Evidence of Chain Folding in a Synthetic Polypeptide and in Keratin

Ultra-violet and infra-red investigations on muscle

Infra-red absorption spectroscopy of muscle has already been carried out, using the Burch reflecting microscope (Barer, Cole & Thompson 1949: Barer, Thompson & Williams unpublished). There are considerable difficulties involved in this type of work. In the first place it is rather doubtful whether such measurements will ever be possible on living muscle owing to the presence of water, which possesses intense absorption bands in some of the most useful regions of the infra-red spectrum. It may be possible to overcome this difficulty to some extent by using heavy water which has a different absorption spectrum. It is in principle possible to obtain information similar to that given by infra-red spectroscopy, even in the presence of water, by means of Raman spectroscopy, but the technical difficulties involved, particularly fight scattering by colloids, would seem to preclude this method of attack so far as muscle is concerned. Our infra-red measurements have hitherto been confined to dried material. The results indicate that there is little prospect of working with whole muscles, as even single isolated striated fibres of the frog, rabbit and crab were usually too thick. However, it was possible to obtain good spectra in the chemically important region from 3 to 14/µon exceptionally thin single fibres or on artificially compressed fibres. An attempt was made to detect dichroism by means of polarized infra-red radiation, but to our surprise none was observed throughout the 3 to 14µrange, even though the material used showed strong birefringence in the visible region. Hr Stocken and I have recently examined certain molecular models of muscle, in the fight of the work of Ambrose, Elliott & Temple (1949) on myosin, and it now appears possible that infra-red dichroism of muscle might be expected to manifest itself only under rather special conditions. We hope to put these theoretical deductions to experimental test. As regards measurements on muscle in the ultra-violet region, the position is much more promising. It is quite possible to determine the absorption spectrum of theAorIband in living single fibres. The entire spectrum from about 230 mµin the ultra-violet to over 600 mµ, in the visible can be recorded simultaneously, using the reflecting microscope. This technique can also be used with polarized ultra-violet fight, in order to detect variation of dichroism in crystals at different wave-lengths (Barer, Jope & Perutz unpublished), and I intend to apply it to the study of dichroism in muscle fibres. Another new possibility is the observation of birefringence, as well as dichroism, in the ultra-violet. I have recently carried out experiments with a view to developing a new type of ultra-violet polarizer and it should now be possible to use the reflecting microscope as an ultra-violet polarizing microscope.

The infra-red spectra of some optically active and meso -synthetic polypeptides

The infra-red spectra of a number of synthetic polypeptides have been examined. Polypeptides which are known from dichroic effects or from X -ray diffraction diagrams to be in a folded configuration have wave numbers for the C═O band lying within a narrow range. Optically active (L) forms have this band lying between 1652 and 1657 cm -1 , and for meso forms a slightly higher range 1661 to 1665 cm -1 is found. The observations cover a wide range of side-chain size and character. A small number of polypeptides in the extended form have been examined, and here the range for the C═O band is 1628 to 1635 cm -1 . It is uncertain whether in the extended polypeptides the difference between optically active and meso forms is significant. No systematic differences between L and DL forms have been found in the band at ca . 1560 cm -1 . The possibility of differences in L and meso forms arising from steric hindrance between β carbon atoms in an α -helix is considered. The effect of absorbed water on the spectra of poly-DL-lysine hydroiodide and poly-DL-alanine has been examined. The effect is chiefly, but not entirely, additive.

Infra-red spectra and structure of proteins

The Infra-Red spectra of thirty-seven proteins have been observed between 650 and 3700 cm -1 . Several of the bands found common to them all had not been reported proviously and some other bands appear to be common only to those believed to exist in a folded configuration. A critical study of the exact positions of the bands near 1650 and 1550 cm -1 indicates that the frequency criteria developed for distinguishing a from β polypeptides cannot be reliably applied to proteins. Spectra obtained with polarized infra-red radiation confirms the view that in proteins where the polypeptide chain has a coiled (α) configuration, the NH stretching and the Co stretching bands have parallel dichroism and that the amide II band (near 1540 cm -1 ) has perpendicular dichroism: However, the counter part of the amide II band near 1240 cm -1 showed little dichroism in α proteins, but marked parallel dichroism in β proteins. The spectra of several deuterated proteins have been investigated; the results are of considerable value in making assignments of fundamental frequencies. Studies of deuterated silkworm gut using polarized infra-red radiation showed dichroic ratios of several key bands in proteins have been used to make quantitative tests of various polypeptide chain configurations which may exist in proteins. Satisfactory agreement was found in some cases with models based on X-ray diffraction data.

