An inquiry into the selective protection of glycine during the radiolysis of glycine-alanine mixtures in aqueous solutions and its implications to the preservation of optically active amino acids in the early earth

Akaboshi et al. (1990) has found an unexpected protection of the achiral amino acid, glycine, towards ionizing radiation at the expense of the selective destruction of the chiral amino acids, alanine and aspartic acid. The present work examines the mechanism of this protection for the case of alanine. We have developed a computer model for the radiolysis of glycine, alanine and glycine-alanine mixtures in aqueous solution. It is established that this protection is due in part to the reaction of the α-radical of glycine with alanine to regenerate a more stable α-radical, according to the following reaction, $$ \cdot CH(NH_3^ + )CO_2^ - + CH_3 CH(NH_3^ + )CO_2^ - \to CH_2 (NH_3^ + )CO_2^ - + CH_3 \dot C(NH_3^ + )CO_2^ -$$ The rate constant of this reaction was estimated to be ≤10⁴M^-1s^-1. The implications for this selective protection of glycine are considered for a hypothetical case in which there would be an enrichment of about 10% ofL-alanine in the primitive ocean and taking the glycine/alanine ratios obtained in CH₄-and CO₂- dominated atmospheres using electric discharge experiments. It is predicted that alanine would be rapidly destroyed and radioracemized in spite of the fact that the concentration of alanine is equal or significantly lower than that of glycine. Assuming that chiral amino acids were a prerequisite for the origin of life, it can be deduced that life could have appeared in a relatively short period of time unless there was a constant supply of optical amino acids from extraterrestrial sources.     

Lactam bridge stabilization of α-helical peptides: Ring size,orientation and positional effects

A series of 14 residue amphipathic α-helical peptides, in which the sidechains of glutamic acid and lysine have been covalently joined, was synthesized in order to determine the effect of spacing, position and orientation of these lactam bridges. It was found that although an (i, i+3) spacing would position the lactam bridge on the same face of the helix, these lactams with 18-member rings were actually helix-destabilizing regardless of position or location. On the other hand, (i, i+4) lactams with 21-member rings were helix-stabilizing but this was dependent on orientation. Glutamic acid-lysine lactams increased the helical content of the peptide when compared with their linear homologue in benign conditions (50 mM KH₂PO₄, 100 mM KCl, pH 7). Two Glu-Lys (i, i+4) lactams located at the N- and C-termini gave rise to a peptide with greater than 99% helical content in benign conditions. Peptides with Lys-Glu oriented lactams were random structures in benign conditions but in the presence of 50% TFE could be induced into a helical conformation. The stability of the single-stranded α-helices, as measured by thermal denaturations in 25% TFE indicated that Glu-Lys oriented lactam bridges stabilized the helical conformation relative to the linear unbridged peptide. One Glu-Lys lactam in the middle of the peptide was more effective at stabilizing helical structure than two Glu-Lys lactams positioned one at each end of the molecule. The lactams with the Lys-Glu orientation were destabilizing relative to the unbridged peptide. This study demonstrates that correct orientation and position of a lactam bridge is critical in order to design peptides with high helical content in aqueous media.     

Conformational differences between cis and trans proline isomers of a peptide antigen representing the receptor binding domain of Pseudomonas aeruginosa as studied by 1H-nmr

A 17-residue disulfide-bridged peptide (PAK 128–144) corresponding to the C-terminus of Pseudomonas aeruginosa pilin strain K has been studied by one- and two-dimensional nmr techniques. This synthetic immunogen has been found to exist as two distinct conformations in solution, which have been demonstrated to arise as a result of the isomerization of the I¹³⁸-P¹³⁹ amide bond. The two isomers occur in the ratio of 3 : 1 trans to cis at 5°C. Sequential assignments for both forms have been accomplished through the use of nuclear Overhauser enhancement spectroscopy (NOESY) spectra and most side-chain resonances have been assigned using a combination of correlated spectroscopy, total correlated spectroscopy, and NOESY spectra. The presence of the cis isomer, which is considerably more predominant in the oxidized peptide, was confirmed by the observation of the characteristic NOEs between P¹³⁹ and the preceding residue. Further corroboration was given by the disappearance of the cis resonances in the spectrum of the P139A analogue of PAK 128–144. From observation of the differences in the chemical shifts and amide proton temperature coefficients of the two isomers, it is apparent that the two forms differ markedly in their solution conformation. The biological implications of the isomerization are discussed. © 1994 John Wiley & Sons, Inc.     

