High-pressure physics: Hydrogen bonds in symmetry
Phys. Rev. Lett. 94, 065505 (2005)
Squeezing crystalline formic acid (HCOOH) turns it into a polymer, Alexander F. Goncharov et al. report.
They find that at about 20 gigapascals (GPa) the hydrogen bonds linking formic acid molecules into chains become symmetric: the hydrogen atoms sit precisely midway between two oxygen atoms on neighbouring molecules. This means that the hydrogens no longer unambiguously ‘belong’ to one molecule or the other. It also puts the hydrogen atoms in a decidedly ‘non-classical’ environment, in which they form chemical bonds to two atoms rather than just one. These symmetric hydrogen bonds, the researchers' quantum-chemical calculations show, have a partially covalent character. Symmetric hydrogen bonds have been proposed to exist in the high-pressure phase of ice known as ice X, but experimental support for them has remained contentious.
Above 40 GPa, Goncharov et al. find that long-range order in their formic acid samples disappears: the solid becomes amorphous, as the hydrogen-bridged chains become fully fledged polymers. This polymeric form persists when the pressure is relaxed, until about 20 GPa.
Microtechnology: Turn on
Small 1, 202–206 (2005)
It's an age-old question for engineers: how can a power source be translated into rotational motion? Jeffrey M. Catchmark and colleagues have been investigating how an existing system of nanorods made of gold at one end and platinum at the other, bathed in a solution of hydrogen peroxide, might be adapted to that purpose.
The platinum catalyses the production of oxygen, which results in a concentration gradient — and so an interfacial tension gradient — along the gold segment of the nanorod. As a result, the rod is propelled forward. An essential feature of the system is that the gold surface is hydrophobic.
To harness this power production for rotational motion, Catchmark et al. manufactured gear-like structures, 150 µm in diameter, made of gold and with platinum implants on one edge of the gear teeth. When fuelled by a hydrogen peroxide solution, the gears refused to spin. The trick, it turned out, was to add small amounts of acids to the solution, making the gold surface hydrophobic and producing motion at a rate of about one rotation per second — in linear terms, much faster than previously observed nanorod velocities.
From other experiments, the authors confirm that the acids maintain hydrophobicity at the gold surface. But there is evidently much still to learn about the surface chemistry involved, and the gears have yet to be coupled up mechanically.
Animal behaviour: Snap responses
Biol. Lett. doi:10.1098/rsbl.2004.0237 (2005)
Eusocial animals exhibit such characteristics as division of reproductive labour between castes, cohabiting generations and cooperative behaviour. There are plenty of examples among the insects; naked mole rats are an instance among the vertebrates.
Some species of shrimp also show eusocial characteristics. Eva Tóth and J. Emmett Duffy now describe a further aspect of shrimp eusociality, that of a collective response of members of a colony in the face of threats. They looked at species of Synalpheus, tiny inhabitants of sponges in the tropics that are fiercely territorial and mark their displeasure by snapping their ‘fighting claw’.
Tóth and Duffy observed the behaviour of Synalpheus when confronted by an alien shrimp of the same species. The initial one-to-one confrontation elicited a snap response from the defender. But if the intruder was brazen enough to push its luck, other colony members joined in with a cacophony of snapping. The aim of this collective sabre-rattling, say the authors, is not to enlist physical help against attack but to provide an unequivocal signal that the sponge is already colonized.
Chemical biology: Pore sequencing
Angew. Chem. Int. Edn 44, 1401–1404 (2005)
Reading the sequence of a single strand of DNA by pulling it through a tiny pore one base at a time may be feasible, Nurit Ashkenasy et al. show. They find that the ion current through a natural pore-forming protein, α-haemolysin, embedded in a lipid membrane, depends on which kind of DNA base is lodged at a key position in the pore neck: a deoxyadenosine (dA) group in this position produces a different signal from a deoxycytosine (dC) group.
It has been proposed previously that this kind of base discrimination could be used for rapid, single-molecule gene sequencing, and single DNA strands containing just purine bases (such as poly-A) have been differentiated from pyrimidine-only (poly-C) strands in conductivity measurements. But can the technique identify bases one at a time?
Ashkenasy et al. have made DNA single strands ‘knotted’ at one end with a hairpin turn, so that they cannot pass right through α-haemolysin but get lodged in the pore. They find that a single A base in a poly-C sequence can be distinguished if it is precisely 20 nucleotides away from the base of the hairpin loop, but not if it is at positions 19 or 21. Thus, there is a critical ‘reading site’ within the protein channel that makes this form of sequencing possible in principle.
Taste: Bitter variations
Curr. Biol. 15, 322–327 (2005)
We truly do inherit our tastes from our parents. For example, the reason that only some people perceive the compound phenylthiocarbamide (PTC) as bitter has a genetic basis. Work by Bernd Bufe and colleagues now shows why those who find it bitter do so to vastly different degrees.
The gene encoding the PTC taste receptor was identified last year, but no functional variations of the bitter receptor had been identified that would account for the taste differences. Bufe et al. now link specific versions of the gene — alleles — to levels of PTC perception. They introduced different alleles into cultured cells, which expressed the taste-receptor protein for the compound. By measuring the response of the receptors to PTC (shown here in crystal form), Bufe et al. identified which alleles conveyed the greatest cellular sensitivity.
Further tests on human subjects enabled the authors to confirm which versions of the PTC bitter-taste gene give people the greatest ability to detect the compound. At the gastronomic level, the findings provide a molecular context for individual fussiness over foods such as broccoli.