Azido alkanes as convenient reporters for mobility within lipid membranes

Varner, Clyde; Xiao, Zhou; Saxman, Zowie K.; Leger, Joel D.; Jayawickramarajah, Janarthanan; Rubtsov, Igor V.

doi:10.1016/j.chemphys.2018.05.020

Cited by 13 publications

(36 citation statements)

References 41 publications

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“… 20 , 21 To determine the vibrational frequencies and anharmonic force constants, DFT anharmonic computations were performed using the Gaussian-09 suite 27 for the 14/15 N 3 -(CH 2 ) 5 - 2000 H compounds, denoted as 14 N 3 C5 and 15 N 3 C5, where a large mass for the chain-terminating hydrogen was used to decouple its motion from other motions in the compounds. In agreement with previous reports for 14 N 3 – initiation, 2 , 23 the ν N≡N relaxation predominantly populates a combination band of modes ν N=N and ν N–C ( Figure 4 A,B), which accounts for >90% of all relaxation channels for both isotopes. To efficiently create a wavepacket in the chain upon the tag relaxation, the relaxation daughter states need to be harmonically mixed with more than one chain state of the same chain band (mechanism B).…”

Section: Resultssupporting

confidence: 92%

“…It is possible to have multiple FRs involving a range of chain states, as reported for the azido end group. 23 Such combination bands can be directly excited with a photon and can, under certain conditions, form a wavepacket within the chain states.…”

Section: Resultsmentioning

confidence: 99%

“…Moreover, the FRs were shown to be a major reason for a large ν N≡N transition width, observed even in nonpolar solvents. 23 Both states of the combination state are involved in mixing with the chain states, wagging for ν N=N and rocking for ν N–C , resulting in more than one FR. The mixing increases with the increase in chain length, and the number of FRs is expected to increase.…”

Section: Resultsmentioning

confidence: 99%

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Competition of Several Energy-Transport Initiation Mechanisms Defines the Ballistic Transport Speed

et al. 2021

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The ballistic regime of vibrational energy transport in oligomeric molecular chains occurs with a constant, often high, transport speed and high efficiency. Such a transport regime can be initiated by exciting a chain end group with a mid-infrared (IR) photon. To better understand the wavepacket formation process, two chemically identical end groups, azido groups with normal, 14 N 3 -, and isotopically substituted, 15 N 3 -, nitrogen atoms, were tested for wavepacket initiation in compounds with alkyl chains of n = 5, 10, and 15 methylene units terminated with a carboxylic acid (-a) group, denoted as 14 N 3 C n -a and 15 N 3 C n -a. The transport was initiated by exciting the azido moiety stretching mode, the ν N≡N tag, at 2100 cm –1 ( 14 N 3 C n -a) or 2031 cm –1 ( 15 N 3 C n -a). Opposite to the expectation, the ballistic transport speed was found to decrease upon 14 N 3 → 15 N 3 isotope editing. Three mechanisms of the transport initiation of a vibrational wavepacket are described and analyzed. The first mechanism involves the direct formation of a wavepacket via excitation with IR photons of several strong Fermi resonances of the tag mode with the ν N=N + ν N–C combination state while each of the combination state components is mixed with delocalized chain states. The second mechanism relies on the vibrational relaxation of an end-group-localized tag into a mostly localized end-group state that is strongly coupled to multiple delocalized states of a chain band. Harmonic mixing of ν N=N of the azido group with CH 2 wagging states of the chain permits a wavepacket formation within a portion of the wagging band, suggesting a fast transport speed. The third mechanism involves the vibrational relaxation of an end-group-localized mode into chain states. Two such pathways were found for the ν N≡N initiation: The ν N=N mode relaxes efficiently into the twisting band states and low-frequency acoustic modes, and the ν N–C mode relaxes into the rocking band states and low-frequency acoustic modes. The contributions of the three initiation mechanisms in the ballistic energy transport initiated by ν N≡N tag are quantitatively evaluated and related to the experiment. We conclude that the third mechanism dominates the transport in alkane chains of 5–15 methylene units initiated with the ν ...

