V,-amylose

S. M., Okada, G. M., Sheldrick, G. M., Saenger, W., V-Amylose at Atomic Resolution X-Ray Structure of a Cycloamylose with 26 Glucose Residues (Cyclomalto-hexaicosaose). Proc.Nat.Acad.Sci.USA, 1999, 96, 4246. 

Polycrystalline and well-oriented specimens of pure amylose have been trapped both in the A- and B-forms of starch, and their diffraction patterns84-85 are suitable for detailed structure analysis. Further, amylose can be regenerated in the presence of solvents or complexed with such molecules as alcohols, fatty acids, and iodine the molecular structures and crystalline arrangements in these materials are classified under V-amylose. When amylose complexes with alkali or such salts as KBr, the resulting structures86 are surprisingly far from those of V-amyloses. 

Fig. 12.—Packing arrangement of shallow, 6-fold, V-amylose (10) helices, (a) Stereo view of two unit cells approximately normal to the fee-plane. The helix at the center (filled bonds) is antiparallel to the two helices at the comers in the back (open bonds). Intrachain hydrogen bonds (3-OH - - 0-2 and 6-OH 0-3) are shown in thin lines.

KOH-amylose complex, 346-347 A-Amylose, 340-342, 407-408 B-Amylose, 342-344, 409 V-Amylose, 345-346, 410 Analytical chemistry, 18 Anhydrides... 

More recent crystallographic work has been directed at inclusion compounds of V-amylose (including more detailed examination of the polyiodide compound) and at the structures of Va-amylose and Vh-amylose. Despite the terminology used, both forms have significant water content and may therefore be classed as inclusion compounds in their own right. 

Electron diffraction by lamellar, single crystals leads to a two-dimensional, tetragonal unit-cell with a = b = 22.9 A (2.29 nm). From X-ray diffraction data obtained from a film of sedimented, lamellar crystals, it was found that the c axis spacing (7.8 A 780 pm) is equivalent to that in 6-fold and 7-fold amylose helices. The true helical diameters of the 1-butanol, isopropyl alcohol, and 1-naphthol complexes were calculated from experimental data. The ratios of 6 7 8 indicated that the 1-naphthol complex has eight D-glucose residues per turn. The diversity of helical orientations in V-amylose crystals was discussed. 

This can be accomplished by more drastic chemical treatments where degradation also takes place and untwining occurs of the short segments (40). There is no support at present for these chains to re-intertwine (40). An untwining is envisaged by Sarko (16,41 ) in the case of A-amylose to V-amylose. 

Structural studies of amylose have, in turn, revealed a wide range of crystalline polymorphy, both in chain conformation and in crystalline packing. An example is the group of V-amyloses that exist as complexes with small organic molecules, water, or iodine. The latter complex is particularly interesting because it displays an intense blue color. The V-amyloses can be prepared by precipitation or drying from solution, and they crystallize readily. Consequently, their crystal structures are of interest in connection with any regenerated form of starch material. 

As will be shown later, these crystal structures differ considerably from the V-amyloses. 

The unit cell dimensions of all crystalline amyloses that have been determined in some detail, are listed in Table I. Also included are some intermediate forms between the va and Vjj amyloses (Ji.) and some V-amylose complexes with n-butanol, which, although not yet completely determined, have been added to illustrate the range of variability in unit cell dimensions. In the case of the Va-BuOH complex, a doubling of one unit cell axis was detected after a careful study of electron diffraction diagrams of single crystals ClO). A consequence of the doubling is that the unit cell now contains four chains, instead of the two normally found in amylose structures. Cln a strict sense, the A- and B-amyloses should also be considered as four-chain unit cells, but their double-helical structure still results in only two helices per cell) (13,1 ). 

Figure 2. X-ray fiber diffraction patterns for ("top, left to right) V -amylose VrnlSo-amylose KOH-amylose (bottom, left to right) B-amylose, amylose triacetate I, triethylamylose I-nitromethane complex...

All V-amylose structures shown in Table I have in common a left-handed, six-rgsidue helix, with h in a very narrow range from 1.32 to 1.36 A, and an 0-2...0-3(2) intramolecular hydrogen... 

Because h is small for the V-amyloses, a wide-diameter helix is characteristic of these structures. Complexing agents, such as DMSO, iodine, or water, are found inside the helix channel. For example, in VQygo-amylose, six DMSO molecules are accommodated inside the channel within one crystallggraphic repeat, which consists of three helix turns (c=24.39 A). This fiber repeat is not the result of the intrachannel DMSO but is caused solely by the packing of the interstitial. DMSO. A non-commensurable fiber repeat for the amylose helix and the intrahelical iodine is observed in V -iodine approximately three iodines occupy the helix channel within one fiber repeat, but the iodines form an almost linear polyiodide chain of an undetermined length. In this respect, the structures of the V -iodine complex and the a-cyclodextrin-iodine complex (30) are similar. 

Even though the inside of the helix channel of V-amyloses is primarily hydrophobic in character, intrahelical water has been found in all of the structures of complexes studied to date. The same was found to be the case in single crystals of hydrated cy-... 

Figure 5. V -amylose in ab projection. Positions of water molecules are shown by circles and hydrogen bonds are shown by dashed lines.

The packing of the VpMgo-s truc ture is somewhat exceptional in that it shows features not found in other V-amyloses included in Table I. The interstitial DMSO molecules prevent any interaction between the corner chains of the unit cell, but allow hydrogen bonds to form between the corner and center chains. The location of the DMSO molecules between the parallel-packing corner chains results in a pseudotetragonal unit cell, but the space group is still P2j 2 2].. The interstitial DMSO molecules are also clearly responsible for the three-turn fiber repeat. 

The molecular conformation and interstrand hydrogen bonding of the A- and B-amyloses are shown in Figd 9 Both strandg of the duplex are relatively extended (h - 3. 7 A for B and 3.51 A for A), although not nearly as extended as the alkali or salt amyloses, or some of the derivative structures. The extension of the helix prevents the formation of the intramolecular 0-2...0-3(2) hydrogen bond that occurs in V-amyloses. Conversely, the extended conformation permits the formation of the interstrand hydrogen bonds that appear instrumental in the stabilization of the structure. 

C-CP-MAS NMR produces a broad resonance with a chemical shift of 31.2 ppm,129 a characteristic of mid-chain methylene carbons of fatty acids in the V-amylose complex. The results showed that up to 43% of amylose in non-waxy rice starch, 33% in oat starch, and 22% in normal maize and wheat starch granules are complexed with lipids at a single helical conformation, and the remaining amylose is free of lipids and is in a random coil conformation.212 Up to 60% of apparent amylose in waxy barley starch is complexed with lipids.212... 

Other WAXD starch patterns, known as V-types, are associated with the amylose fraction. In V-amylose, the chain conformation is a left-handed single helix with six residues per turn (V6) for complexes with aliphatic alcohols and monoacyl lipids with ligands bulkier than a hydrocarbon chain, helices of seven or eight glucose residues per turn are feasible.31 Aliphatic chains within amylose helices are rather locked... 

---