### Abstract

The EPR spectra of nitroxide spin labels have been simulated as a function of microwave field, H_{1}, 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 ∼ H_{1}/√1 + PH^{2}_{1}, in all cases. This result is independent of the degree of inhomogeneous broadening. In general, the fitting parameter, P, depends not only on the T_{1} and T_{2} 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/γ^{2}_{e}T_{1}·T^{eff} _{2}, where T^{eff}_{2} is the effective T_{2}. For molecular reorientation frequencies in the range 2 X 10^{7}-2 X 10^{8} s^{-1}, T^{eff}_{2} is dominated by the molecular dynamics and is only weakly dependent on the intrinsic T^{0}_{2}, allowing a direct estimation of T_{1}. For reorientation frequencies outside this range, the (T_{1}T_{2}) 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 language | English |
---|---|

Pages (from-to) | 79-91 |

Number of pages | 13 |

Journal | Journal of Magnetic Resonance |

Volume | 133 |

Issue number | 1 |

Publication status | Published - Jul 1998 |

### Fingerprint

### 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

*Journal of Magnetic Resonance*,

*133*(1), 79-91.

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

Research output: Contribution to journal › Article

*Journal of Magnetic Resonance*, vol. 133, no. 1, pp. 79-91.

}

TY - JOUR

T1 - Relaxation Time Determinations by Progressive Saturation EPR

T2 - Effects of Molecular Motion and Zeeman Modulation for Spin Labels

AU - Livshits, V. A.

AU - Páli, T.

AU - Marsh, D.

PY - 1998/7

Y1 - 1998/7

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.

AB - 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.

KW - Progressive saturation EPR

KW - Spectral simulation

KW - Spin labels

KW - Spin-lattice relaxation

UR - http://www.scopus.com/inward/record.url?scp=0032110617&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0032110617&partnerID=8YFLogxK

M3 - Article

VL - 133

SP - 79

EP - 91

JO - Journal of Magnetic Resonance

JF - Journal of Magnetic Resonance

SN - 1090-7807

IS - 1

ER -