Solvent‐ and salt‐induced coil–helix transition of alkali metal salts of poly(L‐glutamic acid) in aqueous organic solvents

Abstract

SYNOPSISThe coil-helix transition has been studied for alkali metal salts of poly (L-glutamic acid) ( P L G ) , i.e., PLGLi, -Na, -K, and -Cs, in aqueous organic solvent systems. Dependence of the transition on the solvent composition has been qualitatively discussed in terms of the solvent dielectric constant D , Gutmann's acceptor number AN, and water activity G . The helix formation induced by addition of alkali chlorides has also been studied. The sharpness of the transition has been interpreted as a measure of reduction of electrostatic energy of helical PLG through contact ion-pair formation between a counterion and carboxyl anion.I NTRO DUCT10 N Conformational transitions of charged polypeptides such as poly (L-glutamic acid) ( P L G ) and poly ( Llysine) ( P L L ) in aqueous solutions have been extensively studied as model systems of protein folding and unf01ding.l-l~ It has been well known that addition of organic solvents with low dielectric constants to the polypeptide systems enhances stability of the a-helix conformation. As possible causes for the stabilization, unionized side-chain-side-chain interaction as well as hydrogen bonding between peptides have been suggested, although contribution of counterion binding, especially for fully charged polypeptides, has attracted much less attention. We have studied the coil-helix transition of alkali salts of PLG in aqueous alcohols. 16 We have found that smaller counterions ( N a + and Lif ) stabilize the ahelix conformation rather than larger ones (K' and Cs' ) and that the specificity is observed in the presence of a small amount of alkali, which was added to suppress the protonation of the charged groups by the protic solvents. Although the coil-helix transition of PLG salts in aqueous organic solvents have also been studied by Daoust et al.I7 and others,[8][9][10][11][12][13][14][15][16][17][18][19][20] the transition behaviors observed in these studies seem to reflect not only counterion specificity but also the contribution of protonation by protic solvents.In the present study, we have performed CD measurements on alkali salts of PLG in aqueous alcohols (MeOH, EtOH, 2-PrOH, t-BuOH) and aqueous dioxane, which contain small amount of alkali hydroxides. We have investigated dependencies of the helix content on the counterion species, on the solvent composition, and on salt concentration. We aim at elucidation of the mechanism of these dependencies. In the following, we consider, in advance, possible factors that influence the coil-helix transition of charged polypeptides in aqueous organic solvents.It is apparent that many factors concert, compete, and counterbalance for the helix formation in such mixed solvent systems. These are, however, classified into two groups: one influences the transition through the intramolecular hydrogen bond of polypeptides, i.e., modulation of strength of the hydrogen bond and solvation of the peptide groups. The other controls the transition through the electrostatic interaction between polymer charges. Addition of organi...

Swelling behavior of poly(acrylic acid) gels in aqueous ethanol - effects of counterion species and ionic strength

“…This means that ion-pairs formed between 1COO -groups and Na + are more stable than those with Cs + . In fact, many studies on counterion binding showed that carboxylate anions prefer smaller counterions 13,14) like Na + and Li + to larger ones like K + and Cs + . This has been interpreted as an empirical rule 15) , i. e., interactions between two solvent-structure makers (smaller ions) or two structure breakers (larger ions) are stronger (attractive) than those between a structure maker and a breaker.…”

Photoresponsive polypeptides: Photochromism and conformation of poly (L‐glutamic acid) containing spiropyran units

“…Exposure to light and the consequent photoconversion of the side chains to the apolar spiro form makes the macromolecules adopt the a-helix conformation. Addition of methanol to aqueous solutions is known to induce the coil -helix transition in poly ( a-amino acid) s having charged side chains, such as sodium poly ( L-glutamate ) 46 and poly (L-lysine ) hydr~chloride.~' At a sufficiently high concentration of methanol (> 40% ) , therefore, an analogous solvent effect should allow the photochromic polypeptide to stay in a-helix in the dark as well (photochromic units present as charged species 111) .…”

Counterion binding in Na poly(acrylate) gel. An 23Na‐NMR study

“…Counterion binding of polyelectrolytes is an essential factor for ordered structures of biopolymers such as DNA,1 proteins,2 and charged polysaccharides3 and their functions in vivo as well as for the volume phase transition of synthetic polymer gels4, 5 in vitro. We have been studying counterion binding of some synthetic6, 7 and biopolyelectrolytes8–10 in water/organic solvent mixtures and found marked ion‐ and solvent‐specificities for binding and the resultant structure transition, such as gel collapse11–13 due to ion‐cluster formation and the coil–helix transition of charged polypeptides. For example, α ‐helix conformation of sodium poly(L‐glutamate) (PLGNa) was stabilized in aqueous solutions containing an organic solvent in a higher ratio, for example, ≧ 70 vol % ethanol, while the stability was significantly reduced for the potassium salt or in aqueous t ‐butanol 8.…”

“…We have been studying counterion binding of some synthetic6, 7 and biopolyelectrolytes8–10 in water/organic solvent mixtures and found marked ion‐ and solvent‐specificities for binding and the resultant structure transition, such as gel collapse11–13 due to ion‐cluster formation and the coil–helix transition of charged polypeptides. For example, α ‐helix conformation of sodium poly(L‐glutamate) (PLGNa) was stabilized in aqueous solutions containing an organic solvent in a higher ratio, for example, ≧ 70 vol % ethanol, while the stability was significantly reduced for the potassium salt or in aqueous t ‐butanol 8. The ion‐specificity (Cs + ≪ K + < Li + < Na + ) for the helix stabilization was comparable to that in cation–carboxyl anion binding,14, 15 and the solvent specificity was explained in terms of the dielectric constant, water activity, and Gutmann–Mayer's Acceptor Number (AN)16 of the relevant solvents.…”