilayer. 2DIR spectra of νN3 showed different extents of inhomogeneity at different depths in the bilayer, with the smallest inhomogeneity in the middle of the leaflet. The spectral diffusion dynamics of the N3 peak was found to be dependent on the depth of the N3 group location in the bilayer. The obtained results enhance our understanding of the bilayer dynamics and can be extended to investigate membranes with more complex compositions.

Anisotropic dynamics of an interfacial enzyme active site observed using tethered substrate analogs and ultrafast 2D IR spectroscopy

Enzymes accelerate the rates of biomolecular reactions by many orders of magnitude compared to bulk solution, and it is widely understood that this catalytic effect arises from a combination of polar pre-organization and electrostatic transition state stabilization. A number of recent reports have also implicated ultrafast (femtosecond-picosecond) timescale motions in enzymatic activity. However, complications arising from spatially-distributed disorder, the occurrence of multiple substrate binding modes, and the influence of hydration dynamics on solvent-exposed active sites still confound many experimental studies. Here we use ultrafast two-dimensional infrared (2D IR) spectroscopy and covalently-tethered substrate analogs to examine dynamical properties of the promiscuous Pyrococcus horikoshii ene-reductase (PhENR) active site in two binding configurations mimicking proposed "inactive" and "reactive" Michaelis complexes. Spectral diffusion measurements of aryl-nitrile substrate analogs reveal an end-to-end tradeoff between fast (sub-ps) and slow (>5 ps) motions. Fermi resonant aryl-azide analogs that sense interactions of coupled oscillators are described. Lineshape and quantum beat analyses of these probes reveal characteristics that correlate with aryl-nitrile frequency fluctuation correlation functions parameters, demonstrating that this anisotropy is an intrinsic property of the water-exposed active site, where countervailing gradients of fast dynamics and disorder in the reactant ground state are maintained near the hydration interface. Our results suggest several plausible factors leading to state-selective rate enhancement and promiscuity in PhENR. This study also highlights a strategy to detect perturbations to vibrational modes outside the transparent window of the mid-IR spectrum, which may be extended to other macromolecular systems.

Effects of the Intramolecular Group and Solvent on Vibrational Coupling Modes and Strengths of Fermi Resonances in Aryl Azides: A DFT Study of 4-Azidotoluene and 4-Azido-N-phenylmaleimide

The high transition dipole strength of the azide asymmetric stretch makes aryl azides good candidates as vibrational probes (VPs). However, aryl azides have complex absorption profiles due to Fermi resonances (FRs). Understanding the origin and the vibrational modes involved in FRs of aryl azides is critically important toward developing them as VPs for studies of protein structures and structural changes in response to their surroundings. As such, we studied vibrational couplings in 4-azidotoluene and 4-azido-N-phenylmaleimide in two solvents, N,N-dimethylacetamide and tetrahydrofuran, to explore the origin and the effects of intramolecular group and solvent on the FRs of aryl azides using density functional theory (DFT) calculations with the B3LYP functional and seven basis sets, 6-31G(d,p), 6-31+G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311+G(d,p), 6-311++G(d,p), and 6-311++G(df,pd). Two combination bands consisting of the azide symmetric stretch and another mode form strong FRs with the azide asymmetric stretch for both molecules. The FR profile was altered by replacing the methyl group with maleimide. Solvents change the relative peak position and intensity more significantly for 4-azido-N-phenylmaleimide, which makes it a more sensitive VP. Furthermore, the DFT results indicate that a comparison among the results from different basis sets can be used as a means to predict more reliable vibrational spectra.

