Local Orientational Mobility of Collapsed Dendrimers

Dendrimers are regular hyperbranched macromolecules. The complex structure of the relaxation spectrum is one of the many features of dendrimers that does not fit into the classical concepts of polymers. The relaxation spectrum is manifested in various dynamic processes, including NMR relaxation. In this work, we study the collapsed and swollen dendrimers using NMR relaxation experiments and atomistic simulations of the modified lysine-based dendrimer with double histidine insertions (Lys-2His). This dendrimer is considered most suitable because the imidazole groups are not charged at a normal pH and become protonated at a low pH. We found that the orientational mobilities of segments and CH2 groups inside a collapsed dendrimer are opposite to those in the swollen conformation. Volume interactions suppress the internal local mobility, and reorientation occurs due to the rotation of the dendrimer as a whole. A comparison of the orientational mobility of the segments and CH2 groups for these conformations reveals that taking the segmental mobility into account gives an interpretation pitfall of the mobility of CH2 groups observed in NMR experiments. The studies of the Lys-2His dendrimers are of independent interest for biomedical research. However, our findings should be general for other collapsed dendrimer macromolecules.