Noncontact dipole effects on channel permeation. III. Anomalous proton conductance effects in gramicidin

Proton transport on water wires, of interest for many problems in membrane biology, is analyzed in side-chain analogs of gramicidin A channels. In symmetrical 0.1 N HCl solutions, fluorination of channel Trp(11), Trp-(13), or Trp(15) side chains is found to inhibit proton transport, and replacement of one or more Trps with Phe enhances proton transport, the opposite of the effects on K(+) transport in lecithin bilayers. The current-voltage relations are superlinear, indicating that some membrane field-dependent process is rate limiting. The interfacial dipole effects are usually assumed to affect the rate of cation translocation across the channel. For proton conductance, however, water reorientation after proton translocation is anticipated to be rate limiting. We propose that the findings reported here are most readily interpreted as the result of dipole-dipole interactions between channel waters and polar side chains or lipid headgroups. In particular, if reorientation of the water column begins with the water nearest the channel exit, this hypothesis explains the negative impact of fluorination and the positive impact of headgroup dipole on proton conductance.     

Fluorometric measurement of furfural and 5-hydroxymethylfurfural

Zine(II) forms highly fluorescent chelates with the aroyl hydrazones of furfural and 5-hydroxymethylfurfural, with the latter being more fluorescent than the former. The choice of aromatic acid hydrazide (aroyl hydrazine) as analytical reagents for furfurals was examined; 4-toluenesulfonic acid hydrazine was the most sensitive reagent examined, giving a uv fluorescence 10 times as sensitive as 4-hydroxygenzoic acid hydrazide (PAHBAH). More convenient visible fluorescence was given by PAHBAH and related compounds, and these are capable of detecting less than 500 pmol 5-hydroxymethylfurfural. The condensation reaction is complete in 30 mm ethanolic hydrochloric acid within 2 min at 60°C, and the stable product forms the fluorescent chelate on mixing with a zinc-diethanolamine solution in ethanol.     

Sugar transporters from bacteria, parasites and mammals: structure–activity relationships

Sugar transport across the plasma membrane is one of the most important transport processes. The cloning and expression of cDNAs from a superfamily of related sugar transporters that all adopt a 12-membrane-spanning-domain structure has opened new avenues of investigation, including pre-steady-state kinetic analysis. Structure–function analyses of mammalian and bacterial sugar transporters, and comparisons of these transporters with those of parasitic trypanosomatids, indicate that different environmental pressures have tailored the evolution of the various members of the sugar-transporter superfamily. Subtle distinctions in the function of these proteins can be related to particular amino acid residue substitutions.     

Physical Assay of Competence for Specific Mating-Pair Formation in Escherichia coli

The fractions of Escherichia coli female and male bacteria that are competent to form specific mating pairs at any time during normal exponential growth have been measured by a physical assay. The competencies of a Hayes- and a Cavali-type donor were 35 and 50%, respectively, in agreement with piliation assays reported by others. The competencies of two recipient strains, PA309 and P678, were found to be 33 and 29%, respectively.