Cluster Size of Amylopectin and Nanosized Amylopectin Fragments Characterized by Pyrene Excimer Formation.

Amylopectin from waxy corn and the three nanosized amylopectin fragments (NAFs)-NAF(56), NAF(20), and NAF(8)-from waxy corn starch with a hydrodynamic diameter of 227, 56, 20, and 8 nm, respectively, were randomly labeled with 1-pyrenebutyric acid. The efficiency of these pyrene-labeled amylopectin-based polysaccharides (Py-AbPS) for pyrene excimer formation (PEF) upon diffusive encounter between an excited and a ground-state pyrene increased with increasing concentration of unlabeled NAF(56) in Py-AbPS dispersions in DMSO. Fluorescence decay analysis of the Py-AbPS dispersions in DMSO prepared with increasing [NAF(56)] yielded the maximum number (N(blob)(exp)) of anhydroglucose units (AGUs) separating two pyrene-labeled AGUs while still allowing PEF. Comparison of N(blob)(exp) with N(blob)(theo), obtained by conducting molecular mechanics optimizations on helical oligosaccharide constructs with HyperChem, led to a relationship between the interhelical distance (d(h-h)) in a cluster of oligosaccharide helices, [NAF(56)], and the number of helices in a cluster. It was found that the AbPSs were composed of building blocks made of 3.5 (±0.9) helices that self-assembled into increasingly larger clusters with increasing [NAF(56)]. The ability of PEF-based experiments to yield the cluster size of AbPSs provides a new experimental means to probe the interior of AbPSs at the molecular level.