Amino acid relaxation behavior

These dendritic boxes (Figure 13.7) were synthesized by the conjugation of a chiral shell of protected amino acids onto a flexible polypropylene imine) dendrimer with 64 amino end groups. In solution, the shell was highly hydrogen-bonded and dense-packed, displaying a solid-phase behavior, which was indicated by the low NMR relaxation time of the surface groups [11]. 

The y-dispersion is solely due to water and its relaxational behavior near about 20 GHz. A minor additional relaxation (6) between 3 and y-dispersion is caused in part by rotation of amino acids, partial rotation of charged side groups of proteins, and the relaxation of protein bound water which occurs somewhere between 300 and 2000 MHz. 

Using fs resolution, two residence times of water at the surface of two proteins have been reported (Fig. 7.6) [21]. The natural probe tryptophan amino acid was used to follow the dynamics of water at the protein surface. For comparison, the behavior in bulk water was also studied. The experimental result together with the theoretical simulation-dynamical equilibrium in the hydration shell, show the direct relationship between the residence time of water molecules at the surface of proteins and the observed slow component in solvation dynamics. For the two biological systems studied, a bimodal decay for the hydration correlation function, with two primary relaxation times was observed an ultrafast time, typically 1 ps or less, and a longer one typically 15-40 ps (Fig. 7.7) [21]. Both times are related to the residence period of water at the protein surface, and their values depend on the binding energy. Measurement of the OH librational band corresponding to intermolecular motion in nanoscopic pools of water and methanol... 

The existence of a hollow core as well as a densely packed exterior layer in PAMAM dendrimers was proven by studying their conformational behavior [3]. Meijer et al. introduced the concept of dendritic boxes by synthesis and characterization of dendrimer with flexible core and solid shell structure. The flexible core was based on poly(propyleneimine) dendrimer. The rigid shell was obtained by modification of terminal groups with bulky amino acid derivatives, that is (r-Boc)-protected L-phenylalanine. Dendrimer structure was fully studied by H and CNMR, IR and UV techniques. Additionally, solid-phase behavior of the shell was confirmed by spin-lattice (Ti) and spin-spin (T2) relaxation measurements and molecular mechanics calculations [3,18,19]. 

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