Side-chains bulky

In the methacrylate homologous series, the effect of side-chain bulkiness is just the opposite. In this case, however, the pendant groups are flexible and offer less of an obstacle to free rotation than the phenyl group in polystyrene. As chain bulk increases, molecules are wedged apart by these substituents, free volume increases, and Tg decreases. 

Flexible side chains Bulky or rigid side chains [8,9]... 

The effect of the side chain bulkiness has been further studied on a series of chloro derivatives of poly(ethyl methacrylate)(PEMA). Though poly(2-chloroethyl methacrylate) exhibits69 a pronounced peak at Ty = 117 K, poly(2,2,2-trichloroethyl methacrylate), poly(2,2,2-trichloro-l-methoxyethyl methacrylate), and poly(2,2,2-trichloro-l-ethoxyethyl methacrylate) do not show (Fig. 6) any low-temperature loss maximum above the liquid nitrogen temperature157. However, these three polymers probably display a relaxation process below 77 K as indicated by the decrease in the loss modulus with rising temperature up to 100 K. Their relaxation behavior seems to be similar to that of PEMA rather than of poly(2-chloroethyl methacrylate) which is difficult to explain. 

As a first step toward this purpose, we have studied the chelation effect of tetrapeptides of sequences Cys-X-Y-Cys, by preparation of metal complexes of mainly the first transition series. The hydrophobic effect of the peptides was also studied by utilizing the side chain bulkiness of the amino acid residues interposed between the two cysteine residues. A special effect of aromatic side chains of tyrosine, phenylalanine, and tryptophan has also been examined in order to assess their ability to ease electron transfer to and from the nearby iron core. 

Kowalewska A, Kupcik J, Pola J, Stanczyk WA (2008) Laser irradiation of oligosiloxane copolymer thin films functionalized with side chain bulky carbosilane moieties. Polymer, doi 10.1016/j.polymer.2007.12.022... 

Kowalewska, A. Kupcik, J. Pola, J. Stanczyk, W. A., Laser Irradiation of Oli-gosiloxane Copolymer Thin Films Functionalized with Side Chain Bulky Car-bosilane Moieties. Polymer 2008,49, 857-866. 

In the foregoing discussions of theoretical models and experimental results, we have focused on linear polymers. We have seen the effect of chain substituents on viscosity. All other things being equal, bulky substituents tend to decrease f and thereby lower 17. The effect is primarily due to the opening up of the liquid because of the steric interference with efficient packing arising from the substituents. With side chains of truly polymeric character, the picture is quite different. 

Increa sing the bulkiness of the alkyl group from the esterifying alcohol in the ester also restricts the motion of backbone polymer chains past each other, as evidenced by an increase in the T within a series of isomers. In Table 1, note the increase in T of poly(isopropyl methacrylate) over the / -propyl ester and similar trends within the butyl series. The member of the butyl series with the bulkiest alcohol chain, poly(/-butyl methacrylate), has a T (107°C) almost identical to that of poly(methyl methacrylate) (Tg = 105° C), whereas the butyl isomer with the most flexible alcohol chain, poly( -butyl methaciylate), has a T of 20°C. Further increase in the rigidity and bulk of the side chain increases the T. An example is poly(isobomyl methacrylate)... 

The separation of Hquid crystals as the concentration of ceUulose increases above a critical value (30%) is mosdy because of the higher combinatorial entropy of mixing of the conformationaHy extended ceUulosic chains in the ordered phase. The critical concentration depends on solvent and temperature, and has been estimated from the polymer chain conformation using lattice and virial theories of nematic ordering (102—107). The side-chain substituents govern solubiHty, and if sufficiently bulky and flexible can yield a thermotropic mesophase in an accessible temperature range. AcetoxypropylceUulose [96420-45-8], prepared by acetylating HPC, was the first reported thermotropic ceUulosic (108), and numerous other heavily substituted esters and ethers of hydroxyalkyl ceUuloses also form equUibrium chiral nematic phases, even at ambient temperatures. 

This is nicely illustrated by members of the chymotrypsin superfamily the enzymes chymotrypsin, trypsin, and elastase have very similar three-dimensional structures but different specificity. They preferentially cleave adjacent to bulky aromatic side chains, positively charged side chains, and small uncharged side chains, respectively. Three residues, numbers 189, 216, and 226, are responsible for these preferences (Figure 11.11). Residues 216... 

