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...
lithiummain group element aggregate has the topology of a distorted rhombododecahedron. The Ge atoms are each bound to three As atoms in the molecular framework and are thus almost ideally tetrahedrally surrounded. The Ge-As distances (average 2.444(2) A) differ very little from one another and are similar to the values found in the Zintl phase B~, G~A s , [~] and in 1,3-diarsa-4-~ila-2-germacyclobutanes.[~]The Ge-Li distances of3.18-3.19 A areconsiderably longer than the sum of the van-der-Wads radii (ca. 2.02 A) and are thus not significant for the interpretation of the bonding in the aggregate. Each As atom is bound to three Li atoms, one Ge atom, and one exocyclic SiiPr, group; thus, it is five coordinated. The coordination geometry at the arsenic atom, however, does not correspond to an ideal tetragonai pyramid: the silyl group is pushed out of its ideal position as a result of the steric hindrance caused by the neighboring tert-butyl group at the germanium atom (Fig. 1). The Li-As distances (2.54(2)-2.62(1) A) also differ considerably in length, probably likewise for steric reasons. In the only lithiosilylarsanes structurally characterized to date, [LiAs-(SiMe,), .dme] (dimeric in the crystal)[6a] and Is,SiF-As(SiiPr,)Li(thf), (Is = 2,4,6-iPr3C,H,) (monomeric in the mhH, D-6945/ Weinheim, 1993 0570-0833/93/1010-/441 S 10.00+ .25 0 1441
Commonly a key element enabling proteins to function is an amino acid residue or residues with functional side chains having shifted pKa values. This article reports the results on a set of protein-based polymers (model proteins) that exhibit hydrophobic folding and assembly transitions, and that have been designed for the purpose of achieving large hydrophobic-induced pKa shifts by selectively replacing Val residues by Phe residues. The high molecular weight polypentapeptides, actually poly(tricosapeptides) with six varied pentamers in fixed sequence, were designed and synthesized to have the same amino acid compositions but different proximities between a single aspartic acid residue and 5 Phe residues per 30 residues. With the 5 Phe residues distal from the Asp residue, the observed pKa shift was 2.9 when compared to the Val-containing reference. With the 5 Phe residues within 1 nm of the Asp residue, the pKa shift was 6.2. This represents a free energy of interaction of 8 kcal/mole. To our knowledge, this is the largest pKa shift documented for an Asp residue in a polypeptide- or protein-water system. Data are reviewed that do not support the usual electrostatic arguments for pKa shifts of charge-charge repulsion and/or unfavorable ion self-energies arising from displacement of water by hydrophobic moieties, but rather the data are interpreted to indicate the presence of an apolar-polar repulsive free energy of hydration, which results from a potentially highly cooperative competition between apolar and polar species for hydration.
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