N -Doped Nonalternant Nanoribbons with Excellent Nonlinear Optical Performance

Two novel N-doped nonalternant nanoribbons (NNNR-1 and NNNR-2) featuring multiple fused N-heterocycles and bulky solubilizing groups were prepared via bottom-up solution synthesis. NNNR-2 achieves a total molecular length of 33.8 Å, which represents the longest soluble N-doped nonalternant nanoribbon reported to date. The pentagon subunits and doping of N atoms in NNNR-1 and NNNR-2 have successfully regulated their electronic properties, achieving high electron affinity and good chemical stability enabled by the nonalternant conjugation and electronic effects. When applied a laser pulse of 532 nm, the 13-rings nanoribbon NNNR-2 shows outstanding nonlinear optical (NLO) responses, with the nonlinear extinction coefficient of 374 cm GW-1, much higher than those of NNNR-1 (96 cm GW-1) and the well-known NLO material C60 (153 cm GW-1). Our findings indicate that the N-doping of nonalternant nanoribbons is an effective strategy to access another type of excellent material system for high-performance NLO applications, which can be extended to construct numerous heteroatom-doped nonalternant nanoribbons with fine-tunable electronic properties.

Pentagon Inside the Circle of Willis and the Golden Ratio

The Golden ratio ("phi" or "Ф") has been known to us for ages and its use in art and architectural designs has enhanced its exquisiteness. Even some of the astounding creations of nature do follow this principle of Golden or divine proportions. Three centuries ago the circle of Willis at the base of the brain was first described and illustrated by Thomas Willis. Classically it was described as being circular in shape, however, the gross anatomy reveals a slightly different picture. The components of this complex neurovascular structure are so arranged that it appears more like a pentagon than a circle. A regular pentagon, unlike a circle, is a Golden shape that mathematically obeys the laws of Golden proportions. Like most other marvelous structures in nature, the close resemblance of the circle of Willis to a pentagon is more of a conscious effort of nature to establish consonance with aesthetic perception, rather than just a mere coincidence.

AFM of self-assembled lambda DNA-histone networks

Atomic force microscopy (AFM) was used to investigate the self-assembly behavior of λ-DNA and histones at varying histone:DNA ratios. Without histones and at the lowest histone:DNA ratio (less than one histone per 1000 base pairs of DNA), the DNA appeared as individual (uncomplexed), double-stranded DNA molecules. At increasing histone concentrations (one histone per 500, 250 and 167 base pairs of DNA), the DNA molecules started to form extensive polygonal networks of mostly pentagons and hexagons. The observed networks might be one of the naturally occurring, stable DNA-histone structures. The condensing effects of the divalent cations Mg(2+) and Ca(2+) on the DNA-histone complexes were also investigated. The networks persisted at high Mg(2+) concentration (20mM) and the highest histone concentration. At high Ca(2+) concentration and the highest histone concentration, the polygonal network disappeared and, instead, individual, tightly condensed aggregates were formed.