Guest residues

Figure 8a shows the turbidity measurements for different guest residues in ELP [VgX2l. Lower transition temperatures (Tt) correlate with increased hydrophobicity of the guest residue [24, 25]. This data was extrapolated to other ratios of VahXaa (Fig. 8b). The transition temperature could also be influenced by the molecular weight of the ELP. The Tt was shown to increase with decreasing polymer length (Fig. 8c) [23, 26].

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. 

CD spectra of (Pro)X(Pro) at 5°C. (a) (Pro)7 peptide (b) (Proly peptide (solid line) with Gin (long-and short-dashed line), Ala (long-dashed line), Asn (short-dashed line), and Val (Medium-dashed line) single guest residue peptides. Inset shows maxima (Chellgren and Creamer, 2004). 

Figwe 2 Dependence of inverse temperature, 7j, on chain length for a series of three elastin-like polypeptides (ELPs) based on (Val-Pro-Gly-Xaa-Gly) repeat units. The letters within the brackets indicate the identity and relative number of the guest residues, Xaa, within the pentapeptide repeat units of the respective ELPs (V, valine A, alanine and G, glycine). Data were acquired from libraries of the indicated concatemers at an ELP concentration of 25 pM in phosphate-buffered saline (PBS). The predicted values for the Tt are plotted for comparison. Reprinted from Meyer, D. E. Chilkoti, A. BiomacromoleculesZOM, 5(3), 846. Copyright 2004, with permission from the American Chemical Society. 

Figure 3 (a) Comparison of the turbidimetric profiles for a series of chemosynthetic elastin polypeptides based on random copolymers of the repeat sequence [(Val-Pro-Gly-Val-Gly)i x(Val-Pro-Gly-Xaa-Gly)J, where fx refers to the mole fraction of variant pentapeptides within the polymer sequence. " (b) Hydrophobicity scale based on ft values for the copolymers described above as a function of guest residue identity and mole fraction, f. Note that hydrophobic amino acids shift the position of 7 toward lower temperature and that more polar guest residues shift the position of 7i toward higher temperatures in a manner commensurate with the level of substitution within the polypeptide sequence. Reprinted from Uriy, D. W. Gowda, D. C. Parker, T. M. etal. Biopolymers 99Z, 32(9), 1243 20(b) Copyright 1992, with permission from Wiley. 

Data were estimated to (,=1.0, that is, lull occupancy ol guest residues within the pentapeptide sequences, Irom spectroscopic or calorimetric measurements ol elastin-mimetic protein polymers in which the guest residue occupancy, (,=0.2. 

The approach, therefore, is to introduce a guest residue in place of the V residue in the GVG sequence by preparing the pentamer GXGVP where X is any of the naturally occurring amino acid residues or a chemical modification of biological interest. (See Table 5.1 for identification of each of the R-groups of the 20 natural amino acid residues.) The pentamer with the guest... 

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. )...

Jones 6-12 potential or the Buckingham potential functions. It will be interesting, in future work, to determine the relative magnitude of the endothermic and exothermic components for each of the amino acid residues and for other biologically relevant chemical modifications, as they contribute as guest residues to the inverse temperature transition of (GVGVP) and of other informative host model proteins. 

With regard to structures, L-Pro was foimd to exert a destabilizing influence due to the introduction of structural irregularities. Glycine as a guest residue was shown to hamper the formation of periodic secondary structures such as helices and p-sheets because of its peculiar conformational flexibility. 

Both Pro and Gly have to be considered unusual among the 20 naturally occurring a-amino acids. Their significant influence on the conformation of a host peptide could a priori be expected from a look at the conformational energy maps (Rama-chandran maps) of fiiese two residues. To make it possible to extoid the host-guest principle to amino acids with less dramatic conformational behavior, the original approach was modified such as to insert two vicinal guest residues into the host sequence. 

Host-guest peptides with guest residues having a pronounced preference for the... 

Correspondingly, host-guest peptides containing guest residues with a high P-sheet potoitial ( H ) are to form a p-structure. 

Interpreting the CD spectra with regard to the a-helix content of the host-guest sequence in a semi-quantitative way, the following tentative scale for the a-helix potential in TFE of the guest residues examined could be derived ... 

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