Inhibition of vibrational energy flow within an aromatic scaffold via heavy atom effect

The regulation of intramolecular vibrational energy redistribution (IVR) to influence energy flow within molecular scaffolds provides a way to steer fundamental processes of chemistry, such as chemical reactivity in proteins and design of molecular diodes. Using two-dimensional infrared (2D IR) spectroscopy, changes in the intensity of vibrational cross-peaks are often used to evaluate different energy transfer pathways present in small molecules. Previous 2D IR studies of para-azidobenzonitrile (PAB) demonstrated that several possible energy pathways from the N3 to the cyano-vibrational reporters were modulated by Fermi resonance, followed by energy relaxation into the solvent [Schmitz et al., J. Phys. Chem. A 123, 10571 (2019)]. In this work, the mechanisms of IVR were hindered via the introduction of a heavy atom, selenium, into the molecular scaffold. This effectively eliminated the energy transfer pathway and resulted in the dissipation of the energy into the bath and direct dipole-dipole coupling between the two vibrational reporters. Several structural variations of the aforementioned molecular scaffold were employed to assess how each interrupted the energy transfer pathways, and the evolution of 2D IR cross-peaks was measured to assess the changes in the energy flow. By eliminating the energy transfer pathways through isolation of specific vibrational transitions, through-space vibrational coupling between an azido (N3) and a selenocyanato (SeCN) probe is facilitated and observed for the first time. Thus, the rectification of this molecular circuitry is accomplished through the inhibition of energy flow using heavy atoms to suppress the anharmonic coupling and, instead, favor a vibrational coupling pathway.

Genetically encoded non-canonical amino acids reveal asynchronous dark reversion of chromophore, backbone and side-chains in EL222

Photoreceptors containing the light-oxygen-voltage (LOV) domain elicit biological responses upon excitation of their flavin mononucleotide (FMN) chromophore by blue light. The mechanism and kinetics of dark-state recovery are not well understood. Here we incorporated the non-canonical amino acid p-cyanophenylalanine (CNF) by genetic code expansion technology at forty-five positions of the bacterial transcription factor EL222. Screening of light-induced changes in infrared (IR) absorption frequency, electric field and hydration of the nitrile groups identified residues CNF31 and CNF35 as reporters of monomer/oligomer and caged/decaged equilibria, respectively. Time-resolved multi-probe UV/Visible and IR spectroscopy experiments of the lit-to-dark transition revealed four dynamical events. Predominantly, rearrangements around the A'α helix interface (CNF31 and CNF35) precede FMN-cysteinyl adduct scission, folding of α-helices (amide bands), and relaxation of residue CNF151. This study illustrates the importance of characterizing all parts of a protein and suggests a key role for the N-terminal A'α extension of the LOV domain in controlling EL222 photocycle length. This article is protected by copyright. All rights reserved.

Versatile Vibrational Energy Sensors for Proteins

Vibrational energy transfer (VET) is emerging as key mechanism for protein functions, possibly playing an important role for energy dissipation, allosteric regulation, and enzyme catalysis. A deep understanding of VET is required to elucidate its role in such processes. Ultrafast VIS-pump/IR-probe spectroscopy can detect pathways of VET in proteins. However, the requirement of having a VET donor and a VET sensor installed simultaneously limits the possible target proteins and sites; to increase their number we compare six IR labels regarding their utility as VET sensors. We compare these labels in terms of their FTIR, and VET signature in VET donor-sensor dipeptides in different solvents. Furthermore, we incorporated four of these labels in PDZ3 to assess their capabilities in more complex systems. Our results show that different IR labels can be used interchangeably, allowing for free choice of the right label depending on the system under investigation and the methods available.

Population Dynamics of Stretching Excitations of p-Azido-phenylalanine Incorporated in Calmodulin-Peptide Complexes

We genetically incorporated the unnatural amino acid p-azido-phenylalanine (AzF) into the ubiquitous Ca²⁺ sensor protein calmodulin (CaM) in complex with different peptides to explore the response of the azido stretching line shape to varying binding motifs with femtosecond infrared spectroscopy. The dynamic response of the azido stretching mode varies in different CaM-peptide complexes. We model these dynamics as coherent excitations of Fermi resonances and extract a lifetime of the azido stretching vibration of about 1 ps. The resulting model parameters are commensurate with the linear infrared absorption lineshapes which suggests that the conformation-sensitive vibrational lineshape could be composed of Fermi resonances that differ between the protein-peptide complexes.

