Intrinsic noise modulation in closed oligomerization-type systems⋆

How random fluctuations impact on biological systems and what is their relationship with complexity and energetic cooperativity are challenging questions that are far from being elucidated. Using the stochastic differential equation formalism, we studied analytically the effect of fluctuations on a series of oligomerization processes, in which several molecules of the same or different species interact to form complexes, without interaction with the environment. The conservation of the total number of molecules within the systems imposes constraints on the stochastic quantities, among which the negativity of the covariances and the vanishing of the determinant of the covariance matrix. The intrinsic noise on the number of molecules of each species is represented by the Fano factor, defined as the variance to mean ratio. At the equilibrium steady states, the sum of the Fano factors of all molecular species is equal to the rank of the system, independently of the parameters. The Fano factors of the individual molecular species are, however, parameter dependent. We found that when the free energy cooperativity of the reactions increases, the intrinsic noise on the oligomeric product decreases, and is compensated by a higher noise on the monomeric reactants and/or intermediate states. The noise reduction is moreover more pronounced for higher complexity systems, involving oligomers of higher degrees.

A modified polyurethane elastomer using polyfunctional HTPB synthesized by in-situ nitroxide mediated polymerization of 1,3-butadiene

In-situ nitroxide mediated polymerization (NMP) of 1,3-butadiene was carried out in the presence of tetraethylenepentamine (TEPA) as a tri-functional nitroxide precursor, which furnished a high degree of hydroxy functionality. The multi-modal molar mass distribution of the product was elucidated by reductive hydrolyzation of the oligomeric product. The new polyfunctional hydroxyl-terminated polybutadiene (PFHTPB) was characterized and used in preparation of an HTPB-based polyurethane (PU). The effects of this new HTPB on mechanical and dynamic-mechanical properties of its corresponding polyurethane elastomer were studied. The mechanical and dynamic-mechanical characterization of the elastomer was evaluated by tensile and dynamic mechanical thermal analysis (DMTA) tests. A significant enhancement in mechanical properties of the elastomeric product with a small increase in the glass transition temperature was achieved using PFHTPB. In addition, there was a significant decrease in loss of the tangent value over the use of PFHTPB, compared to industrial HTPB, due to an efficient increase in crosslink density of the PFHTPB-based PU elastomer. The thermal stability of PFHTPB-based PU was comparable with its corresponding HTPB-based PU elastomer.

Resistance to amino acid analogues in Coprinus: Dominance modifier genes and dominance reversal in dikaryons and diploids

SUMMARYWild-type strains of Coprinus lagopus are sensitive to para-fluoro-phenylalanine and ethionine. Resistant mutants to these two analogues are known but all these mutants are recessive in a heterozygous dikaryon except for F7 (pfpr-3) which is semi-dominant. Resistance to two other analogues, however – canavanine sulphate and azetidine-2-carboxylic acid – were found to be wild-type features. One strain of C. lagopus sensitive to canavanine was identified. Selection for canavanine resistance in monokaryons always yielded only dominant resistance, while selection for para-fluorophenylalanine resistance in monokaryons gave only recessive resistance. Canavanine-resistant mutants were due to a single gene mutation which, like the wild-type resistance, were dominant in heterozygous dikaryons. The wild-type resistance was also dominant in a diploid but the mutant resistance was recessive. Selection for resistance to para-fluorophenylalanine in auxotrophically balanced dikaryons resulted in the identification of two new loci (pfpr-10 and pfpr-ll), and two specific dominance modifiers (mod+-10 and mod+-ll). In the absence of the specific modifier, pfpr-10 and pfpr-ll were recessive while, in the presence of even one dose of the specific modifier, resistance was dominant in the dikaryon. The pfpr-10 and pfpr-ll even in the presence of two doses of modifier were fully recessive in the diploid. The action of the modifier genes and the reversal of dominance in dikaryon and diploid is discussed in terms of negative complementation in an oligomeric product of the pfpr gene and localized translation of the relevant mRNA in the cell with the modifier acting as a reinforcer of localization.