Specific Cross-Relaxation Enhancement by Active Motions under DNP (SCREAM-DNP) is a method which relies on direct polarization transfer in solid-state DNP at typical DNP temperatures. The mechanism is based on cross relaxation between 1H and 13C. It is generated by the internal dynamics of methyl groups resulting in negative enhancement and inverted 13C MAS NMR signal in a direct DNP experiment. Furthermore, this effect can be suppressed by 1H saturation. Through mathematical subtraction, we can exclusively observe magnetization which was generated by cross relaxation. Therefore, the use of methyl groups as a specific promotor for polarization transfer opens new applications in DNP.
In this work, we show the application of SCREAM-DNP on a tetracycline-binding RNA aptamer. Here, CH3 groups were introduced into the biomolecular complex by non-covalent interaction between the aptamer and its highly specific ligand. Thereby, we use tetracycline which carries three CH3-groups as a source of cross-relaxation enhancement for complex formation studies. Moreover, we can influence the reorientation dynamics of methyl groups to a significant degree by changing the temperature, resulting in an increase of cross-relaxation efficiency. In conclusion, SCREAM-DNP is a promising method for different applications, especially in site-specific DNP-studies.
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