An investigation of the aggregation and water-air interfacial behaviour of a cationic tentacle molecule

The aggregation and air-water interface properties of a three-legged tentacle molecule tris (11-pyridinium undec-1-yl)benzene-1, 3, 5-tricarboxylate tribromide have been investigated by measurement of the molar conductance, , and equilibrium surface tensin,, over a wide range of bulk concentration (8×10^–6 – 0.3 M). In contrast to the single chain analogue, dodecyl pyridinium bromide, DDPB, the tentacle species shows no evidence for a conventional micellar transition in solutions up to 100 fold more concentrated than the critical micelle concentration of DDPB, though aggregation of small numbers of molecules cannot be excluded. The conductance behaviour suggests the occurrence of ion-pair (or higher) equilibria, which are likely to complicate the interpretation of the data. Also, in contrast to a previous tentacle molecule described in the literature, the present species is significantly surface active, and in the high concentration limit reduces to <30 mNm^–1, significantly lower than the values obtained for simple ionic amphiphiles. In this region the tentacle species seems to stand like a tripod at the water-air interface with both its hydrocarbon legs and its aromatic body lifted clear of the water surface. At much lower concentrations two small plateau regions in are apparent ( 60 and 52 mNm^–1). Here, the tentacle molecule occupies a much larger surface area, and seems to adopt a crab-like crouch with its aromatic body and polar ester substituents lowered into the water surface.     

The formation of a liquid bridge during the coalescence of drops

This paper examines a mathematical model for the coalescence of two viscous liquid volumes in an inviscid gas or in a vacuum which removes the pressure singularity at the instant of impact inherent in the classical formulation of the continuum model. The very early stages of coalescence are examined in order to study the formation of the liquid bridge in two cases: (i) for two infinitely long, coalescing liquid cylinders; and (ii) for two coalescing spheres. Numerical solutions are computed for the velocity and pressure fields in the flow in both cases, and they confirm the removal of the pressure singularity. Also, the free-surface position at small times is determined.     

Diversity oriented synthesis: substitution at C5 in unreactive pyrimidines by Claisen rearrangement and reactivity in nucleophilic substitution at C2 and C4 in pteridines and pyrido[2,3-d]pyrimidines

Diversity oriented synthesis of fused pyrimidines leads to scaffolds with many biological activities. In the case of the preparation of pyrido[2,3-d]pyrimidines from 2-alkylthiopyrimidines, the formation of a new carbon-carbon bond at C5 is required, a reaction, that is, very limited in scope. However Claisen type rearrangement of simple 4-allylic ethers affords C5 substituted pyrimidines readily; in cases with an ester substituent, rearrangement occurs at room temperature. Subsequent cyclisation to afford 6-methylpyrido[2,3-d]pyrimidin-7(8H)-ones was achieved in high yield. Using allylic ethers derived from 3-chloromethyl-4-arylbut-3-en-2-ones as substrates, a new titanium[IV]chloride catalysed reaction affording 6-arylmethyl-7-methylpyrido[2,3-d]pyrimidines was discovered. In contrast, 2-alkylthiopteridines are readily available. In both cases, substitution at C2 and C4 to generate diversity has been carried out and the reactivity compared; yields of substitution products were generally higher with pteridine substrates. In biological assays unexpected hits were found for activity against the Gram positive bacterium, Nocardia farcinia, and against the parasite Trypanosoma brucei brucei, illustrating the value of diversity oriented synthesis in the discovery of biologically active compounds.     

The Cyclopropyl Group in Studies of Enzyme Mechanism and Inhibition

The cyclopropyl group—particularly in compounds such as cyclopropylmethanol, phenylcyclopropylamine, and cyclopropanone hydrate—has proved to be very useful in the study of enzyme reactions. Whether the cyclopropane ring is opened to form a radical or remains intact can provide much information about reactions occurring at the active site.