Spermidine-induced two-dimensional DNA condensations on mica surfaces: A different pathway from condensations in solution

Abstract

With atomic force microscopy (AFM) we systematically studied the DNA condensations on mica surfaces induced by multivalent cation spermidine. The pattern of the DNA condensates is a flat single layer, with a core in the centre and DNA wrapping around it at high density. We assume this to be a two-dimensional condensation of free coiled DNA onto negatively charged mica surfaces by the multivalent cation. The DNA molecules condense on mica surfaces via a pathway different from the formation of toroids, rods or globules in bulk solutions. We give an explanation to why toroid structures are difficult to be observed by AFM, and further discuss the relationship between DNA condensations in solutions and on mica surfaces. The present work will be helpful for understanding the behaviors of DNA on charged surfaces, which might be significantly different from that in solutions. DNA condensation, spermidine, two-dimension, condensing pathway, AFMThe condensation of DNA has become a fascinating research subject. Studies on DNA condensations are beneficial to the understanding of the physical behaviors of charged biopolymers and the process of virus packing in vivo, as well as to the development of technologies for gene therapy [1,2] . Experimental evidence has shown that DNA condensation occurs when ~90% of its charge is neutralized by multivalent cations, leading to a weak short range attractive force between different DNA segments [3] . DNA condensing agents include spermidine, spermine, cobalt hexamine and proteins such as protamine. The condensation can also be induced by crowding polymers such as PEG or weak electrolyte such as ethanol [4,5] . Although appearing as being continuous in ensemble assays, the coil-globule transition of DNA is markedly discrete for individual molecules, which is considered as a first-order phase transition [6] . In other words, each DNA molecule exists either in coiled or folded states. Fluorescence microscopy observations have revealed that, in bulk solutions, DNA molecules are mainly elongated when the spermidine concentration is less than 100 µM; elongated coils and folded compact states coexist in 150 µM spermidine; and all DNA molecules are in folded state when the spermidine concentration is increased to 200 µM [7] .Toroid is the most frequently observed folded structure of DNA condensates in solutions [8] . The size of the toroids is largely independent of the DNA length, reflecting that several DNA molecules may be incorporated into a single particle. Toroids are usually observed with electron microscopy. In such an observation, DNA is condensed in solution in advance, and then deposited onto a hydrophobic carbon grid and fixed with ethanol [9] . Some very well developed toroids have been recorded using this method.As a convenient and high-resolution instrument, AFM

Electrostatic interactions in biological DNA-related systems

“…Some mechanical studies of toroidal compression between wetting and nonwetting surfaces have been performed in the past, 127 without accounting however for ES effects inside condensates and on condensate-surface interphase. DNA condensation in 2D on positively charged surfaces 128 and membranes is expected to follow a different pathway and results in different final morphologies, as compared to 3D DNA aggregation.…”

Modeling DNA condensation on freestanding cationic lipid membranes

“…Dense DNA adsorption onto 2D positively charged surfaces 36,37 and DNA condensation on cationic lipid membranes 38 follow different pathways and lead to the formation of different final structures, as compared to 3D DNA condensates. In this situation, the positive surface charges serve as DNA counterions, compensating the DNA charge along its contact with the interface.…”