Observation of color-transparency in diffractive dissociation of pions

We have studied the diffractive dissociation into dijets of 500 GeV/c pions scattering coherently from carbon and platinum targets. Extrapolating to asymptotically high energies (where t(min)-->0), we find that when the per-nucleus cross section for this process is parametrized as sigma = sigma0Aalpha, alpha has values near 1.6, the exact result depending on jet transverse momentum. These values are in agreement with those predicted by theoretical calculations of color-transparency.     

Direct measurement of the pion valence-quark momentum distribution, the pion light-cone wave function squared

We present the first direct measurements of the pion valence-quark momentum distribution which is related to the square of the pion light-cone wave function. The measurements were carried out using data on diffractive dissociation of 500 GeV/c pi(-) into dijets from a platinum target at Fermilab experiment E791. The results show that the /q&q> light-cone asymptotic wave function describes the data well for Q2 approximately 10 (GeV/c)(2) or more. We also measured the transverse momentum distribution of the diffractive dijets.     

Understanding protease catalysed solid phase peptide synthesis

A protease (thermolysin) was used to directly synthesise a number of dipeptides from soluble Fmoc-amino acids onto a solid support (PEGA1900) in bulk aqueous media, often in very good yields. This shift in equilibrium toward synthesis is remarkable because for soluble dipeptides in aqueous solution hydrolysis rather than synthesis is observed. Three possible reasons for the equilibrium shift were considered: (i) using a solid support makes it easy to use an excess of reagents, so mass action contributes towards synthesis; (ii) reduction in the unfavourable hydrophobic hydration of the Fmoc group within the solid support compared with the free amino acid in solution and (iii) suppression of the ionization of amino groups linked to the solid phase due to mutual electrostatic repulsion. It was found that under the conditions studied the second effect was most important.     

Dramatic Specific‐Ion Effect in Supramolecular Hydrogels

We report on a pronounced specific‐ion effect on the intermolecular and chiral organization, supramolecular structure formation, and resulting materials properties for a series of low molecular weight peptide‐based hydrogelators, observed in the presence of simple inorganic salts. This effect was demonstrated using aromatic short peptide amphiphiles, based on fluorenylmethoxycarbonyl (Fmoc). Gel‐phase materials were formed due to molecular self‐assembly, driven by a combination of hydrogen bonding and π‐stacking interactions. Pronounced morphological changes were observed by atomic force microscopy (AFM) for Fmoc‐YL peptide, ranging from dense fibrous networks to spherical aggregates, depending on the type of anions present. The gels formed had variable mechanical properties, with G′ values between 0.8 kPa and 2.4 kPa as determined by rheometry. Spectroscopic analysis provided insights into the differential mode of self‐assembly, which was found to be dictated by the hydrophobic interactions of the fluorenyl component, with comparable H‐bonding patterns observed in each case. The efficiency of the anions in promoting the hydrophobic interactions and thereby self‐assembly was found to be consistent with the Hofmeister anion sequence. Similar effects were observed with other hydrophobic peptides, Fmoc‐VL and Fmoc‐LL. The effect was found to be less pronounced for a less hydrophobic peptide, Fmoc‐AA. To get more insights into the molecular mechanism, the effect of anions on sol–gel equilibrium was investigated, which indicates the observed changes result from the specific‐ion effects on gels structure, rather than on the sol–gel equilibrium. Thus, we demonstrate that, by simply changing the ionic environment, structurally diverse materials can be accessed providing an important design consideration in nanofabrication via molecular self‐assembly.