The cytosine (C)-rich sequences that can fold into tetraplex structures known as i-motif are prevalent in genomic DNA. Recent studies of i-motif–forming sequences have shown increasing evidence of their roles in gene regulation. However, most of these studies have been performed in short single-stranded oligonucleotides, far from the intracellular environment. In cells, i-motif–forming sequences are flanked by DNA duplexes and packed in the genome. Therefore, exploring the conformational dynamics and kinetics of i-motif under such topologically constrained environments is highly relevant in predicting their biological roles. Using single-molecule fluorescence analysis of self-assembled DNA duplexes and nanocircles, we show that the topological environments play a key role on i-motif stability and dynamics. While the human telomere sequence (C3TAA)3C3 assumes i-motif structure at pH 5.5 regardless of topological constraint, it undergoes conformational dynamics among unfolded, partially folded and fully folded states at pH 6.5. The lifetimes of i-motif and the partially folded state at pH 6.5 were determined to be 6 ± 2 and 31 ± 11 s, respectively. Consistent with the partially folded state observed in fluorescence analysis, interrogation of current versus time traces obtained from nanopore analysis at pH 6.5 shows long-lived shallow blockades with a mean lifetime of 25 ± 6 s. Such lifetimes are sufficient for the i-motif and partially folded states to interact with proteins to modulate cellular processes.

Nanopore-based resistive pulse sensing with biological nanopores has traditionally been applied to biopolymer analysis, but more recently, interest has grown in applying the technique to characterizing water-soluble metallic clusters. This paper reports on the use of α-hemolysin (αHL) for detecting a variety of thiolate-capped gold nanoclusters. The ligands studied here are p-mercaptobenzoic acid (p-MBA), tiopronin (TP), and thiolated PEG7 (S-PEG7). Individual clusters trapped in the cis-side of an αHL pore for extended periods (>10 s) exhibit fluctuations between numerous substates. We compare these current steps between the three different ligands and find that they scale with the mass of the corresponding ligand, which suggests that nanopore sensing could be used to characterize intraparticle surface modifications.

A single focus optical tweezer is formed when a laser beam is launched through a high numerical aperture immersion objective. This objective focuses the beam down to a diffraction-limited spot, which creates an optical trap where cells suspended in aqueous solutions can be held fixed. Spermatozoa, an often probative cell type in forensic investigations, can be captured inside this optical trap and dragged one by one across millimeter-length distances in order to create a cluster of cells which can be subsequently drawn up into a capillary for collection. Sperm cells are then ejected onto a sterile cover slip, counted, and transferred to a tube for DNA analysis workflow. The objective of this research was to optimize sperm cell collection for maximum DNA yield, and to determine the number of trapped sperm cells necessary to produce a full STR profile. A varying number of sperm cells from both a single-source semen sample and a mock sexual assault sample were isolated utilizing optical tweezers and processed using conventional STR analysis methods. Results demonstrated that approximately 50 trapped spermatozoa were required to obtain a consistently full DNA profile. A complete, single-source DNA profile was also achieved by isolating sperm cells via optical trapping from a mixture of sperm and vaginal epithelial cells. Based on these results, optical tweezers are a viable option for forensic applications such as separation of mixed populations of cells in forensic evidence.