Films oriented, polypeptides

The anisotropy of piezoelectricity in oriented polymer films is quite different among polymers. The piezoelectric effect in oriented polypeptide films is greatest when elongated along a direction at 45° to the draw-axis. On the contrary, the effect is most remarkable for elongation along the draw-axis for roll-drawn poly(vinylidene fluoride) film. 

The self condensation of esters of long-chain a-amino acids (methyl 2-aminoocta-decanoate (39), docosanyl 2-aminooctadecanoate (40), methyl 2-aminohexacosanoate (41), docosanyl 2-aminohexacosanoate (42) is used to prepare oriented polypeptide films, which are models for biological membranes.52 ... 

Reliable circular dichroism spectra cannot be obtained from crystalline, oriented polypeptide material that is birefringent or scatters light. However, monolayers of protein do not appear to provide anomalous spectra (4,15), probably because of the relative uniformity of the film and the minimal refractive index increment at the protein/water interface. Nevertheless, previous results from this and other laboratories (3,16,17) have shown that albumin and fibrinogen molecules tend to lie flat, that is, with their long axes parallel to the surface, whereas globulins lie perpendicular to the surface. 

It may be ctmsidered that the molecular cluster in which rod-lihe molecules align neatly parallel to the cluster axis, itself forms a crystallite. The intensity of the X-ray diffraction for the incident beam that is perpendicular to the magnetically oriented film of polypeptide, /, is well expresred in the form (38) ... 

Table 4. Maximum induced dipole moments of the molecular cluster in magnetically oriented polypeptide films and its estimated birefringences at complete orientation...

The angular distribution of polarized components of fluorescence intensity of electrically or numerically oriented films of polypeptides show the pattern similar to those typical of the uniaxially oriented film. The degree of orientation of the films as expected from the contour of the pattern, is not so good despite the excellent orientation of the films, which may indicate that most of the fluorescent molecules are introduced into the gap regions that may possibly form amorphous regions. 

Fig. 10.9. Infrared spectra of oriented polypeptide films obtained with polarized light. (Krull et at., 1965.)...

Polypeptides that are characterized by helical structures are, at the molecular level, noncentrosymmetric. However, in randomly oriented polypeptide films such as might be obtained by solvent evaporation, noncentrosymmetry is lost and the sample does not exhibit nonlinear optical (NLO) properties such second harmonic generation (SHG). Alignment of the molecular helices by application of an electric field across the solution with simultaneous evaporation of the solvent yields SHG-active films with relaxation times reported to be at least 6 months [1]. 

Some spectra of oriented polypeptides obtained with polarized infrared radiation (Ambrose and Elliott, 1951a) are shown in Fig. 111. In the case of poly -benzyl-L-glutamate, the NH stretching mode at 3292 cm. and the C=0 stretching mode at 1653 cm. show parallel dichroism while the band associated with NH deformation at 1549 cm. shows perpendicular dichroism. The ester C=0 band at 1730 cm. does not show much dichroism. This is the behavior to be expected of the a form since the axes of the helices would be oriented preferentially in the direction of stretching, and the NH and CO bonds are essentially parallel to the axes of the helices. In some cases, the solvent used for casting films favors the extended /8 form, with the opposite dichroic behavior since the NH and CO bonds in the ]3 form are essentially normal to the backbone chain. Such is the case... 

Now the polypeptide chain depicted above is in the extended or /3-form and in protein films (and possibly in the protein mono-layers of cell membranes) this form can be stabilised by hydrogen bonding between the carbonyl (> C = O) and imide (HN <) groups of the backbones of parallel orientated polypeptide chains to give a pleated-sheet structure (Fig. 3.3(A)). This is the structure of several fibrous proteins, e.g. j8-keratin. From the primary polypeptide unit of structure, a secondary structure has arisen. 

Solid-state spectral studies relating to proteins have not been numerous, but those associated with the peptide-bond absorption are of special interest. In particular, we note Peterson and Simpson s (1957) singlecrystal spectroscopy of myristamide, and the demonstration of dichroism in oriented films of helical polypeptides by Gratzer et al. (1961). The results of these studies are discussed in Section IV, . Since great care and exacting technique is required for solid-state studies, we may do no better than to refer interested readers to the original papers for critical discussions of experimental aspects. 

