Assessing performance of single‐sample molecular genetic methods to estimate effective population size: empirical evidence from the endangered Gochu Asturcelta pig breed

Estimating effective population size (N_e) using linkage disequilibrium (LD) information (N_e(LD)) has the operational advantage of using a single sample. However, N_e(LD) estimates assume discrete generations and its performance are constrained by demographic issues. However, such concerns have received little empirical attention so far. The pedigree of the endangered Gochu Asturcelta pig breed includes individuals classified into discrete filial generations and individuals with generations overlap. Up to 780 individuals were typed with a set of 17 microsatellites. Performance of N_e(LD) was compared with N_e estimates obtained using genealogical information, molecular coancestry (N_e(M)) and a temporal (two‐sample) method (N_e(JR)). Molecular‐based estimates of N_e exceeded those obtained using pedigree data. Estimates of N_e(LD) for filial generations F3 and F4 (17.0 and 17.3, respectively) were lower and steadier than those obtained using yearly or biannual samplings. N_e(LD) estimated for samples including generations overlap could only be compared with those obtained for the discrete filial generations when sampling span approached a generation interval and demographic correction for bias was applied. Single‐sample N_e(M) estimates were lower than their N_e(LD) counterparts. N_e(M) estimates are likely to partially reflect the number of founders rather than population size. In any case, estimates of LD and molecular coancestry tend to covary and, therefore, N_e(M) and N_e(LD) can hardly be considered independent. Demographically adjusted estimates of N_e(JR) and N_e(LD) took comparable values when: (1) the two samples used for the former were separated by one equivalent to discrete generations in the pedigree and (2) sampling span used for the latter approached a generation interval. Overall, the empirical evidence given in this study suggested that the advantage of using single‐sample methods to obtain molecular‐based estimates of N_e is not clear in operational terms. Estimates of N_e obtained using methods based in molecular information should be interpreted with caution.     

Factors driving spatial and temporal variation in production and production / biomass ratio of stream‐resident brown trout (Salmo trutta) in Cantabrian streams

1. The objective was to identify the factors driving spatial and temporal variation in annual production (P_A) and turnover (production/biomass) ratio (P/B_A) of resident brown trout Salmo trutta in tributaries of the Rio Esva (Cantabrian Mountains, Asturias, north‐western Spain). We examined annual production (total production of all age‐classes over a year) (P_A) and turnover (P/B_A) ratios, in relation to year‐class production (production over the entire life time of a year‐class) (P_T) and turnover (P/B_T) ratio, over 14 years at a total of 12 sites along the length of four contrasting tributaries. In addition, we explored whether the importance of recruitment and site depth for spatial and temporal variations in year‐class production (P_T), elucidated in previous studies, extends to annual production. 2. Large spatial (among sites) and temporal (among years) variation in annual production (range 1.9–40.3 g m^?2 per year) and P/B_A ratio (range 0.76–2.4 per year) typified these populations, values reported here including all the variation reported globally for salmonids streams inhabited by one or several species. 3. Despite substantial differences among streams and sites in all production attributes, when all data were pooled, annual (P_A) and year‐class production (P_T) and annual (P/B_A) and year‐class P/B_T ratios were tightly linked. Annual (P_A) and year‐class production (P_T) were similar but not identical, i.e. P_T = 0.94 P_A, whereas the P/B_T ratios were 4 + P/B_A ratios. 4. Recruitment (Rc) and mean annual density (N_A) were major density‐dependent drivers of production and their relationships were described by simple mathematical models. While year‐class production (P_T) was determined (R² = 70.1%) by recruitment (Rc), annual production (P_A) was determined (R² = 60.3%) by mean annual density (N_A). In turn, variation in recruitment explained R² = 55.2% of variation in year‐class P/B_T ratios, the latter attaining an asymptote at P/B_T = 6 at progressively higher levels of recruitment. Similarly, variations in mean annual density (N_A) explained R² = 52.1% of variation in annual P/B_A, the latter reaching an asymptote at P/B_A = 2.1. This explained why P/B_T is equal to P/B_A plus the number of year‐classes at high but not at low densities. 5. Site depth was a major determinant of spatial (among sites) variation in production attributes. All these attributes described two‐phase trajectories with site depth, reaching a maximum at sites of intermediate depth and declining at shallower and deeper sites. As a consequence, at sites where recruitment and mean annual density reached minimum or maximum values, annual (P_A) and year‐class production (P_T) and annual (P/B_A) and year‐class P/B_T ratios also reached minimum and maximum values.     

