Poly homopolypeptides

Fig. 26. The preparation of well-defined peptidic surfaces [65,66]. A Using a thiol spacer, homopolypeptides (poly-A in this case) that adopt an a-helical conformation are synthesized on a gold surface. B Using an aminopeptidase, the longer peptidic chains are hydrolyzed to yield a more homogeneous surface. (Reproduced with the permission of Ref. 65,66)...

Homopolypeptides provide useful secondary structural models for spectroscopic studies on proteins and the ROA spectra of poly(L-lysine) in the three most important conformers are shown in Fig. 7.4. Poly(L-lysine) at alkaline pH... 

Early attempts used data obtained from homopolypeptides, such as poly(Lys), for their basis spectra [87, 88]. In the past fifteen years, approaches using data from globular proteins have emerged [18, 89-101]. Basically, a data base comprised of proteins with known secondary structure compositions is assembled and far UV CD spectra recorded. The choice of the proteins to be included is critical and various combinations have been examined. Mathematical matrix methods can be used to extract basis spectra which represent the contributions from the various secondary structures. Typically, four or five basis spectra can be obtained (corresponding to a helices, jj sheets, p turns, and random coil structures). In some approaches, such as those developed by Johnson and co-workers [11, 12, 51, 52, 102], separate basis spectra can be obtained for parallel and antiparallel p sheets. These basis spectra are then linearly combined to reconstruct the CD spectrum of the protein of interest The proportion of the basis spectra used to provide the best fit to the spectrum corresponds to the percentage of that secondary structure in the protein of interest Complete details of the mathematical algorithms that have been employed can be found elsewhere [10, 12, 17, 89, 103]. 

While earlier basis sets employed CD spectra of model homopolypeptides such as poly(Lys) [87, 88], more developed approaches use CD spectra of proteins with known secondary structure composition. These data sets attempt to span the range of protein types (a, P, a+p, and a/p). The approaches also differ in the algorithm used to reconstruct the experimental CD spectrum in question [10,12,103]. Johnson and co-workers even developed methods for systematically eliminating unlike proteins from the basis set to achieve an improved fit [10,12, 102]. 

Hydrophobic homopolypeptides are stable in their a-helical conformation at the air/water surface. a-Helices are adsorbed with their long axis parallel to the water surface, the long axis being predominantly perpendicular to the direction of compression. Poly-L-alanine and poly-L-leucine pack together in the form of a-helices on the water surfaces. Inflection points appeared at 12.8 A and 17.0 A per residue. The a-helices are organized in such a way that their side-chains are interdigitated (Figure 6.16) . ... 

A variety of homopolypeptides were synthesized by the N-carboxy a-amino acid anhydride (NCA), activated ester, or azide methods 5 except for deuterium labelled samples and polyglycines. The physical state of these samples is in general a semicrystalline state. Poly(glycine) (MW = 5200) was purchased from Sigma Chemical Company. 

Foerster et alf 1 measured the first 15N CP/MAS NMR spectra of some 15N-labelled homopolypeptides in the solid state. Figure 16 shows the 30.5-MHz 15N CP/MAS NMR spectra of poly(L-leucine), poly(L-phenylalanine) and poly (glycine). The 15N chemical shifts of solid homopolypeptides47 are... 

Fig. 23. Correlation of the l5N chemical shifts of Ala of copolypeptides, [Ala, X] , with those of host homopolypeptides [X] in the solid state. The arrow indicates the N chemical shift of poly(L-alanine).

Table 22.4 shows the values of Rn for some solid polypeptides, determined by using Equations (22.1a-e) through the observation of the amide carbonyl-carbon chemical shift as listed are solid polyglycine[(Gly) ], poly(L-alanine)[(Ala) ], poly(L-valine)[(Val) ], and poly(L-leucine)[(Leu) ] with several conformations such as righthanded a-helix (aR-helix), j8-sheet, 3i- and (UL-helix. In these homopolypeptides, the Rn o values determined for the /3-sheet form are constant in the range of 3.0-3.1 A, regardless of amino acid residue species. The Rn-.-o value for the 3i-helix in (Gly) is... 

The calculation of p j i as a function of / provides an estimate of the distance that end effects penetrate into a long unperturbed chain. This calculation shows that end effects for polyethylene are confined to the first few bmids at the end of the chain [10]. The end effects can extend much further into the chain when the second-order interactirMis become more severe, as is frequently the case for the probability of a helical conformation, p/,, in a long homopolypeptide near the midpoint of its helix-coil transition [132]. In proton NMR, the values of p, , are helpful in understanding the values of the spin-spin coupling constants, using the Karplus relationship [111-114], and in understanding the y effect on the chemical shift in C NMR spectra [110,133]. The values of p, , have also been used to interpret the optical activity exhibited by chiral poly(a-olefins) [115] and other polymers [134,135]. 

These were the foundations on which our work was to be built. Joe Speyer and I were at the preparatory stage of the project (we had received all labeled amino acids ordered for the experiments and were determining the size of our polynucleotides) when the rumor of Nirenberg and Matthaei s momentous discovery reached us in August 1961. While studying the effect of RNAs from different sources on protein synthesis in a cell-free system from E. coli, they discovered that the homopolyribonucleotide poly(U) promoted the formation of the homopolypeptide polyphenylalanine. Thereby, the genetic code was broken. 

By this chemistry, polymers with one amine end group as well as a,co-diamine-functionalized polymers can be used to prepare AB or ABA copolymers, respectively. The method gives copolymers with well-controlled polypeptide segments. Furthermore, no unreacted homopolymers or homopolypeptides could be detected. Several examples of the polymer B block have been reported poly(octenamer) prepared by acylic diene metathesis polymerization [67], poly(methyl acrylate) prepared by atom transfer radical polymerization (ATRP) [70], poly(ethylene glykol) PEG, and PDMS [68]. The method was expanded for the synthesis of... 

The findings by Sakamoto are in agreement with earlier observations by Yaron and Berger, who also identified linear homopolypeptide by-products when NCA graft copolymerizations were carried out in dry dioxane or DMF. Tewksbury and Stahmann, in contrast, reported that the synthesis of multichain poly(amino acids) using poly(L-lysine) initiator and o/L-phenylalanine NCA, L-leucine NCA, or Bn-Glu NCA in anhydrous DMSO was not accompanied by the formation of linear by-products. These contradictory observations are characteristic for primary amine-initiated NCA... 

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