Transparent window 2D IR spectroscopy of proteins

Proteins are complex, heterogeneous macromolecules that exist as ensembles of interconverting states on a complex energy landscape. A complete, molecular-level understanding of their function requires experimental tools to characterize them with high spatial and temporal precision. Infrared (IR) spectroscopy has an inherently fast time scale that can capture all states and their dynamics with, in principle, bond-specific spatial resolution. Two-dimensional (2D) IR methods that provide richer information are becoming more routine but remain challenging to apply to proteins. Spectral congestion typically prevents selective investigation of native vibrations; however, the problem can be overcome by site-specific introduction of amino acid side chains that have vibrational groups with frequencies in the "transparent window" of protein spectra. This Perspective provides an overview of the history and recent progress in the development of transparent window 2D IR of proteins.

Site-by-site tracking of signal transduction in an azidophenylalanine-labeled bacteriophytochrome with step-scan FTIR spectroscopy

Signal propagation in photosensory proteins is a complex and multidimensional event. Unraveling such mechanisms site-specifically in real time is an eligible but a challenging goal. Here, we elucidate the site-specific events in a red-light sensing phytochrome using the unnatural amino acid azidophenylalanine, vibrationally distinguishable from all other protein signals. In canonical phytochromes, signal transduction starts with isomerization of an excited bilin chromophore, initiating a multitude of processes in the photosensory unit of the protein, which eventually control the biochemical activity of the output domain, nanometers away from the chromophore. By implementing the label in prime protein locations and running two-color step-scan FTIR spectroscopy on the Deinococcus radiodurans bacteriophytochrome, we track the signal propagation at three specific sites in the photosensory unit. We show that a structurally switchable hairpin extension, a so-called tongue region, responds to the photoconversion already in microseconds and finalizes its structural changes concomitant with the chromophore, in milliseconds. In contrast, kinetics from the other two label positions indicate that the site-specific changes deviate from the chromophore actions, even though the labels locate in the chromophore vicinity. Several other sites for labeling resulted in impaired photoswitching, low structural stability, or no changes in the difference spectrum, which provides additional information on the inner dynamics of the photosensory unit. Our work enlightens the multidimensionality of the structural changes of proteins under action. The study also shows that the signaling mechanism of phytochromes is accessible in a time-resolved and site-specific approach by azido probes and demonstrates challenges in using these labels.

A Genetically Encoded Two-Dimensional Infrared Probe for Enzyme Active-Site Dynamics

While two-dimensional infrared (2D-IR) spectroscopy is uniquely suitable for monitoring femtosecond (fs) to picosecond (ps) water dynamics around static protein structures, its utility for probing enzyme active-site dynamics is limited due to the lack of site-specific 2D-IR probes. We demonstrate the genetic incorporation of a novel 2D-IR probe, m-azido-L-tyrosine (N3Y) in the active-site of DddK, an iron-dependent enzyme that catalyzes the conversion of dimethylsulfoniopropionate to dimethylsulphide. Our results show that both the oxidation of active-site iron to Fe^III , and the addition of denaturation reagents, result in significant decrease in enzyme activity and active-site water motion confinement. As tyrosine residues play important roles, including as general acids and bases, and electron transfer agents in many key enzymes, the genetically encoded 2D-IR probe N3Y should be broadly applicable to investigate how the enzyme active-site motions at the fs-ps time scale direct reaction pathways to facilitating specific chemical reactions.

Two-dimensional IR spectroscopy reveals a hidden Fermi resonance band in the azido stretch spectrum of β-azidoalanine

Azido stretch modes in a variety of azido-derivatized nonnatural amino acids and nucleotides have been used as a site-specific infrared (IR) probe for monitoring changes in their conformations and local electrostatic environments. The vibrational bands of azide probes are often accompanied by complex line shapes with shoulder peaks, which may arise either from incomplete background subtraction, Fermi resonance, or multiple conformers. The isotope substitution in the infrared probe has thus been introduced to remove Fermi resonances without causing a significant perturbation to the structure. Here, we synthesized and labeled the mid-N atoms of aliphatic azide derivatives with 15N to study the effects of isotope labelling on their vibrational properties. The FT-IR spectra of the aliphatic azide with asymmetric lineshape became a single symmetric band upon isotope substitution, which might be an indication of the removal of the hidden Fermi resonance from the system. We also noticed that the 2D-IR spectrum of unlabeled aliphatic azide has cross-peaks, even though it is not apparently identifiable. The 1D slice spectra obtained from the 2D-IR spectra reveal the existence of a hidden Fermi resonance peak. Furthermore, we show that this weak Fermi resonance does not produce discernible oscillatory beating patterns in the IR pump-probe spectrum, which has been used as evidence of the Fermi resonance. Therefore, we confirm that isotope labelling combined with 2D-IR spectroscopy is the most efficient and incisive way to identify the origin of small shoulder peaks in the linear and nonlinear vibrational spectra of various IR probe molecules.