CD studies of double-stranded polydeoxynucleotides composed of repeating units of contiguous homopurine residues

Double-stranded synthetic polydeoxynucleotides of the general form poly[d(G_nC_n)] · poly[d(G_nC_n)], poly[d(G_nC)] · poly[d(GC_n)], and poly[d(A_nT_n)] · poly[d(A_nT_n)] have been synthesized. When n = 4 or larger, the CD spectra of polymers of the form poly[d(G_nC_n)] · poly[d(G_nC_n)] or poly[d(G_nC)] · poly[d(GC_n)] closely resemble the spectrum of poly[dG] · poly[dC], suggesting that a string of four continguous guanosine residues is sufficient to induce a conformation resembling that of the polypurine · polypyrimidine. With polymers of the form poly[d(A_nT_n)] · poly[d(A_nT_n)], however, the CD spectrum only gradually approaches that of poly[dA] · poly[dT].     

Comparison of parameters estimated from A/Ci and A/Cc curve analysis

The parameters estimated from traditional A/C _i curve analysis are dependent upon some underlying assumptions that substomatal CO₂ concentration (C _i) equals the chloroplast CO₂ concentration (C _c) and the C _i value at which the A/C _i curve switches between Rubisco- and electron transport-limited portions of the curve (C _i-t) is set to a constant. However, the assumptions reduced the accuracy of parameter estimation significantly without taking the influence of C _i-t value and mesophyll conductance (g _m) on parameters into account. Based on the analysis of Larix gmelinii’s A/C _i curves, it showed the C _i-t value varied significantly, ranging from 24 Pa to 72 Pa and averaging 38 Pa. t-test demonstrated there were significant differences in parameters respectively estimated from A/C _i and A/C _c curve analysis (p<0.01). Compared with the maximum ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation rate (V_cmax), the maximum electron transport rate (J_max) and J_max/V_cmax estimated from A/C _c curve analysis which considers the effects of g _m limit and simultaneously fits parameters with the whole A/C _c curve, mean V_cmax estimated from A/C _i curve analysis (V_cmax-C _i) was underestimated by 37.49%; mean Jmax estimated from A/C _i curve analysis (J_max-C _i) was overestimated by 17.8% and (J_max-C _i)/(V_cmax-C _i) was overestimated by 24.2%. However, there was a significant linear relationship between V_cmax estimated from A/C _i curve analysis and V_cmax estimated from A/C _c curve analysis, so was it J_max (p<0.05).     

Additive genetic variance within populations derived by single-seed descent and pod-bulk descent

Summary Breeders of self-pollinated legumes commonly use single-seed descent (SSD) or pod-bulk descent (PBD) to produce segregating populations of highly inbred individuals. We presented equations for the expected value of the additive genetic variance within populations derived by SSD (E(V _A)_SSD) and PBD (E(V _A)_PBD) in terms of the initial population size (N ₀), the number of seed harvested per pod (M), the probability of survival of an individual (), and the generation at which the population is evaluated (S _t). Differences between (E(V _A)_SSD) and (E(V _A)_PBD) are due to differences in the expected amount of random drift which occurs with the two methods after the S ₀ generation. With both methods, random drift occurs when progeny are sampled from heterozygous parents. An additional component of random drift occurs when sampled progeny fail to survive during SSD, or when sampling occurs amoung families during PBD. For values of N ₀, M, , and S _t that are typical of soybean (Glycine max (L.) Merr.) breeding programs, (E(V _A)_SSD) will be greater than (E(V _A)_PBD). The ratio of (E(V _A)_SSD) to (E(V _A)_PBD) will: (1) increase as M and increase; (2) approach a value of 1.00 as N ₀ increases; and (3) be a curvilinear function of S _t. Plant breeders should compare SSD and PBD based upon values of (E(V _A)_SSD) and (E(V _A)_PBD) and the expected cost of carrying out the two methods.Contribution No. 2910 of the South Carolina Agricultural Experiment Station, Clemson University     

Effects of stand age and tree species on canopy transpiration and average stomatal conductance of boreal forests

