Glucose residue crystal structures

Figure 2. A contour diagram of the conformational energy of p-cellobiose computed from eqn. (6) holfing constant all variables except < ), v see ref. 5 for details. The rigid glucose residue geometry was taken from ref. 23, and the valence angle p at 04 was chosen as 116 in accordance with the results of pertinent crystal structure determinations. Contours are drawn at 2,4, 6, 8,10,25, and 50 kcal/mol above the absolute minimum located near ( ), v = -20 , -30 higher energy contours are omitted.

In this study, we assume that crystal structures will have the lowest possible total of intra- and inter-molecular potential energy. However, the partitioning of the potential energy between intra- and inter-molecular terms will vary among crystal structures, distorting the glucose residues away from the shape of lowest energy in a way that will reflect more-or-less random... 

Glucagon is a 29-residue hormone whose primary biological role is to stimulate glucose release and production. In dilute aqueous solution the peptide is in a random conformation, but it can be induced to adopt a largely helical conformation under a variety of conditions. Under basic conditions it aggregates to form an a-helical trimer, and the crystal structure of this trimer is known (Sasaki et al., 1975). It also adopts a partially helical conformation when it forms mixed micelles with detergents (Braun et al., 1983). 

Single crystal X-ray diffraction data for the crystalline portion of A type starch have been reported and the crystallographic data are summarized in Table 2. The unit cell contains 12 glucose residues located in two left-handed, parallel-stranded double helices packed in a parallel fashion. Four water molecules are located between these helices. It has been reported that the B type starch also contains chains arranged in double heli-ces. The currently accepted hexagonal unit cell dimensions are in Table 2. The A and B structures differ in crystal packing of the chains and in moisture content. 

It has been known for almost 200 years that starch gives a deep blue color when a solution of potassium iodide and iodine is added [47]. More than a century later it was suggested that the complex consisted of a helical polysaccharide, with triiodide in the center of the helix [48]. Using flow dichroism, it was demonstrated that the triiodide was stacked in a linear structure, as required for the helical model [49]. Another study of the optical properties of crystals of the amylose-triiodide complex showed it to be consistent with a helical structure [50] and X-ray diffraction showed the triiodide complex gave the dimensions of a unit-cell of a helix with six glucose residues per turn [51]. This confirmed a helical structure for the amyiose complex with triiodide that predated the helical models proposed by Pauling for polypeptides [52] and the double helical model for DNA by Watson and Crick [53] by 10 years. 

Glycogenin, the autocatalytic initiator of starch biosynthesis, catalyses the initial glycosylation of one of its own tyrosine residues, followed by the first few a-glucosyl residues of the starch molecule the stereochemistry of the transfer to tyrosine is unknown, but that to the 4-OH of glucose residues is retentive. Again, the X-ray crystal structure revealed no likely nucleophile except Asp 102, which would require a conformation change to be correctly placed." ... 

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