Robert B. Corey

An investigation of the structure of silk fibroin

Abstract An investigation based on new X-ray diffraction data, including quantitative spectrometric measurements of X-ray intensities, has led to the derivation of the fundamental structural features of silk fibroin. The structure consists of extended polypeptide chains bonded together by lateral N—H…O hydrogen bonds to form antiparallel-chain pleated sheets. The sequence-G-X-G-X-G-X-in which G represents glycyl and X alanyl or seryl residues predominates throughout the structure, so that adjacent sheets pack together at distances of about 3.5 and 5.7 A. Longer inter-sheet distances are explained by the presence in the structure of the larger amino-acid residues, such as tyrosine.

Molecular Models of Amino Acids, Peptides, and Proteins

A set of accurate scale models has been developed for use in studies of the structures of amino acids, peptides, and proteins. Models representing atoms or groups of atoms built from hard wood to the scale 1 in.=1A are connected by a clamping device which maintains desired molecular configurations. These accurate models have been used as substitutes for calculation in investigations of the probable configuration of the polypeptide chain in proteins. Analogous models constructed of rubber‐like plastic to the scale 1 in.=2A and connected by snap fasteners are designed for qualitative studies of protein structure.

Specific hydrogen-bond formation between pyrimidines and purines in deoxyribonucleic acids

Abstract From crystal structure data for purines and pyrimidines it is concluded that in Watson and Cricks structure for DNA cytosine and guanine should form three hydrogen bonds. This conclusion strengthens the arguments of Watson and Crick as to the role of complementariness of structure of two DNA polynucleotide chains in the duplication of the gene.

Stable configurations of polypeptide chains

Several configurations of polypeptide chains involving planar amide groups with the dimensions found by experiment for simple substances, and hydrogen bonds between the NH groups and the carbonyl oxygen atoms, have been discovered. One of these structures, the α-helix, with about 3·7 amino-acid residues per turn of the helix, has been assigned to synthetic polypeptides and proteins that give X-ray diagrams of the α-keratin type. The evidence supporting this assignment is reviewed. Other configurations of polypeptide chains, including they γ-helix, three pleated-sheet structures, and the three-chain helical structure proposed for collagen, are described, and evidence bearing on their possible presence in proteins is discussed.

Preliminary X-ray diffraction studies of crystal forms of free and inhibited chymotrypsin

Abstract 1. 1. Crystals of α- and γ-chymotrypsin (EC 3.4.4.5) have been prepared, and also the monoclinic and tetragonal forms of the diisopropylphosphorofluoridate-inhibited enzyme. Crystals of a new trigonal form of diisopropylphosphorochymotrypsin have also been obtained. The unit cells and space groups of all of these crystal forms have been determined from X-ray diffraction data. 2. 2. The X-ray diffraction patterns of monoclinic and tetragonal diisopropylphosphorochymotrypsin are almost identical with those of α- and γ-chymotrypsin, respectively. The interconversion of these forms of diisopropylphosphorochymotrypsin previously described has been confirmed. In addition, the corresponding interconversion of α- and γ-chymotrypsin has been carried out at room temperature by proper control of the pH of the crystallizing solution, and the identity of the crystal forms has been established by their X-ray diffraction patterns.

X-Ray Diffraction Patterns from Reprecipitated Connective Tissue

For a number of years 1 it has been known that tendon gives excellent crystalline reflections. Recently 2 large spacings have been observed which provide strong evidence for the view that collagenous structures may be fibrous arrays of true crystals composed of very large molecules. Most forms of tendon from adult animals cannot be put into solution without obvious chemical disintegration. The work of Nageotte and of others 3 has, however, demonstrated that optically active fibrous precipitates can be obtained from weakly acid solutions made from the tail tendon of adult rats and from many connective tissues of foetal animals. The optical appearance of these precipitates suggests that they are at least similar to the tissue from which they were dissolved. We have carried out a series of experiments to determine: (a) whether reprecipitated tendon is really crystalline and (b) whether its X-ray diffraction pattern is like that of the original tissue. Reprecipitated connective tissue was obtained according to the procedures outlined by Nageotte. Pieces of freshly excised tail tendon from adult rats were placed in dilute (1 to 25,000) acetic acid. In such acid the tendon swells and a portion of it dissolves to form a clear but highly viscous solution. The reprecipitated tissue is obtained from this solution by salting out with 5% NaCl. Series of X-ray diffraction studies have been made of a number of these preparations. Some of these diffraction photographs were made with the usual short crystal-to-film distances; others were made with very small slit systems and with the large camera radii needed to record spacings of the order of 100 A.

