Intramolecular conformational transitions

The thermochromic effect in solutions of poly(3-alkylthiophene)s has been attributed to an intramolecular conformational transition to an extended chain conformation, principally on the basis of an observed isosbestic point <1987PSB1071, 1992MM2141>, but evidence for supramolecular aggregates was noted <1987PSB1071>. 

Abstract Aqueous solutions of star-like polyelectrolytes (PEs) exhibit distinctive features that originate from the topological complexity of branched macromolecules. In a salt-free solution of branched PEs, mobile counterions preferentially localize in the intramolecular volume of branched macroions. Counterion localization manifests itself in a dramatic reduction of the osmotic coefficient in solutions of branched polyions as compared with those of linear PEs. The intramolecular osmotic pressure, created by entrapped counterions, imposes stretched conformations of branches and this leads to dramatic intramolecular conformational transitions upon variations in environmental conditions. In this chapter, we overview the theory of conformations and stimuli-induced conformational transitions in star-like PEs in aqueous solutions and compare these to the data from experiments and Monte Carlo and molecular dynamics simulations. 

Chee CK, Riuuner S, Soutar I, Swanson L (1997) Time-resolved fluorescence anisotropy studies of the temperature-induced intramolecular conformational transition of poly (N-isopropylacrylamide) in dilute aqueous solution. Polymer 38(2) 483 86. doi 10.1016/ s0032-3861(96)00636-2... 

The quantity of primary interest in our thermodynamic construction is the partial molar Gibbs free energy or chemical potential of the solute in solution. This chemical potential reflects the conformational degrees of freedom of the solute and the solution conditions (temperature, pressure, and solvent composition) and provides the driving force for solute conformational transitions in solution. For a simple solute with no internal structure (i.e., no intramolecular degrees of freedom), this chemical potential can be expressed as... 

The reactions of intramolecular cross-linking is a rather poorly investigated area in the field of macro-molecular reactions. However, the problems of regularities of such processes are related to such important problems of polymer chemistry as chemical modification of polymers, networks formation, sorption of low molecular reagents by polymers, intramolecular catalysis, conformational transitions and so on. In spite of the great importance of the study of regularities of cross-linking reactions, the experimental and theoretical analysis of such processes is complicated by many difficulties. ... 

Pressure provokes transition of the linear (extended) conformation into the bent (V-like) one. (The V-like form is more compact and occupies a smaller volume.) It is obvious that the V-like form is favorable in respect of intramolecular electron transfer from the donor (the aniline part) to the acceptor (the pyrene part). In the utmost level of the phenomenon, the donor part transforms into the cation-radical moiety, whereas the acceptor part passes into the anion-radical moiety. Such transformation is impossible in the case of the extended conformation because of the large distance between the donor and acceptor moieties. The spectral changes observed reflect this conformational transition at elevated pressures. 

D. Shemesh and R. B. Gerber. Classical trajectory simulations of photoionization dynamics of tryptophan intramolecular energy flow, hydrogen-transfer processes and conformational transitions, J. Phys. Chem. A, 110 8401-8408 (2006). 

The dynamic RIS model, which was proposed before to investigate the dynamics of local conformational transitions in polymers, is elaborated to formulate the increase in the number of excimer-forming sites through rotational sampling. Application of the model to the meso and racemic diads in PS confirms the fact that conformational mobility of the chain plays a major role in intramolecular exclmer formation. Comparison with experiments demonstrates that the decay of the monomer fluorescence in styrene dimers is predominantly governed by the process of conformational transitions. 

From these experimental results it was assumed that conformational transitions within single polymer chains — controlled via intramolecular hydrogen bonds — are causing the viscosity decrease. Chain scission could be excluded. 

The study of the reasons for the formation of intramolecular structures reveals that the enhanced interaction of cholesterol groups occuring on cooling is very important. The dilution of the sequence of cholesterol-containing monomeric units by butylmethacrylate units (copolymers of ChMA-11 with butylmethacrylate) leads gradually to the degeneration of the conformational transition (Fig. 31, curves 3-5). 

Heijboer [28] has reported the dynamic mechanical properties of poly(nethacrylate)s with different size of the saturated ring as side chain. The y relaxation in these polymers is attributed to a conformational transition in the saturated ring. In the case of poly(cyclohexyl methacrylate), the transition is between the two chair conformations in the cyclohexyl ring. However, this type of internal motion in hindered by rather high intramolecular barriers, which can reach about 11 kcal mol-1. 

For compact proteins with molecular masses of greater than 10,000 and saturation of native structure by intramolecular hydrogen bonds of about 0.75 0.10 mole of bonds per mole of amino acid residues, the asymptotic values of enthalpy and entropy of the conformational transition, calculated per amino acid residue, amount to A%H(TX) = (6.25 0.2) kJ mol-1 and A 5(7 x) = (17.6 0.6) J K-1 mol-1. For some noncompact proteins (e.g., histones) or small globular proteins with molecular masses... 

In the 1960 s and 1970 s, much indirect evidence was obtained in favour of protein intramolecular mobility, i.e. the entropy and energy specificity of enzyme catalysis (Likhtenshtein, 1966, 1976a, b, 1979, 1988 Lumry and Rajender, 1970 Lumry and Gregory, 1986). The first observations made concerned the transglobular conformational transition during substrate-protein interaction (Likhtenshtein, 1976), the reactivity of functional groups inside the protein globule, and proteolysis. 

The effect on the conformational-aggrega-tlonal properties of pectate in aqueous solution brought by the addition of specific ions (H+, Ca +. Cu +) was studied by osmometric, microcalorimetric, dilato-metric, and rheological methods. Evidence is provided for the intramolecular nature of the pH induced conformational transition. The addition of divalent ions brings about at the same time a conformational change of the chain of pectate and chain-chain association. 

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