Studies on the factors influencing larval settlement in Balanus amphitrite and Mytilus galloprovincialis

Understanding of factors influencing settlement(attachment and metamorphosis) of marine invertebratelarvae is of great importance in aquaculture andcontrol of biofouling. The influence of two factors onsettlement of larvae was assessed from two separateinvestigations: 1, the influence of age (endogenousfactor) on cyprids of the barnacle Balanusamphitrite; and 2, the influence of a microbial film(exogenous factor) on pediveligers of the mussel Mytilus galloprovincialis.The settlement response of cypris larvae of B.amphitrite was found to be age-dependent. Oldercyprids responded more readily to settlement factorsthan newly molted ones. In M.galloprovincialis, competent pediveligers settled inresponse to a microbial filmed surface but not toan unfilmed surface. Moreover, a factor with MW of lessthan 5000 dalton, derived from culture medium of abacterial strain C1.1 (Pseudomonas-Alteromonasgroup), induced the settlement of M. galloprovincialis larvae.Thus, marine invertebrate larvae may require a periodof competence acquisition, during which they arepoorly responsive to settlement inducers. Uponacquisition of competence, larvae readily respondto external cues (e.g. microbial film, bacterialextracellular products).     

Purification and properties of malate dehydrogenase from the extreme thermophileBacillus caldolyticus

The enzyme malate dehydrogenase (EC 1.1.1.37) from an extreme thermophileB. Caldolyticus was purified to about 91% homogeneity. The molar mass of the enzyme was determined as 73 000 daltons and it is composed of two subunits, each with a molar mass of 37 000. Initial velocity studies with oxaloacetic acid and NADH as substrates at pH 8.1, over a range of temperatures, indicate that the enzyme operates via a sequential type mechanism. Van't Hoff plots of the kinetic parameters displayed sharp changes in slope at characteristic temperatures, whereas the Arrhenius plot exhibited no such breaks over the temperature interval investigated. The enzyme was found to be stable at 41°C and lower temperatures. At 51°C and 59°C an almost immediate 20% reduction in activity was obtained, but no further inactivation occurred during the 60 min of incubation. At 59°C the enzyme lost 50% of its initial activity in about 38 s. High concentration of NADH was observed to greatly stabilize the enzyme at that temperature.It is suggested that the slope changes in the Van't Hoff plots and the stability profies at 51°C and 59°C are representative of a temperature induced conformational change in the enzyme.Proceedings of the Fourth College Park Colloquium on Chemical Evolution:Limits of Life, University of Maryland, College Park, 18–20 October 1978.     

Amino acids in a carbonaceous chondrite from Antarctica

Summary A carbonaceous chondrite from the Antarctic, referred to as the Allan Hills meteorite 77306, appears to be free from terrestrial organic contamination. The presence of both protein and non-protein amino acids and an equal abundance of D- and L-enantiomers of amino acids, is testimony to the extraterrestrial nature of these compounds.     

Effects of the bark beetle nematode, Contortylenchus reversus, on gallery construction,fecundity, and egg viability of the Douglas Fir beetle, Dendroctonus pseudotsugae (Coleoptera: Scolytidae)

Healthy and Contortylenchus reversus infected Douglas Fir beetles were colonized in logs in the laboratory. At 7, 14, and 23 days after introduction of the beetles into the logs, the bark was removed from some of the logs and the length and shape of the primary gallery, the number of eggs laid, and the egg viability were determined for each pair of parent beetles. Parasitism caused a 25, 28, and 27% reduction in the length of the primary egg gallery built by the infected female at 7, 14, and 23 days, respectively, after the start of colonization. In addition, the number of eggs laid by the infected females by 7, 14, and 23 days after the start of colonization were 50, 33, and 45%, respectively, lower than that of healthy female beetles. Despite the differences in gallery length between control and infected females, linear regression analysis showed that the rates of gallery construction were not significantly different. However, infected female beetles showed significantly lower rates of egg-laying than did healthy female beetles. Nematode parasitism did not affect gallery shape and egg viability of female Douglas Fir beetles. Mating of healthy female beetles with infected males affected neither the gallery length nor the fecundity of the uninfected females.