Residue 189 is at the bottom of the specificity pocket. In trypsin the Asp residue at this position interacts with the positively charged side chains Lys or Arg of a substrate. This accounts for the preference of trypsin to cleave adjacent to these residues. In chymotrypsin there is a Ser residue at position 189, which does not interfere with the binding of the substrate. Bulky aromatic groups are therefore preferred by chymotrypsin since such side chains fill up the mainly hydrophobic specificity pocket. It has now become clear, however, from site-directed mutagenesis experiments that this simple picture does not tell the whole story. 

A further < ) rotation of 120° removes the bulky carbonyl group as far as possible from tire side chain... 

In the first step, a resin-bound secondary amine is acylated with bromoacetic acid, in the presence of N,N-diisopropylcarbodiimide. Acylation of secondary amines is difficult, especially when coupHng an amino acid with a bulky side chain. The sub-monomer method, on the other hand, is facilitated by the use of bromoacetic acid, which is a very reactive acylating agent Activated bromoacetic acid is bis-reactive, in that it acylates by reacting with a nucleophile at the carbonyl carbon, or it can alkylate by reacting with a nucleophile at the neighboring ah-phatic carbon. Because acylation is approximately 1000 times faster than alkylation, acylation is exclusively observed. 

If peptide residues are converted to peptoid residues, the conformational heterogeneity of the polymer backbone is likely to increase due to cis/trans isomerization at amide bonds. This will lead to an enhanced loss of conformational entropy upon peptoid/protein association, which could adversely affect binding thermodynamics. A potential solution is the judicious placement of bulky peptoid side chains that constrain backbone dihedral angles. 

The introduction of bulky side chains that contain adamantyl groups to poly(p-phenylenevinylene) (PPV), a semiconducting conjugated polymer, decreases the number of interchain interactions. This action will reduce the aggregation quenching and polymer photoluminescence properties would be improved [93]. 

Chemical modifications of PPO by electrophilic substitution of the aromatic backbone provided a variety of new structures with improved gas permeation characteristics. It was found that the substitution degree, main chain rigidity, the bulkiness and flexibility of the side chains and the polarity of the side chains are major parameters controlling the gas permeation properties of the polymer membrane. The broad range of solvents available for the modified structures enhances the possibility of facile preparation of PPO based membrane systems for use in gas separations. 

The crystallographic structure of rubredoxin from Clostridium pasteurianum at 2.5 A, a resolution sufficient to reveal the sequence of several of the bulky amino acid side chains, shows the iron coordinated to two pairs of cysteine residues located rather near the termini of the polypeptide chain (Fig. 1). A related rubredoxin, with a three times larger molecular weight, from Pseudomonas oleovorans is believed to bind iron in a similar fashion. This conclusion is based on physical probes, especially electron paramagnetic resonance spectroscopy, all of which indicate that the iron is in each case situated in a highly similar environment however, the proteins display some specificity in catalytic function. 

Each residue has its own propensity to adopt the PPII conformation, with the backbone propensity being modulated by the side chain (Kelly et al., 2001). Short, bulky side chains occlude backbone from solvent and thus disfavor the PPII conformation, while the lack of a side chain or long, flexible side chains tend to favor the conformation (Kelly et al., 2001). Again, the described calculations support this. Steric interactions... 

Aryl alcohol oxidase from the ligninolytic fungus Pleurotus eryngii had a strong preference for benzylic and allylic alcohols, showing activity on phenyl-substituted benzyl, cinnamyl, naphthyl and 2,4-hexadien-l-ol [103,104]. Another aryl alcohol oxidase, vanillyl alcohol oxidase (VAO) from the ascomycete Penicillium simplicissimum catalyzed the oxidation of vanillyl alcohol and the demethylation of 4-(methoxymethyl)phenol to vanillin and 4-hydro-xybenzaldehyde. In addition, VAO also catalyzed deamination of vanillyl amine to vanillin, and hydroxylation and dehydrogenation of 4-alkylphenols. For the oxidation of 4-alkylphenol, the ratio between the alcohol and alkene product depended on the length and bulkiness of the alkyl side-chain [105,106]. 4-Ethylphenol and 4-propylphenol, were mainly converted to (R)-l-(4 -hydroxyphenyl) alcohols, whereas medium-chain 4-alkylphenols such as 4-butylphenol were converted to l-(4 -hydroxyphenyl)alkenes. 

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