Vibrational Spectroscopic Map, Vibrational Spectroscopy, and Intermolecular Interaction

Vibrational spectroscopy is an essential tool in chemical analyses, biological assays, and studies of functional materials. Over the past decade, various coherent nonlinear vibrational spectroscopic techniques have been developed and enabled researchers to study time-correlations of the fluctuating frequencies that are directly related to solute-solvent dynamics, dynamical changes in molecular conformations and local electrostatic environments, chemical and biochemical reactions, protein structural dynamics and functions, characteristic processes of functional materials, and so on. In order to gain incisive and quantitative information on the local electrostatic environment, molecular conformation, protein structure and interprotein contacts, ligand binding kinetics, and electric and optical properties of functional materials, a variety of vibrational probes have been developed and site-specifically incorporated into molecular, biological, and material systems for time-resolved vibrational spectroscopic investigation. However, still, an all-encompassing theory that describes the vibrational solvatochromism, electrochromism, and dynamic fluctuation of vibrational frequencies has not been completely established mainly due to the intrinsic complexity of intermolecular interactions in condensed phases. In particular, the amount of data obtained from the linear and nonlinear vibrational spectroscopic experiments has been rapidly increasing, but the lack of a quantitative method to interpret these measurements has been one major obstacle in broadening the applications of these methods. Among various theoretical models, one of the most successful approaches is a semiempirical model generally referred to as the vibrational spectroscopic map that is based on a rigorous theory of intermolecular interactions. Recently, genetic algorithm, neural network, and machine learning approaches have been applied to the development of vibrational solvatochromism theory. In this review, we provide comprehensive descriptions of the theoretical foundation and various examples showing its extraordinary successes in the interpretations of experimental observations. In addition, a brief introduction to a newly created repository Web site (http://frequencymap.org) for vibrational spectroscopic maps is presented. We anticipate that a combination of the vibrational frequency map approach and state-of-the-art multidimensional vibrational spectroscopy will be one of the most fruitful ways to study the structure and dynamics of chemical, biological, and functional molecular systems in the future.

2D-IR studies of cyanamides (NCN) as spectroscopic reporters of dynamics in biomolecules: Uncovering the origin of mysterious peaks

Cyanamides (NCN) have been shown to have a larger transition dipole strength than cyano-probes. In addition, they have similar structural characteristics and vibrational lifetimes to the azido-group, suggesting their utility as infrared (IR) spectroscopic reporters for structural dynamics in biomolecules. To access the efficacy of NCN as an IR probe to capture the changes in the local environment, several model systems were evaluated via 2D IR spectroscopy. Previous work by Cho [G. Lee, D. Kossowska, J. Lim, S. Kim, H. Han, K. Kwak, and M. Cho, J. Phys. Chem. B 122(14), 4035-4044 (2018)] showed that phenylalanine analogues containing NCN show strong anharmonic coupling that can complicate the interpretation of structural dynamics. However, when NCN is embedded in 5-membered ring scaffolds, as in N-cyanomaleimide and N-cyanosuccinimide, a unique band structure is observed in the 2D IR spectrum that is not predicted by simple anharmonic frequency calculations. Further investigation indicated that electron delocalization plays a role in the origins of the band structure. In particular, the origin of the lower frequency transitions is likely a result of direct interaction with the solvent.