X-ray studies of amino acids and peptides.

Publisher Summary Amino acids and peptides are considered the fundamental components of nearly all biological systems. Protein molecules are accepted as long peptide chains capable of combining in a more or less extended configuration or of folding in definite and unknown ways, which are dependent upon the chemical structure of the molecules and upon the forces that act upon them from within and outside. This chapter discusses some of the facts provided by X-ray studies concerning the structure of some amino acids and related compounds and the contribution that these facts make to an understanding of protein structure. X-ray diffraction studies of the molecular structure of solid proteins can be divided into two categories: (1) investigations made directly on protein material, both fibrous and crystalline and (2) determinations of atomic positions in crystals of amino acids and other compounds related to proteins. The chapter describes the latter investigations on atomic positions. The attack on the constitution and configuration of protein molecules and on the forces that hold them together in natural proteins is thus being carried out from both the top and the bottom. Single-crystal X-ray diffraction data obtained from crystalline proteins provides a basis for the determination of the dimensions of the crystallographic unit cells and the space groups of the crystals.

The Configuration of Polypeptide Chains in Proteins

The properties of proteins are determined not only by the sequence of amino-acid residues in the polypeptide chains, but also by the configuration of the chains—the way in which the chains are coiled or folded. It is probable that denaturation, the loss of some of the specific properties of a native protein, may in many cases be the result simply of a change in configuration of the polypeptide chains, without any change whatever in the sequence of amino-acid residues.

Spectrometric Measurements on Hexamethylene Tetramine and Urea

In order to use quantitative intensity methods in determining the crystal structures of organic compounds it is important to know how near alike are the atomic i-curves (uncorrected for thermal effects) in different crystals. Hexamethylene tetramine, CeH12N4, and urea, CO(NH2)2, are among the few organic substances with structures simple enough to lead fairly directly to determinations of scattering powers. The crystal structures of both compounds are well known from previous studies. The unit cube of hexamethylene tetramine)) has an edge length a0 = 7.02 Á and contains two molecules of C6H12N4. Its atoms are in the following special positions of either T¿ or T¿: C: (12a) u00; (M); 00ι>; ®00; (M); OChl; ν + --, etc. Ν: (8a) www; www; www; ûuû\ u + w + w + \ \ etc. with) w = 0.12, ν = 0.23-, or; w = 0.13, ν = 0.26 as determined from L a u e and spectrographs reflections. Urea) is tetragonal with a0 = 5.670, c0 = 4.726. The atoms in this two molecule ccll are in the following special positions of V¿ :

The crystal structure of zinc hydroxide

Zinc hydroxide differs from all the other anhydrous divalent hydroxides thus far studied by X-ray methods in having orthorhombicsymmetry. Two earlier studies have been made. I t is evident from the data published^) in one of them that the substance investigated was a different modification or, more probably, a hydrate of Zn{OH)^. The few reflections recorded in a recent powder study do not apply to the Compound analysed in the present paper. For the crystals used in this investigation we are greatly indebted to Dr. J o h n J o h n s t o n , Director of the Research Laboratory of the United States Steel Corporation. They were grown®) from a Solution of precipitated gelatinous zinc hydroxide dissolved in concentrated ammonium hydroxide from which ammonia was gradually removed. The resulting clear and colorless prisms, about one millimeter in diameter and two millimeters long, gave excellent X-ray reflections. Measured with the optical goniometer they had axial ratios which agreed with previous description*): a:h:c = 0.6048: 1: 0.5763. Data for the structure analysis have been fumished by series of L a u e and oscillation photographs about the three crystallographic axes and by spectrometric measurements of intensities from (hkO) and (hol) planes. Typical reflections occurring on a L a u e photograph and some of the data from oscillation photographs are listed in Tables I and II . Integrated intensities of reflections of the Cu—Ka line for all (hkO) and (hOl) planes having 20 < ca. 100° are recorded in Tables I I I and IV. Measurements of (hkO) reflections were made upon a crystal of approximately uniform diameter elongated in the direction of the