Covalent carbon-carbon bonds are hard to break. Their strength is evident in the hardness of diamonds 1,2 and tensile strength of polymeric fibres [3][4][5][6] ; on the single-molecule level, it manifests itself in the need for forces of several nanonewtons to extend and mechanically rupture one bond. Such forces have been generated using extensional flow [7][8][9] , ultrasonic irradiation 10 , receding meniscus 11 and by directly stretching a single molecule with nanoprobes [12][13][14][15][16] . Here we show that simple adsorption of brush-like macromolecules with long side chains on a substrate can induce not only conformational deformations 17 , but also spontaneous rupture of covalent bonds in the macromolecular backbone. We attribute this behaviour to the fact that the attractive interaction between the side chains and the substrate is maximized by the spreading of the side chains, which in turn induces tension along the polymer backbone. Provided the side-chain densities and substrate interaction are sufficiently high, the tension generated will be strong enough to rupture covalent carbon-carbon bonds. We expect similar adsorption-induced backbone scission to occur for all macromolecules with highly branched architectures, such as brushes and dendrimers. This behaviour needs to be considered when designing surface-targeted macromolecules of this typeeither to avoid undesired degradation, or to ensure rupture at predetermined macromolecular sites.A series of brush-like macromolecules with the same number average degree of polymerization of a poly(2-hydroxyethyl methacrylate) backbone, N n ¼ 2,150^100, and different degrees of polymerization of poly(n-butyl acrylate) (pBA) side chains ranging from n ¼ 12^1 to n ¼ 140^12 were synthesized by atom transfer radical polymerization (see 'Polymer Characterization' in the Methods) 18 . Owing to the high grafting density, the side chains repel each other and thereby stretch the backbone into an extended conformation. Placing these macromolecules on a surface enhances the steric repulsion between the side chains, which results in both an extension of the polymer backbone and an increase of the persistence length.The effect is illustrated in Fig. 1, which shows atomic force microscopy (AFM) micrographs of monolayers of pBA brushes with short ( Fig. 1a) and long side chains (Fig. 1b). Measurements on both types of molecules yielded a number average contour length per monomeric unit of the backbone of l ¼ L n =N n ¼ 0:23^0:02 nm (see 'Atomic Force Microscopy' in Methods), which is close to l 0 ¼ 0.25 nm, the length of the tetrahedral C-C-C section. This means that even for short side chains (n ¼ 12), the backbone is already fully extended and adopts an all-trans conformation. As the side chains become longer, we observe global straightening of the backbone reflected in the increase of the persistence length (Fig. 1c).Chain extension requires a substantial amount of force, which we estimate using simple spreading arguments (Fig. 2). Just as in normal liquids, the polymer...

Recent studies have identified RNA transcripts arising from mammalian telomeres with the transcript of the C-rich strand (r(5-UUAGGG-3) n ) being much more abundant than transcripts of the G-rich strand. Here we used transmission electron microscopy, CD, and nuclease digestion to investigate the structure of ϳ640-nucleotide (nt) RNA transcripts of the C-rich and G-rich strands of mammalian telomeric DNA. The CD spectrum of the C-rich RNA in low salt (10 mM KCl) or high salt (100 mM KCl) was typical of mixed sequence RNA, whereas the CD spectrum for the G-rich RNA differed with changes characteristic of parallel G-quadruplexes at the higher salt concentration. Electron microscopy visualization of the C-rich RNA revealed relatively extended unstructured molecules 59.7 ؎ 17.8 nm in length and with a width consistent with single-stranded RNA following metal coating. In contrast, the G-rich RNA was observed as round particles and short, thick rods with the rods being most prevalent in high salt conditions and absent in low salt. The rods were 22.7 ؎ 4.8 nm in length and 7.6 nm in width. Digestion of the G-rich RNA with T1 RNA nuclease revealed a ladder of bands whose sizes were integral multiples of 24 nt plus a 4-nt overhang. These observations suggest a model in which G-rich telomeric RNA folds into chains of particles each consisting of four (UUAGGG) repeats stabilized by parallel G-quartets and joined by UUA linkers. These chains further condense to form short rods and round particles.Telomeres are the terminal elements of eukaryotic chromosomes. They maintain the integrity of linear chromosomes by blocking misrecognition of the DNA ends as double strand breaks. They also provide a mechanism to overcome the progressive erosion of DNA sequences at chromosome ends due to the end replication problem, and they serve to control transcription of genes near the telomere. These complex functions reflect the interplay between a large set of both telomere-specific and nonspecific factors and the unique structural organization of the telomere that sets it apart from the remainder of the genome. Indeed telomeres have long been known to be heterochromatic and to show telomere position effects in which transcription of genes located near the telomere is repressed (reviewed in Ref. 1).Telomeric DNA of most higher members of the animal kingdom consists of ϳ5-20 kb of the repeat (5Ј-TTAGGG-3Ј) n , whereas in plants, the repeat is (5Ј-TTTAGGG-3Ј) n with plant telomeres frequently exceeding 50 kb (2, 3). The normal cellular core histones comprise the major protein species at plant and animal telomeres. In addition, there are six telomere-specific proteins termed the shelterin proteins that have been identified largely in mammalian cells, and they consist of TRF1 and TRF2, which bind directly to duplex telomeric repeats, Pot1, which binds to the single-stranded 3Ј overhang on the G-rich strand, and additional factors including hRap1, Tin2, and TPP1, which scaffold onto TRF1 and TRF2 (reviewed in Ref. 4).The organization of telomeres...

No abstract