STRUCTURAL UNITS OF STARCH DETERMINED BY X-RAY CRYSTAL STRUCTURE METHOD.
A few brief statements summarizing the foregoing conclusions may make a picture of the structure of the starch grain somewhat clearer. 1. The presence of lines on the negatives indicates a regular arrangement of the planes of atoms. 2. The lines are in close agreement with lines which would be produced by a lattice of the tetragonal system, the elementary cell of which is a square prism with the dimensions 5.94 x 5.94 x 5.05 A.u. 3. The unit of the lattice occupies a space equal to the volume of the starch group, C(6)H(10)O(5). 4. The large number of atoms in the unit makes it highly probable that principal planes and secondary planes of atoms occur for every reflecting position. 5. The effect of the secondary upon the principal planes may readily account for the differences in the density of the lines produced on the negatives. 6. From theoretical considerations, reflections, such as those obtained, would occur if starch grains were built up of concentric layers of units. 7. Two other factors which might affect the density of the lines are thermal agitation and the curvature of the concentric layers. 8. A model of the starch group was constructed to scale based on the accepted sizes of the atoms involved and upon rather meager chemical evidence. The model apparently fulfills the requirements necessary to produce reflections such as were obtained. 9. The model fits the elementary cell loosely enough to suggest a low density and to allow for considerable thermal movement. At the same time, parts of it approach the faces of the cell closely enough to make cohesion seem possible. 10. The model makes clearer the basis for the assumption that reflection from certain positions would be stronger than from others. If the interpretation of the data is correct and if the assumptions made are sound, then the starch grain is built up of units arranged in concentric layers, and the units are groups of atoms, each containing 6 carbon, 10 hydrogen, and 5 oxygen atoms. Such a structure is certainly not an amorphous structure, and on the other hand it is not crystalline in the common sense of the term. Parts of the grain, it is true, act as crystals in that for certain distances the layers of units are in planes, but taken as a whole the layers are curved. As to the validity of the conclusions, those pertaining to the type of lattice and to the size of the unit may be accepted as sound in our present knowledge of x-rays and crystal structure; those, however, pertaining to the nature and the spherical arrangement of the units, while they seem convincing, need the support of further investigation into the various structures deposited by living protoplasm. In conclusion, the assumption that the units form a sort of spherical space lattice, gives a picture of the starch grain which leads us to ponder over the nature of the activity in protoplasm when it is depositing solid substances. Starch, cellulose, and pectic bodies are about the only solid deposits made directly by the living substance of plants, and all three have the same proportional formula, C(6)H(10)O(5). Investigations, as yet incomplete, indicate that cellulose also consists of a regular arrangement of C(6)H(10)O(5) groups, each acting as a unit, but the spacing (6.14 x 6.14 x 5.55) is slightly different from that of starch. Pectin has not been studied. Protoplasm may be thought of as being composed of molecules of many different sizes, polypeptides, or even proteins forming the larger, and amino-acids the smaller, if water and electrolytes are ignored. The smaller molecules, such as those of the amino-acid, leucine, are approximately equal in size to the C(6)H(10)O(5) group of starch. That being the case, what can be the state of affairs at the interface when the starch particles are being deposited? Is it probable that protoplasm is homogeneous to the extent of being able to deposit these particles at 6 A.u. intervals? From quite another view-point a clear picture of the units of structure and their arrangement in cellulose should give a new point of attack on the many problems connected with osmosis. And from still a different view-point, it might lead perhaps, to a solution of problems connected with swelling. Another line of thought is suggested by the uniformity of the groups in the starch grain. Since the C(6)H(10)O(5) group occurs as an individual unit, one is inclined to suspect that it is really the molecule. Generally the starch molecule is considered to be very large, to be composed of several dozens of such groups, and to have a molecular weight of 7,000 or much more. No one figure, however, seems satisfactory to the different authorities. There is already at hand considerable evidence which will be assembled in a later paper favoring the single group, C(6)H(10)O(5), as the molecule. Finally, problems in polarized light may receive more satisfactory explanations through a clearer notion of the molecular structure of the carbohydrates.
Sponsler
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University of California, Southern Branch, Los Angeles.