Type of Document Thesis Author Kim, Hyeongnam Author's Email Address email@example.com URN etd-11242003-213702 Title Solid State NMR Cross Polarization Schemes for Peptide Samples Oriented in Hydrated Phospholipid Bilayers Degree Master of Science Department Chemistry and Biochemistry, Department of Advisory Committee
Advisor Name Title Timothy A. Cross Committee Chair Naresh Dalal Committee Member Oliver Steinbock Committee Member Keywords
- Cross Polarization
- Solid State NMR
Date of Defense 2003-11-20 Availability unrestricted AbstractCross polarization (CP) has been widely used to enhance the polarization of dilute nuclei S with low gyromagnetic ratios from abundant nuclei I with higher gyromagnetic ratios. Efficient CP transfer is generally achieved by spin locking both the I and S spins with radiofrequency (RF) amplitudes that fulfill the so–called Hartmann-Hahn match condition ù1I = ù1S, where ù1I and ù1S refer to the amplitudes of the RF fields applied to the I and S spins, respectively. The matching bandwidth is dictated by the I-S heteronuclear interactions (including dipolar and scalar interactions) as well as the 1H homonuclear dipolar interactions. Here, cross polarization suitable for samples in anisotropic phases, such as liquid crystals and membrane proteins oriented in hydrated lipid environments, is discussed.
Continuous-wave, ramped amplitude and frequency modulated cross polarization schemes (abbreviated as CWCP, RACP and FMCP, respectively) are evaluated for samples in anisotropic phases, such as peptides oriented in lipid environments. It is shown experimentally that both RACP and FMCP give rise to 20% higher polarized signal intensity in comparison to CWCP. The CP matching bandwidths for CWCP and RACP are about the same. Because of its adiabaticity, FMCP has a much broader CP matching bandwidth than CWCP and RACP. In addition, the 15N RF amplitude used at the center of the FMCP matching profile is much lower than that of the CWCP and RACP matching profiles. A sample of [15N]Leu4 labeled gramicidin A oriented in lipids was used in these experiments.
The effect of the Hartmann–Hahn mismatch during polarization inversion spin exchange at the magic angle (PISEMA) has been investigated. During the PISEMA evolution period, the exact Hartmann–Hahn match condition yields a maximum dipolar scaling factor of 0.816 for PISEMA experiments, while any mismatch results in two different effective fields for the first and second half of each frequency switched Lee–Goldburg (FSLG) cycle. The mismatch effect on the scaling factor depends strongly on the transition angle from one effective field to the other within each FSLG cycle as well as on the cycle time. At low RF spin-lock amplitudes in which the FSLG cycle time is relatively long, the scaling factor rapidly becomes smaller as w1S becomes greater than weff. On the other hand, when ww1S < eff, there is relatively little effect on the scaling factor with variation in delta. As a result, the presence of RF inhomogeneities may significantly broaden the line-width in the dipolar dimension because of the mismatch effect. Higher RF spin-lock amplitudes result in a relatively small variation for the scaling factor. Furthermore, ramped amplitude of the 15N RF spin-lock field in synchronization with the flip-flop of the FSLG sequence minimizes the transition angle between the two effective fields within the FSLG cycle. It is shown experimentally that such a ramped amplitude not only gives rise to the same scaling factor but also results in a narrower dipolar line-width in comparison with the rectangular amplitude.
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