Relaxation Time Determinations by Progressive Saturation EPR

Effects of Molecular Motion and Zeeman Modulation for Spin Labels

V. A. Livshits, T. Páli, D. Marsh

Research output: Contribution to journalArticle

20 Citations (Scopus)

Abstract

The EPR spectra of nitroxide spin labels have been simulated as a function of microwave field, H1, taking into account both magnetic field modulation and molecular rotation. It is found that the saturation of the second integral, S, of the first harmonic in-phase absorption spectrum is approximated by that predicted for slow-passage conditions, that is, S ∼ H1/√1 + PH21, in all cases. This result is independent of the degree of inhomogeneous broadening. In general, the fitting parameter, P, depends not only on the T1 and T2 relaxation times, but also on the rate of molecular reorientation and on the modulation frequency. Calibrations for determining the relaxation times are established from the simulations. For a given modulation frequency and molecular reorientation rate, the parameter obtained by fitting the saturation curves is given by 1/P = a + 1/γ2eT1·Teff 2, where Teff2 is the effective T2. For molecular reorientation frequencies in the range 2 X 107-2 X 108 s-1, Teff2 is dominated by the molecular dynamics and is only weakly dependent on the intrinsic T02, allowing a direct estimation of T1. For reorientation frequencies outside this range, the (T1T2) product may be determined from the calibrations. The method is applied to determining relaxation times for spin labels undergoing different rates of rotational reorientation in a variety of environments, including those of biological relevance, and is verified experimentally by the relaxation rate enhancements induced by paramagnetic ions.

Original languageEnglish
Pages (from-to)79-91
Number of pages13
JournalJournal of Magnetic Resonance
Volume133
Issue number1
Publication statusPublished - Jul 1998

Fingerprint

Spin Labels
Relaxation time
retraining
Paramagnetic resonance
relaxation time
Modulation
Frequency modulation
saturation
modulation
Calibration
frequency modulation
Magnetic Fields
Molecular Dynamics Simulation
Microwaves
Molecular dynamics
Absorption spectra
molecular rotation
Ions
Magnetic fields
molecular dynamics

Keywords

  • Progressive saturation EPR
  • Spectral simulation
  • Spin labels
  • Spin-lattice relaxation

ASJC Scopus subject areas

  • Molecular Biology
  • Physical and Theoretical Chemistry
  • Spectroscopy
  • Radiology Nuclear Medicine and imaging
  • Condensed Matter Physics

Cite this

Relaxation Time Determinations by Progressive Saturation EPR : Effects of Molecular Motion and Zeeman Modulation for Spin Labels. / Livshits, V. A.; Páli, T.; Marsh, D.

In: Journal of Magnetic Resonance, Vol. 133, No. 1, 07.1998, p. 79-91.

Research output: Contribution to journalArticle

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N2 - The EPR spectra of nitroxide spin labels have been simulated as a function of microwave field, H1, taking into account both magnetic field modulation and molecular rotation. It is found that the saturation of the second integral, S, of the first harmonic in-phase absorption spectrum is approximated by that predicted for slow-passage conditions, that is, S ∼ H1/√1 + PH21, in all cases. This result is independent of the degree of inhomogeneous broadening. In general, the fitting parameter, P, depends not only on the T1 and T2 relaxation times, but also on the rate of molecular reorientation and on the modulation frequency. Calibrations for determining the relaxation times are established from the simulations. For a given modulation frequency and molecular reorientation rate, the parameter obtained by fitting the saturation curves is given by 1/P = a + 1/γ2eT1·Teff 2, where Teff2 is the effective T2. For molecular reorientation frequencies in the range 2 X 107-2 X 108 s-1, Teff2 is dominated by the molecular dynamics and is only weakly dependent on the intrinsic T02, allowing a direct estimation of T1. For reorientation frequencies outside this range, the (T1T2) product may be determined from the calibrations. The method is applied to determining relaxation times for spin labels undergoing different rates of rotational reorientation in a variety of environments, including those of biological relevance, and is verified experimentally by the relaxation rate enhancements induced by paramagnetic ions.

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