The matter has been carried further in the studies of Gratzer et al. (1961) with measurements of the absorption of polarized radiation by oriented films of a-helical polypeptides. Their polarization spectra of poly-L-ala-nine, and of poly-7-methyl-L-glutamate (Fig. 8) clearly show the predicted opposite polarization for the two peptide absorption bands of the -helix. Taking these band positions as 1910 and 2060 A, we obtain a band separation of 3800 cm . [The value of 2700 cm in the paper of Gratzer et al. 

The role of the supramolecular order of LBK films on the photoreorientation of azobenzene moieties upon polarized irradiation was investigated in detail. It was found that the photo(re)orientation proceeds reaclily in disordered, spin-coated films of azobenzene polymer 34 or azobenzene-containing polypeptide 38, whereas the photoreorientation is hindered in highly ordered LBK films of these polymers. In the case of polyacrylaerylat 34 and polypeptides 38 with shorter spacers (n = 2), the photoreorientation proceeds readily after the structure of the LBK film has been randomized by UV irradi-... 

A number of infrared absorption studies on the purified epidermal protein are described. In combination with the X-ray diffraction data, they entirely confirm the recent advances in the definition of absorption spectra, which are characteristic of a- and /3-type proteins and polypeptides, in terms of the kinds of dichroism and the frequency changes associated with the (3 form. It is pointed out that as the dichroism changes with the a — change it is not possible to read off the relative amounts of a and from the absorption spectra because of the differences in the orientation of the absorbing bonds with reference to the plane of the film. In the absorption spectra of highly stretched epidermin, where... 

Some of the most interesting results which have come from a study of the infrared spectra of synthetic polypeptides, have been obtained by investigating the polarization properties of the NH and CO bands in an oriented film and correlating these results with X-ray diffraction studies on the same films. Ambrose and Hanby (1949) showed in this way that the polymer (IV) formed by the polymerization of the AT-carbonic anhydride of L-glutamic 7-methyl ester could not have a fully extended structure such as nylon. In (IV) the NH and CO bands at 3 /i and 6 /z had polarization properties indicating that these groups must be oriented mainly parallel to the direction of extension (orientation produced by... 

From the ultraviolet dichroism at 2500 A. of oriented silk fibroin films it was inferred that the axes of the C=N peptenol chromophors were inclined at angles of ca. 45° to the stretching axis of the films, which was also taken to be the direction of the extended polypeptide chains. 

In the molecular cluster of PBLG the main chain and the side chains of the polypeptide molecule orient in a similar way as in the mechanically oriented film however, the rotation of the side chains becomes marked at high temperature, say... 

Solid films have been examined frequently for infrared analysis in biochemical work, for example, in structural studies of proteins, polypeptides, and polysaccharides. Such films have been of particular value for studying polarization spectra of macromolecules in intact films and in oriented ones (stretched, rolled, or stroked), thereby permitting knowledge to be gained concerning spatial arrangements within the molecule and conformational effects among molecules. (See The Use of Polarized Infrared Radiation and the Measurement of Dichroism, p. 73, for a detailed discussion.) A few workers have discussed the film technique (Lecomte, 1948 Randall et al., 1949 Hacskaylo, 1954). 

Bradbury et al. (1967) have studied partial nucleoproteins produced by removal of histone fractions from nucleohistone. Figure 12.6 shows a polarized infrared spectrum of one of the nucleoproteins oriented by shearing. The spectrum is that of a film deuterated by D2O vapor. The bands of the in-plane vibrations of cytosine and guanine moieties are highly polarized when the electric vector is placed perpendicular to the fiber axis. The 1452 cm absorption is that of the amide IF band (Miyazawa et al., 1956 Miyazawa, 1962) for the readily deuterated fraction of the partial nucleoprotein. As seen in Fig. 12.6, the 1452 cm band is polarized considerably in the direction parallel to the fiber axis, giving evidence that the extended polypeptide chain is situated so that its axis lies between the angle of the groove in the DNA double helix and the axis of the DNA helices (Bradbury et al., 1967). 

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