Comparison of fast backbone dynamics at amide nitrogen and carbonyl sites in dematin headpiece C-terminal domain and its S74E mutant

We perform a detailed comparison of fast backbone dynamics probed at amide nitrogen versus carbonyl carbon sites for dematin headpiece C-terminal domain (DHP) and its S74E mutant (DHPS74E). Carbonyl dynamics is probed via auto-correlated longitudinal rates and transverse C′/C′-C^α CSA/dipolar and C′/C′–N CSA/dipolar cross-correlated rates, while ¹⁵N data are taken from a previous study. Resulting values of effective order parameters and internal correlation times support the conclusion that C′ relaxation reports on a different subset of fast motions compared to those probed at N–H bond vectors in the same peptide planes. ¹³C′ order parameters are on the average 0.08 lower than ¹⁵N order parameters with the exception of the flexible loop region in DHP. The reduction of mobility in the loop region upon the S74E mutation can be seen from the ¹⁵N order parameters but not from the ¹³C order parameters. Internal correlation times at ¹³C′ sites are on the average an order of magnitude longer than those at ¹⁵N sites for the well-structured C-terminal subdomains, while the more flexible N-terminal subdomains have more comparable average internal correlation times.