Characterization of Waters of Hydrophobic Hydration by Microwave Dielectric Relaxation

Abstract

In the present paper, a temperature dependent dielectric relaxation near 5 GHz (at a frequency just lower than that of bulk water) is observed in aqueous solutions of hydrophobic elastic protein-based polymers, such as (GVGVP) 251 and (GV-GIP) 260 . On dilution at low temperatures of the solution, this relaxation becomes more intense approaching different hydrophobicity dependent limits as the hydrophobicity increases from Val (V) with the side chain -CH(CH 3 ) 2 to Ile (I) with the addition of a CH 2 moiety (i.e., -CH(CH 3 )CH 2 CH 3 ). The relaxation decreases in intensity to near 0 as the temperature of solutions of the elastic protein-based polymers are raised from below to above their respective inverse temperature transitions of hydrophobic folding and assembly. Furthermore, using the polymers (GEGXP GVGVP GVGVP GVGVP GVGVP GVGX-P) n where the two X residues are either two V or two Phe (F) residues with the aromatic phenyl side chain of -CH 2 C 6 H 5 , ionization of glutamic acid (E) side chains (i.e., the formation of COOfrom COOH) destroys the majority of the waters of hydrophobic hydration in a charge density dependent manner down to a limit suggestive of remaining pentagonally arranged waters previously observed in crystal structures 1,2 adjacent to hydrophobic moieties. This paper characterizes, for the first time, waters of hydrophobic hydration (N hh ) in terms of the variables of dilution, temperature and polymer charge density. In the absence of charge, N hh appears to be more extensive than the first shell of pentagonally arranged waters. The significance of this characterization resides in the widely held view that the thermodynamics of waters of hydrophobic hydration is central to the hydrophobic folding and function of proteins and proteinbased polymers. [3][4][5][6][7] Previous dielectric relaxation studies extending into the microwave (supra gigahertz) range have been reported on proteins such as myoglobin, 8,9 lysozyme, 10 and collagen, 11 and the ca. 10 GHz relaxation was, indeed, recognized as arising from protein hydration. For several reasons, however, the previous protein studies were unable to correlate with hydrophobic hydration any part of the relaxations ascribed to protein hydration. First, only a very small part of the hydration could be due to the presence of hydrophobic groups on the surface of these native proteins. Second, transitions to hydrophobically unfolded states (e.g., cold denaturation) would have to occur under conditions where a substantial part of the total water was hydrophobic hydration. Third, the transitions would have to be thermally accessible and well-characterized as dominantly hydrophobic. Finally, the variables that favor or disrupt hydrophobic hydration have not been identified in more complex proteins as they have for the elastic protein-based polymers.Characterization of waters of hydrophobic hydration becomes possible with elastic protein-based polymers, because these model proteins exhibit phase transitional behavior. 7 When the temperature is raised...

Quantitative Observation of Backbone Disorder in Native Elastin

“…They are also consistent with recent MD simulations of an elastin-like polypeptide, (VPGVG) 18 . The discrepancy with the somewhat larger MD backbone amide order parameters (S ϭ 0.51) (25) is likely due to the much shorter time scale (ϳ10 ns) of the MD calculations compared with that of the NMR experiment (ϳ50 s).…”

“…polypeptide chains are completely disordered with a gaussian distribution of end-to-end distances. Backbone motions were confirmed by dielectric (18) and NMR (19 -21) relaxation studies that focused on the time scale of the dynamics. In addition, from the absence of intrinsic birefringence, it was concluded that elastin is isotropic and devoid of structure at the molecular level (22,23).…”

“…Support for this picture includes the observation of a slow, thermally induced decrease in thermoelasticity that was attributed to unfolding of the ␤-spiral (24). This model is not, however, supported by recent molecular dynamics (MD) 1 simulations of hydrated (VPGVG) 18 (25)(26)(27). Starting with a ␤-spiral structure, a high degree of disorder inconsistent with a well defined 2°structure develops on a time scale of ϳ10 ns.…”

Intramolecular Charge Transfer near a Hydrophobic Surface. 2,6-p-Toluidinonaphthalene Sulfonate in a Reverse Micelle

“…Recently several groups used the time dependent Stokes shift 5-10 , pulsed NMR and dielectric relaxation techniques [11][12][13][14] to unravel the dynamics of the water molecules in various organized media such as cyclodextrins 5,6 , reverse micelle 4,7-9 and micelles. 10 The results of the dynamic Stokes shift studies indicate that while ordinary water molecules relax in the subpicosecond time scale, inside these organized assemblies the solvation dynamics becomes several thousand times slower and occurs in the nanosecond timescale.…”

“…[1][2][3] The behavior of water molecules present at such interfaces and perturbed by local electrostatic and hydrogen bond interactions are of special interest because of the unique role played by the water molecules in many biological processes. Recently several groups used the time dependent Stokes shift [5][6][7][8][9][10] , pulsed NMR and dielectric relaxation techniques [11][12][13][14] to unravel the dynamics of the water molecules in various organized media such as cyclodextrins 5,6 , reverse micelle 4,7-9 and micelles. 10 The results of the dynamic Stokes shift studies indicate that while ordinary water molecules relax in the subpicosecond time scale, inside these organized assemblies the solvation dynamics becomes several thousand times slower and occurs in the nanosecond timescale.…”