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Section: Resultssupporting

confidence: 92%

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

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Competition of Several Energy-Transport Initiation Mechanisms Defines the Ballistic Transport Speed

et al. 2021

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“…In saturated lipids, the only feasible modes used to probe the bilayer core are the CH 2 or CH 3 stretching modes. Vibrational coupling makes it difficult to obtain an atomistic interpretation of measurements in this frequency region, and for that reason, extrinsic probes such as metal carbonyls have been introduced to measure dynamics within the hydrophobic core. , In past studies, the combination of vibrational probes with ultrafast spectroscopy has provided numerous insights into protein environments and dynamics, and the same methods can be directly translated to membrane studies . Probes become useful when the intrinsic vibrational modes are weak, insensitive to their environment, or when spectra are difficult to interpret due to spectral congestion or other factors.…”

Section: Model Systems and Case Studiesmentioning

confidence: 99%

Ultrafast Dynamics at Lipid–Water Interfaces

2020

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Metrics & MoreArticle Recommendations * sı Supporting Information CONSPECTUS: Lipid membranes are more than just barriers between cell compartments; they provide molecular environments with a finely tuned balance between hydrophilic and hydrophobic interactions that enable proteins to dynamically fold and self-assemble to regulate biological function. Characterizing dynamics at the lipid−water interface is essential to understanding molecular complexities from the thermodynamics of liquid−liquid phase separation down to picosecond-scale reorganization of interfacial hydrogen-bond networks. Ultrafast vibrational spectroscopy, including two-dimensional infrared (2D IR) and vibrational sum-frequency generation (VSFG) spectroscopies, is a powerful tool to examine picosecond interfacial dynamics. Two-dimensional IR spectroscopy provides a bond-centered view of dynamics with subpicosecond time resolutions, as vibrational frequencies are highly sensitive to the local environment. Recently, 2D IR spectroscopy has been applied to carbonyl and phosphate vibrations intrinsically located at the lipid−water interface. Interface-specific VSFG spectroscopy probes the water vibrational modes directly, accessing H-bond strength and water organization at lipid headgroup positions. Signals in VSFG arise from the interfacial dipole contributions, directly probing headgroup ordering and water orientation to provide a structural view of the interface.In this Account we discuss novel applications of ultrafast spectroscopy to lipid membranes, a field that has experienced significant growth over the past decade. In particular, ultrafast experiments now offer a molecular perspective on increasingly complex membranes. The powerful combination of ultrafast, interface-selective spectroscopy and simulations opens up new routes to understanding multicomponent membranes and their function. This Account highlights key prevailing views that have emerged from recent experiments: (1) Water dynamics at the lipid−water interface are slow compared to those of bulk water as a result of disrupted H-bond networks near the headgroups. (2) Peptides, ions, osmolytes, and cosolvents perturb interfacial dynamics, indicating that dynamics at the interface are affected by bulk solvent dynamics and vice versa.(3) The interfacial environment is generally dictated by the headgroup structure and orientation, but hydrophobic interactions within the acyl chains also modulate interfacial dynamics. Ultrafast spectroscopy has been essential to characterizing the biophysical chemistry of the lipid−water interface; however, challenges remain in interpreting congested spectra as well as designing appropriate model systems to capture the complexity of a membrane environment.

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“…The single carbon atom separation between the azido-group and the benzene ring is likely decoupling the ring modes of benzene and the azido-transition. 78 The experimental relative IR intensity ratio between the azido-to cyano-stretches was determined to be 3.4:1 and 4.7:1 for PAMB and PAB, respectively. The observed trend in the intensity ratios is in agreement with with the DFT calculated trend for PAMB (5.9:1) and PAB (8.8:1).…”

Section: Resultsmentioning

confidence: 94%

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

et al. 2019

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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 timescales 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 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.

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Azido alkanes as convenient reporters for mobility within lipid membranes

Cited by 13 publications

References 41 publications

Competition of Several Energy-Transport Initiation Mechanisms Defines the Ballistic Transport Speed

Competition of Several Energy-Transport Initiation Mechanisms Defines the Ballistic Transport Speed

Ultrafast Dynamics at Lipid–Water Interfaces

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

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