Model-free nuclear magnetic resonance study of intermolecular free energy landscapes in liquids with paramagnetic Ln3+ spotlights: Theory and application to Arg-Gly-Asp

Fries, Pascal H.

doi:doi:10.1063/1.3671990

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Prediction of reaction barriers and force-induced instabilities under mechanochemical conditions with an approximate model: A case study of the ring opening of 1,3-cyclohexadiene

Mechanochemistry, the use of mechanical stresses to activate chemical reactions, has emerged as a topic of significant interest. The present study examines the use of an approximate model for the prediction of reaction barriers under mechanochemical conditions using the ring opening of 1,3-cyclohexadiene along conrotatory and disrotatory directions as a specific test case. To do this, reaction barriers are evaluated using quantum chemical methods with an external force applied between various pa...

Consequences of local inter-strand dehybridization for large-amplitude bending fluctuations of double-stranded DNA

The wormlike chain model of DNA bending accurately reproduces single-molecule force-extension profiles of long (kilobase) chains. These bending statistics over large scales do not, however, establish a unique microscopic model for elasticity at the 1–10 basepair (bp) scale, which holds particular interest in biological contexts. Here, we examine a class of microscopic models which allow for disruption of base pairing (i.e., a “melt” or “kink”, generically an “excitation”) and consequently enhanc...

We propose an easily applicable method for investigating the pair distribution function of a lanthanide Ln³⁺ complex LnL (L = ligand) with respect to any solvent or solute molecule A carrying observable nuclear spins. Let r be the distance of Ln³⁺ to the observed nuclear spin I. We derive a simple expression of the experimental value of the configurational average of 1∕r⁶ in terms of longitudinal paramagnetic relaxation (rate) enhancements (PREs) of the spin I measured on a standard high-resolution NMR spectrometer and due to well-chosen concentrations of LnL complexes in which Ln³⁺ is a fast-relaxing paramagnetic lanthanide or the slowly-relaxing gadolinium Gd³⁺. The derivation is justified in the general case of a molecule A which is by turns in a bound state where it follows the complex and a free state where it moves independently. It rests on the expression of the underlying PRE theory in terms of the angle-dependent pair distribution function of LnL and A. The simplifications of this theory in the high-field regime and under the condition of fast exchange between bound and free states are carefully discussed. We also show that original information on the angle dependence of the molecular pair distribution function can be gained from the measured paramagnetic dipolar shifts induced by complexed fast-relaxing Ln³⁺ ions. The method is illustrated by the case study of the anionic Lnttha³⁻ = [Ln³⁺(ttha)]³⁻ (ttha⁶⁻ = triethylene tetraamine hexacetate) complex interacting with the biologically important tripeptide Arg-Gly-Asp (RGD) which carries peripheral ionic groups. The usefulness of an auxiliary reference outer sphere probe solute is emphasized.

Article Outline

INTRODUCTION
THE MOLECULAR PAIR DISTRIBUTION FUNCTION
1. Full and radial site-site structural information
2. Formation of molecular adducts
INTERMOLECULAR RELAXATION DUE TO A GIVEN Ln ³⁺ ION
1. Simple algebraic expressions of the relaxivity
  1. The Curie spin
  2. The outer sphere relaxivity
  3. The inner sphere relaxivity
  4. Fast-exchange limit
2. Link with the general theory of liquid structure
3. Direct measurement of the fluctuating relaxivity at low field
MIXTURES OF ISO-STRUCTURAL COMPLEXES OF Ln ³⁺ IONS
1. Dipolar shifts induced by fast relaxing Ln ³⁺ ions
  1. Instantaneous configuration
  2. Configurational average for a given fast-relaxing Ln ³⁺ ion
  3. Configurational average for a mixture of fast-relaxing Ln ³⁺ ions
2. Fluctuating relaxivity in solutions containing mixtures of isostructural Ln ³⁺ complexes
  1. Overall relaxivity
  2. Experimental determination
IONIC RECOGNITION OF THE TRIPEPTIDE Arg-Gly-Asp
1. NMR peak assignment of RGD in diamagnetic solution
2. Conformational studies
3. Lanthanide induced shifts
4. Magnetic moments of the complexed fast-relaxing Ln ³⁺
5. Relaxivity measurements
6. Reference outer sphere probe solute
CONCLUSION AND PROSPECTS

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KEYWORDS and PACS

Keywords

free energy, intermolecular forces, magnetic relaxation, molecular configurations, negative ions, nuclear magnetic resonance, organic compounds, paramagnetism, rare earth compounds, solvent effects

PACS

33.25.+k

Nuclear resonance and relaxation
34.20.Gj

Intermolecular and atom-molecule potentials and forces
33.15.Bh

General molecular conformation and symmetry; stereochemistry

ARTICLE DATA

Digital Object Identifier

http://dx.doi.org/10.1063/1.3671990

PUBLICATION DATA

ISSN

1931-9223 (online)

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American Institute of Physics

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