The cluster modelId of the structure of amylopectin restss7 on (a) the isolation of -35% of branch points in multiply branched oligosaccharides in which (1+6)-linked units are separated by only one glucose residue, (6) the formation, by acid hydrolysis of ungelatinised starch, of residual "Nageli" and "Lintner" amylodextrins, (c) the distribution of the lengths of enzymically debranched chains into discrete ranges, (d) the formation of macrodextrins on partial degradation with alpha-amylase or cyclodextrin glycosyltransferase, and (e) the observation of discrete periodicities of 50-lOO A by electron microscopy and X-ray diffraction. The sizes and arrangements of the clusters remain uncertain, although the models present them as intermolecularly hydrogenbonded aggregates of 5-10 chains of d.p. 15-20, corresponding to 30 000-80 000 entities of relative molecular mass (RMM) 12 000-30 000 per amylopectin molecule. Hizukur? suggested 22-25 chains of d.p. 12-16 corresponding to 15 000-23 000 clusters of RMM 43 00&66 000 per molecule of RMM 10'. Burchard and Thurn' demonstrated excellent agreement of the angular dependence of scattered light with a modified Robin-Mercier3 model containing clusters of d.p. -50 with 3-4 branches and connected by chains carrying 1.4 clusters on average. Bertoft", from the results ofpartial alpha-amylolysis of waxy maize amylopectin, described two types of cluster containing 19 or 26 chains and 256 or 348 "anhydroglucose" units, respectively.
We now propose that the unusual pattern of action of the maltotetraose-forming amylase from Pseudomonas stutzeri may provide a new approach to analysing sizes and distributions of clusters in amylopectins.
P. stutzeri amylase"~'2 catalyses the removal of maltotetraose units from the non-reducing chain ends of amylopectins and has" an amylolysis limit towards potato amylopectin of 52% (@I, 57% for beta-amylase). However, the enzyme possesses some endo-activity as shown by its ability to release soluble coloured products from insoluble dyed starch'3-'5 and amylose16. We now report that the limit dextrins, obtained by the * Dedicated to Professor David Manners.