Systematic Errors Associated with the CPMG Pulse Sequence and Their Effect on Motional Analysis of Biomolecules
✍ Scribed by A. Ross; M. Czisch; G.C. King
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 288 KB
- Volume
- 124
- Category
- Article
- ISSN
- 1090-7807
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✦ Synopsis
A theoretical approach to calculate the time evolution of magfor the divergent signals. One extended approach uses an netization during a CPMG pulse sequence of arbitrary parameter additional correlation time describing motion on a time scale settings is developed and verified by experiment. The analysis that is intermediate between the overall and internal correlareveals that off-resonance effects can cause systematic reductions tion times (5,6). Other authors have postulated a ''breathin measured peak amplitudes that commonly lie in the range 5ing'' motion of the molecule caused by conformational flex-25%, reaching 50% in unfavorable circumstances. These errors, ibility, with the related chemical-shift fluctuations making a which are finely dependent upon frequency offset and CPMG selective contribution to T 2 relaxation (7). In a third apparameter settings, are subsequently transferred into erroneous proach, deviations from the simple model have been inter-T 2 values obtained by curve fitting, where they are reduced or preted in terms of anisotropic molecular tumbling (8). The amplified depending upon the magnitude of the relaxation time.
Subsequent transfer to Lipari-Szabo model analysis can produce common feature of each of these extended treatments is that significant errors in derived motional parameters, with t e internal poor fits to the basic formalism are primarily caused by T 2 correlation times being affected somewhat more than S 2 order data, leading us to question the extent to which such deviaparameters. A hazard of this off-resonance phenomenon is its tions must be caused by genuine molecular motions or oscillatory nature, so that strongly affected and unaffected sigwhether another property of the T 2 experiment may be renals can be found at various frequencies within a CPMG specsponsible in some cases. In this context, we note that all trum. Methods for the reduction of the systematic error are diserror analysis of relaxation data conducted to date has been cussed. Relaxation studies on biomolecules, especially at high based solely on spectral imperfections due to thermal noise field strengths, should take account of potential off-resonance (9), ignoring the potential for systematic errors.
contributions. ᭧ 1997 Academic Press
The Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence (10) given by 90Њ y -(D-180Њ x -D) 2n is the most widely used method for measuring transverse relaxation