Application of a transparent window vibrational probe (azido probe) to the structural dynamics of model dipeptides and amyloid β-peptide

The azido asymmetric stretching motion is widely used for the elucidation of the intrinsic conformational preference and folding mechanism of protein since it has strong vibrational absorbance in the spectral transparent windows. However, the possible secondary structural disturbance induced by the insertion of azido group in the side chain of polypeptides should be carefully evaluated. Here, DFT calculation and enhanced sampling method were employed for model dipeptides with or without azido substitution, and the outcome results show that the lower potential energy basins of isolated model dipeptides are consistent with the preferred structural distributions of model dipeptides in aqueous solution. The azido asymmetric stretching frequency shows its sensitivity to the backbone configurations just like amide-I vibration does, and the azido vibration exhibits great potential as a structural reporter in the transparent window. For the evaluation of the application of azido group in biologically related system, the structural dynamics of Aβ_37-42 and N₃-Aβ_37-42 fragments and the self-assemble process of their protofiliments in aqueous solution were demonstrated. The outcome results show that the structural fluctuations of Aβ_37-42 and its protofilament in aqueous solution are quite similar with or without azido substitution, and the dewetting transitions of Aβ_37-42 and N₃-Aβ_37-42 β-sheet layers are both complete within 30 ns and assemble into stable protofilaments. Therefore, the azido asymmetric vibrational motion is a minimally invasive structural probe and would not introduce much disturbance to the structural dynamics of polypeptides.

Tuning Molecular Vibrational Energy Flow within an Aromatic Scaffold via Anharmonic Coupling

From guiding chemical reactivity in synthesis or protein folding to the design of energy diodes, intramolecular vibrational energy redistribution harnesses the power to influence the underlying fundamental principles of chemistry. To evaluate the ability to steer these processes, the mechanism and time scales of intramolecular vibrational energy redistribution through aromatic molecular scaffolds have been assessed by utilizing two-dimensional infrared (2D IR) spectroscopy. 2D IR cross peaks reveal energy relaxation through an aromatic scaffold from the azido- to the cyano-vibrational reporters in para-azidobenzonitrile (PAB) and para-(azidomethyl)benzonitrile (PAMB) prior to energy relaxation into the solvent. The rates of energy transfer are modulated by Fermi resonances, which are apparent by the coupling cross peaks identified within the 2D IR spectrum. Theoretical vibrational mode analysis allowed the determination of the origins of the energy flow, the transfer pathway, and a direct comparison of the associated transfer rates, which were in good agreement with the experimental results. Large variations in energy-transfer rates, approximately 1.9 ps for PAB and 23 ps for PAMB, illustrate the importance of strong anharmonic coupling, i.e., Fermi resonance, on the transfer pathways. In particular, vibrational energy rectification is altered by Fermi resonances of the cyano- and azido-modes allowing control of the propensity for energy flow.

Conformation-specific detection of calmodulin binding using the unnatural amino acid p-azido-phenylalanine (AzF) as an IR-sensor

Calmodulin (CaM) is a very conserved, ubiquitous, eukaryotic protein that binds four Ca²⁺ ions with high affinity. It acts as a calcium sensor by translating Ca²⁺ signals into cellular processes such as metabolism, inflammation, immune response, memory, and muscle contraction. Calcium binding to CaM leads to conformational changes that enable Ca²⁺/CaM to recognize and bind various target proteins with high affinity. The binding mode and binding partners of CaM are very diverse, and a consensus binding sequence is lacking. Here, we describe an elegant system that allows conformation-specific detection of CaM-binding to its binding partners. We incorporate the unnatural amino acid p-azido-phenylalanine (AzF) in different positions of CaM and follow its unique spectral signature by infrared (IR)-spectroscopy of the azido stretching vibration. Our results suggest that the AzF vibrational probe is sensitive to the chemical environment in different CaM/CaM-binding domain (CaMBD) complexes, which allows differentiating between different binding motifs according to the spectral characteristics of the azido stretching mode. We corroborate our results with a crystal structure of AzF-labelled CaM (CaM108AzF) in complex with a binding peptide from calmodulin-dependent protein kinase IIα identifying the structural basis for the observed IR frequency shifts.