On the microscopic origins of relaxation processes in aqueous peptide solutions undergoing a glass transition

Abstract

We combine broadband dielectric spectroscopy (BDS) with 1 H and 2 H nuclear magnetic resonance (NMR) to study molecular dynamics in mixtures of ε-polylysine with H 2 O or D 2 O. In BDS, four relaxation processes can be attributed to molecular dynamics. While the fastest process P1 obeys the Arrhenius law, the slowest process P4 shows prominent non-Arrhenius behavior typical of structural α relaxation. For the intermediate processes P2 and P3, the temperature dependence changes at the glass transition temperature Tg. The 1 H and 2 H NMR results yield insights into the molecular origins of these relaxation phenomena. In these NMR analyses, we exploit, in addition to the isotope selectivity of the method, the possibility to distinguish between various types of motion based on their respective line-shape effects and the capability to single out specific molecular moieties based on different spin-lattice relaxation behaviors. In this way, we reveal that process P1 results from the rotation of side and end groups of the peptide, while process P2 is caused by a reorientation of essentially all water molecules, which are quasi-isotropic and survive well below Tg. As for the peptide backbone dynamics, we find evidence that rotational motion of polar groups is involved in process P3 and that nonpolar regions show a dynamical process, which is located between P3 and P4. Thus, the NMR analyses do not yield evidence for coexisting fast peptide-decoupled and slow peptide-coupled water species, which contribute to BDS processes P2 and P3, respectively, but minor bimodality of water motion may remain undetected. Finally, it is demonstrated that the proton/deuteron exchange needs to be considered when interpreting experimental results for molecular dynamics in aqueous peptide solutions.

Dynamics of aqueous peptide solutions in folded and disordered states examined by dynamic light scattering and dielectric spectroscopy

“…13 Dielectric data for e-PLL solutions at temperatures above T g , as is common for other hydrated proteins, 23,30,31 are dominated by both polarization and conductivity effects, which prevents a clear observation of the glass transition-related relaxation, even when the bio-solutions are dialyzed and filtered. As analyzed in previous works, 13,14,20,21 the dielectric response of e-PLL reveals three main relaxations (called fast-water relaxation, slow-water relaxation and a-relaxation) as seen in Fig. 2.…”

“…In recent years, we analyzed the dynamics of e-poly(lysine) (e-PLL) using BDS 13,20 and nuclear magnetic resonance 21 (NMR) at pH = 10. Compared with proteins and other peptides, e-PLL has the advantages that it can be completely dissolved in water, its concentrated solution is semi-transparent, and crystallization is avoided at all temperatures for high pH values (Z10).…”

“…The dynamical behaviour of this system is complex and, in particular, it is laborious to determine the segmental (a-) relaxation using BDS, 20 and it cannot be directly observed using NMR. 21 In this work, we analysed the dynamics of aqueous solutions of e-PLL in two different conformations (a pure b-sheet conformation (at pH = 10) and a more elongated chain (at pH = 7)) using DDLS, PDLS and BDS. The combination of these techniques allows us to analyse the dynamics of the solute and the solvent; we can determine both the a-relaxation and the water dynamics around the peptide in the two conformations.…”

Origin of Apparent Slow Solvent Dynamics in Concentrated Polymer Solutions

“…This includes the prominent cases of aqueous peptide and protein solutions. Especially the occurrence of fast and slow water processes in aqueous solutions − has to be critically reviewed for these systems, considering also the discrepancy identified between dielectric and NMR results of an aqueous ε-polylysine solution . The proposed coupling between the slow water process and the solute dynamics in these systems receives a new interpretation as a consequence of the cross-correlation mechanism identified in this work.…”

“…Moreover, a detailed discussion of BDS and NMR results for aqueous ε-polylysine solutions revealed such a discrepancy. Whereas the dielectric analysis yielded a significant slow water contribution, , 2 H NMR measurements did not provide evidence for such water dynamics but rather observed rotational motion of polar peptide groups on the relevant time scale …”