In the literature one encounters also the following scheme :
Primary structure = the rather stable linear concatenation of alpha-amino acids giving polypeptide chains as a result of peptide bonds.
Secondary structure = folding or screw-like twisting, fixed by weaker bonds. After Pauling by means of polypeptide screws stiffened by hydrogen bridges [the alpha-helix, see Figure in main text, above].
Tertiary structure = mode of spatial packing of the molecule into a relatively compact three-dimensional form, with salt-bonds, hydrogen bridges, van der Waals' associations, and covalent lattice formation.
[In addition to these, one often also mentions :
Quarternary structure : This structure of proteins corresponds to the manner in which several protein subunits are associated in larger molecules. All of such units are held together by ionic or polar forces but not by formal covalent bonds.]
Responsible for the variation of the basic scheme of protein structures, are, alongside many other factors, especially the internal sequence of reactive components, and the phenomenon of hydratation. With a molecular weight of 100000 -- absorbed ions not considered -- about 100 positive and 100 negative charges may be encountered (as a result of ionizing side chains of asparagin- and glutamin acids, of arginine, lysine, and histidine [See list of the 20 amino acids in previous document (i.e. part XVh)] ), an alternation probably being extended and varied as a result of internal bipolar subgroups [of the protein molecule]. In actomyosin, the work-producing substance of the muscle cell -- being not proper vital substance, but immediately and truly bonded to it, and also hard to be distinguished from it -- one sees the poly-electrolyte charges being distributed such that mutual repulsion and with it stretching result. Another mode of alternation may promote and fix a rolling up [of the molecule]. In the case of spreading into films, the globular proteins seem to unfold and to lie with their longitudinal axis parallel to the surface. This is accomplished by the hydrophilous members. The energy of unfolding is provided by the change of surface. Beyond the surface, gamma-globuline molecules do not show, after the X-ray-small-angle method, the expected normal form of a circular section, but the external shape of a elliptic cylinder, which one may interpret as statistical loosening of the periphery (especially because the internal part seems to be spherical or circular).
[When protein is dissolved in water, each of its molecules structurally changes the surrounding water layer, and so each possesses a water mantle. This is hydratation.] The hydratation (ca 100 gr H2O per 100 gr protein) partly is a more narrow one [and then is "hydratation in the strict sense"], as such surely mediated and favored by hydrogen bridges [from water molecules to the protein molecule] and therefore stoichiometrically defined (namely 2 mole H2O [+/- 5 percent] per [mole] amino acid), and partly, now referring to the rest of the protein's water mantle, a loose associative water zone, grading into the "solvent". The firmly bonded part of the water resists freezing up to minus 700. From the observation that upon dehydration [of hydratated protein] the inter-molecular distances, and not the intra-molecular distances, change, one concludes that the water indeed forms a deposited shell around the protein molecule, and thus is not truly imbibed by it. ( But also intermediary combinations are conceivable. Good knowledge about atomic distances in bonding in polypeptide chains -- known with a precision of up to +/- 0.02 Angstrom -- and the same about variations of valence angles, only within +/- 2 percent, refer, as always, to "dead" protein).
The firmly determined amino acid sequence [in proteins] -- a hereditary genetically nucleotidically determined property [i.e. coded for in DNA] -- allotting to the amino acids fixed inexchangeable locations, renders the smaller active substances [such as molecularly small enzymes] into chemically uniform substances ( [And] the body can, in the given case, also make enzymatically active proteins with [the amino acid] selenomethionin instead of methionin). Larger proteins often seem to lack absolute homogeneity. From the different behavior of pseudo-fractions one concludes that they are micro-heterogeneous in a certain degree. These, it is true, are conclusions that often will have to be revised, because it is difficult to prove that the presuppositions are unequivocal. Many methods are far too barbaric with respect to the object. There is great distinction between the functional robustness of living organismic substance and the chemical and physical susceptiveness of many isolated, and therefore rendered defenceless, "native" substances.
One also did entertain the idea of not having involved the whole [entire] intact molecular structure in many effects [of proteins] but only a specific "center of activity", whereas the rest of the molecule has the function of side line help and of embedding. In this, one may see an extention and completion of the concept of "prosthetic group" as well as a conceptual compression of all necessities.