. The cell and protoplasm ... Cells; Protoplasm; Cytology; Cytoplasm. 174 THE CELL AND PROTOPLASM carbon-nitrogen, for each amino acid resi- due. This forms a back-bone to ^vhich the remaining parts of the amino acid are attached forming ribs or side chains. The baclv-bone portion of the chain. Fig. 2. Eeproduction of a protein chain model showing about 15 amino acid residues of various kinds. The model was made to scale. The white horizontal lines serve to emphasize the zig-zag back-bone. varies only slightly from one protein to another; the side-chain portion, however, varies to
. The cell and protoplasm ... Cells; Protoplasm; Cytology; Cytoplasm. 174 THE CELL AND PROTOPLASM carbon-nitrogen, for each amino acid resi- due. This forms a back-bone to ^vhich the remaining parts of the amino acid are attached forming ribs or side chains. The baclv-bone portion of the chain. Fig. 2. Eeproduction of a protein chain model showing about 15 amino acid residues of various kinds. The model was made to scale. The white horizontal lines serve to emphasize the zig-zag back-bone. varies only slightly from one protein to another; the side-chain portion, however, varies to a considerably greater extent. This may be seen by referring to Table II and to the model in Fig. 3. The dimensions of the models made to scale were based on radii and valence angles of the respective atoms which have been determined from many measurements obtained from X-ray crystal analyses and from absorption spectra methods. These atomic characteristics are summarized in Table III (Pauling and Huggins 1934; Pauling 1939). There are several points of especial in- terest to us in Fig. 3. One important feature of the protein molecule may be recalled: that every ox^^gen and every nitrogen atom present is potentially a hydration center; or more specifically, that these atoms are capable of forming hydro- gen bridges with water molecules (Huggins 1936; Lassettre 1937) and thus binding the water molecules to the protein in much the same manner tliat water of crystallization is bound in a crj'stal. In a chain of 300 residues there will be approximately 300 oxygens and an equal number of nitrogens in the back-bone alone, almost regardless of the kind of protein; while, in contrast to this, ill tlie side chains the number of these V* V"^. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original American Association f
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