The configuration of the polypeptide chain in small peptides such as gramicidin S

The infra-red spectra of a number of small peptides have been measured and it is concluded that for a degree of polymerization greater than two the absorption bands of the peptide —NH—CO— group are reasonably characteristic. The spectra of various crystalline derivatives of gramicidin S, a cyclic decapeptide, have been examined. Measurements with polarized radiation indicate considerable dichroism which is very similar to that shown by folded synthetic polypeptides and α -keratin. It is possible to construct a model, using a fold with hydrogen-bonded rings, which appears to satisfy the intra-red data and also to represent a stable configuration for the polypeptide chain.

Polypeptide chain configurations in crystalline proteins

Astbury’s studies of α-keratin, and X-ray studies of crystalline haemoglobin and myoglobin by Perutz and Kendrew, agree in indicating some form of folded polypeptide chain which has a repeat distance of about 5·1 Å, with three amino-acid residues per repeat. In this paper a systematic survey has been made of chain models which conform to established bond lengths and angles, and which are held in a folded form by N—H—O bonds. After excluding the models which depart widely from the observed repeat distance and number of residues per repeat, an attempt is made to reduce the number of possibilities still further by comparing vector diagrams of the models with Patterson projections based on the X-ray data. When this comparison is made for two-dimensional Patterson projections on a plane at right angles to the chain, the evidence favours chains of the general type proposed for a-keratin by Astbury. These chains have a dyad axis with six residues in a repeat distance of 10·2 Å, and are composed of approximately coplanar folds. As a further test, these chains are placed in the myoglobin structure, and a comparison is made between calculated and observed F values for a zone parallel to the chains; the agreement is remarkably close taking into account the omission from the calculations of the unknown effect of the side-chains. On the other hand, a study of the three-dimensional Patterson of haemoglobin shows how cautious one must be in accepting this agreement as significant. Successive portions of the rod of high vector density which has been supposed to represent the chains give widely different projections and show no evidence of a dyad axis. The evidence is still too slender for definite conclusions to be drawn, but it indicates that a further intensive study of these proteins, and in particular of myoglobin which has promising features of simplicity, may lead to a determination of the chain structure.

Infra-red spectra and structure of fibrous proteins

The spectra of silk suture, porcupine quill, elephant hair, swan quill, gelatine and rat-tail tendon have been observed with polarized infra-red radiation. The first four materials give spectra which show that they contain both extended ( β ) and folded ( α ) peptide chains. A band at about 4600 cm. -1 is assigned to a C = O combination frequency, whose dichroism indicates the direction of the transition moment for a C = O deformation mode. Gelatine and collagen give similar spectra, and are different from either α or β proteins or polypeptides. A method of folding a peptide chain, based on a suggestion of Huggins, is given which accounts for the known infra-red and X-ray spectra of collagen and gelatine.

The structure of synthetic polypeptides. II. Investigation with polarized infra-red spectroscopy

Methods of producing oriented films of synthetic polypeptides when prepared from various solvents are described. The polarized infra-red spectra of a number of these films have been measured. It is shown, in agreement with earlier work, that the polypeptides can exist in a folded ( α ) and an extended ( β ) configuration. In the α form the N—H and C═O stretching modes of the polypeptide chain show parallel dichroism while the N—H deformation mode shows perpendicular dichroism. In the β form these conditions are reversed. The α form has characteristic frequencies of about 1660 and 1560 cm. -1 , while the β form has characteristic frequencies of about 1640 and 1530 cm. -1 . When highly dispersed in an inert polymer or liquid solvent it is shown that all the hydrogen bonds of the polypeptide chain can be internally satisfied. It is concluded that in the folded state the chain is maintained in the form of seven-membered rings by intra -molecular hydrogen bonds.

Infra-red spectroscopic studies of globular protein structure

The infra-red absorption bands associated with the peptide groups in globular proteins, folded synthetic polypeptides and α -fibrous proteins are very similar. The spectra of denatured globular proteins, extended chain synthetic polypeptides and β-fibrous proteins are also very similar. Denatured fibrous insulin is shown by polarized-beam techniques to consist of extended β chains lying perpendicular to the fibril axis, with a layer structure involving inter-chain hydrogen bonded grids. Rhombohedral insulin crystals show dichroism consistent with an arrangement of folded chains packed perpendicular to the trigonal axis. A mechanism to explain denaturation of proteins is suggested in which precipitation is caused by a change from infra -chain to inter -chain hydrogen bonds. A method for investigating chain configurations of proteins in aqueous solution is described.