Guest amino acid

Since the Zimm-Bragg parameters o and s of the naturally occurring amino acids (In water) cannot be obtained from studies of the helix-coil transition in homopolymers, because of experimental difficulties, a technique Is developed to circumvent these problems. It involves the study of the thermally induced transition curves for random copolymers of "guest amino acid residues in a water-soluble host" po y(amino acid). The data may be interpreted with the aid of suitable theories for the helix-coil transition in random copolymers to obtain a and s for the "guest" residues. It is shown in this paper that, for the usual ranges of parameters found for polylamino acids), one of the two lowest order approximations (corresponding to earlier treatments by Lifson and Allegra) is completely adequate. In essence, the low-order approximations hoid if o and s for the two constituents of the copolymer do not differ appreciably from each other. 

Figure 5.9. Experimental data for development of the T,-based hydrophobicity scale. The general composition for the protein-based polymer is poly [f,(GXGVP),fv(GVGVP)], where X is the guest amino acid residue to be evaluated and fx and E are mole fractions wherein fj -i- E = 1. Part A contains the raw data for a number of guest residues substituted at a mole fraction of 0.2, which means 4 substituted residues per 100 residues of poly(GVGVP). The experimental conditions were 40mg/ml of polymer of a molecular weight of about 100,000 Da in 0.15 N NaCl and 0.01 M phosphate at pH 7.4. Experimental T,-values were obtained as shown in part A for fx = 0.2, and additional polymers were characterized with different fx values such that a plot of fx versus T, could be constructed as in part B. Extrapolation of the linear plots in part B to fx = 1 gave the T,-values that became the basis for the T,-based hydrophobicity scale given in Table 5.1. (Adapted with permission from Urry. )...

For the following estimates, Tb values are available for guest amino acid residues without functional groups. With Equation (5.10b), the calculated values for the residues more hydrophobic than glycine (Gly, G), that is, AGha(GGGVP —> GXGVP) substitutions, in... 

Toniolo and Mutter et al. adopted the host-guest idea to study the impact to the guest amino acids glycine and L-proline on the preferred conformation of various host sequences... 

Two guest amino acids (G) are incorporated into two adjacent positions of a host peptide which is potentially able to exist in a random coil, a-helical and p-sheet conformation, depending on the expo imental conditions. The guest amino adds are supposed to alter the conformational state of the sequence reflecting the strudural preferences of the guest ... 

Guest amino acids with a high tendency to disrupt ordered structures ( B ) should cause a conftnmational transititai to the random coil state. 

Importance of the position which the guest residue (G) occupies in the host peptide. From these investigations asymmetric conformation nucleation properties of the guest amino acid might possibly be derived. 

Bode et al. incorporated pairs of guest amino acids X and Y into an alaninic host peptide resulting in a sequence of the type Boc-(L-Ala)2-X-Y-(L-Ala)2-NH-POE. The choice of X and Y referred to results from statistical investi tirms on protdns which indicated different probabilities of the selected pairs to occur in the relevant positions i + 1 (X) and i + 2 (Y) of a p-tum... 

Figure 1 A. Temperature versus normalized turbidity curves for a series of guest amino acid residues occurring at the frequency of 4 guest residues per 100 residues, i.e., fx = 0.2. The value of Tj, the temperature for the onset of the inverse temperature transition for hydrophobic folding and assembly, is defined as the temperature for 50% of maximal turbidity. Note that more hydrophobic guest residues lower the value of T,. and less hydrophobic, more polar residues raise the value of Ti.

Among various types of chiral stationary phases, the host-guest type of chiral crown ether is able to separate most amino acids completely (58). 

Figure 5. Top Tetraurea calixarene monomers 37 and 38 bearing chiral amino acid ester residues (isoleucine and valine methyl esters, respectively) attached to the urea functions. Norcamphor 39 was the chiral guest used to detect the chirality transfer from the outside to the inner cavity.

Enantioselective self-assembling of amino acids 209 Host-guest inclusion complexes 213 Reactivity of chiral ion-dipole complexes 233... 

Table 14 1 /Is. Dn values of permethylated cyclic oligosaccharide hosts with amino acid ester hydrochloride guests...

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