Aqueous amylodextrins form helical complexes with alcohol, halogenated hydrocarbons, and triiodide known as V-complexes. With the formation of these complexes, the ('13)C-nuclear magnetic resonance (n.m.r.) signals of carbons 1 and 4 are shifted substantially downfield due to rotations about the glycosidic bond necessary to form the helical conformation. ('13)C-N.m.r. studies with amylodextrin, (alpha)-methyl D-glucoside, and (alpha)-methyl maltoside complexes suggest a complexing mechanism in which pyranose ring strain, induced by binding of the complexing agent, is relieved by rotations about the glycosidic bonds.;Retrograded amylodextrin, gives ('1)H and ('13)C n.m.r. spectra identical to those of dissolved amylodextrin. These results, along with X-ray diffraction studies, indicate that retrograded amylodextrin, exists as a double helix, which (a) has a reticulate structure leading to an isoluble aggregate, and (b) because of hydration of the chains retains the conformation of dissolved amylodextrin in solution.;Human salivary (HSA), porcine pancreatic (PPA) and Bacillus subtilis (BSA) (alpha)-amylases have been used to study the structures of amylose-V complexes and retrograded amylose. These (alpha)-amylases hydrolyze the amorphous folding areas on the surfaces of the lamella of packed helices with the formation of resistant amylodextrin fragments. PPA or HSA was used to hydrolyze n-butyl and t-butyl alcohol and (alpha)-naphthol amylose-V complexes to yield resistant fragments of degree of polymerization (d.p.) 75 (+OR-) 4, 90 (+OR-) 3, and 123 (+OR-) 2), respectively. These d.p. values correspond to six, seven, and nine glucose residues per turn, for a folding length of 10 nm.;Acid hydrolysis of retrograded amylose gave resistant fragments with an average d.p. of 32. HSA and PPA hydrolyses each gave resistant fragments of d.p. 43 and BSA gave a fragment of d.p. 50. These slightly larger resistant fragments of the (alpha)-amylase hydrolyses result from the (alpha)-amylase leaving "stubs" on the ends of the resistant fragments. The lengths of the stubs are dependent on the sizes of the binding sites of the individual (alpha)-amylases. Hydrolysis of amylose-V complexes and retrograded amylose by (alpha)-amylases or acid may be used as a new method of preparing amylodextrins of different sizes, with a relatively narrow size distribution.