We quantified the effect of stand age and tree species composition on canopy transpiration (E_C) by analysing transpiration per unit leaf area (E_L) and canopy stomatal conductance (G_S) for boreal trees comprising a five stand wildfire chronosequence. A total of 196 sap flux sensors were used on 90 trees consisting of Betula papyrifera Marsh (paper birch; present in the youngest stand), Populus tremuloides Michx (quaking aspen), Pinus banksiana Lamb. (jack pine), and Picea mariana (Mill.) (black spruce). While fine roots were positively correlated with stand E_C; leaf area index, basal area, and sapwood area were not. Stands less than 70 years old were dominated by Populus tremuloides and Pinus banksiana and stands greater than 70 years old were composed almost entirely of Picea mariana. As Populus tremuloides and Pinus banksiana increased in size and age, they displayed an increasing sapwood to leaf area ratio (A_S : A_L), a constant minimum leaf water potential (Ψ_L), and a constant proportionality between G_S at low vapour pressure deficit (Dj G_Sref) and the sensitivity of G_S to D (–δ). In contrast, A_S : A_L, minimum Ψ_L, and the proportionally between –δ and G_Sref decreased with height and age in Picea mariana. A G_S model that included the effects of D, A_S : A_L, tree height, and for Picea mariana an increasing soil to leaf water potential gradient with stand age, was able to capture the effects of contrasting hydraulic properties of Picea mariana, Populus tremuloides and Pinus banksiana during stand development after wildfire.     

Temperature dependences of carbon assimilation processes in four dominant species from mountain grassland ecosystem

Temperature responses of carbon assimilation processes were studied in four dominant species from mountain grassland ecosystem, i.e. Holcus mollis (L.), Hypericum maculatum (Cr.), Festuca rubra (L.), and Nardus stricta (L.), using the gas exchange technique. Leaf temperature (T _L) of all species was adjusted within the range 13–30 °C using the Peltier thermoelectric cooler. The temperature responses of metabolic processes were subsequently modelled using the Arrhenius exponential function involving the temperature coefficient Q ₁₀. The expected increase of global temperature led to a significant increase of dark respiration rate (R _D; Q ₁₀ = 2.0±0.5), maximum carboxylation rate (V _Cmax; Q ₁₀ = 2.2±0.6), and maximum electron transport rate (J _max; Q ₁₀ = 1.6±0.4) in dominant species of mountain grassland ecosystems. Contrariwise, the ratio between J _max and V _Cmax linearly decreased with T _L [y = −0.884 T _L + 5.24; r ² = 0.78]. Hence temperature did not control the ratio between intercellular and ambient CO₂ concentration, apparent quantum efficiency, and photon-saturated CO₂ assimilation rate (P _max). P _max primarily correlated with maximum stomatal conductance irrespective of T _L. Water use efficiency tended to decrease with T _L [y = −0.21 T _L + 8.1; r ² = 0.87].     

Comparison of the A–Cc curve fitting methods in determining maximum ribulose 1·5-bisphosphate carboxylase/oxygenase carboxylation rate, potential light saturated electron transport rate and leaf dark respiration

A review of the literature revealed that a variety of methods are currently used for fitting net assimilation of CO₂–chloroplastic CO₂ concentration (A–C_c) curves, resulting in considerable differences in estimating the A–C_c parameters [including maximum ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) carboxylation rate (V_cmax), potential light saturated electron transport rate (J_max), leaf dark respiration in the light (R_d), mesophyll conductance (g_m) and triose‐phosphate utilization (TPU)]. In this paper, we examined the impacts of fitting methods on the estimations of V_cmax, J_max, TPU, R_d and g_m using grid search and non‐linear fitting techniques. Our results suggested that the fitting methods significantly affected the predictions of Rubisco‐limited (A_c), ribulose 1,5‐bisphosphate‐limited (A_j) and TPU‐limited (A_p) curves and leaf photosynthesis velocities because of the inconsistent estimate of V_cmax, J_max, TPU, R_d and g_m, but they barely influenced the J_max : V_cmax, V_cmax : R_d and J_max : TPU ratio. In terms of fitting accuracy, simplicity of fitting procedures and sample size requirement, we recommend to combine grid search and non‐linear techniques to directly and simultaneously fit V_cmax, J_max, TPU, R_d and g_m with the whole A–C_c curve in contrast to the conventional method, which fits V_cmax, R_d or g_m first and then solves for V_cmax, J_max and/or TPU with V_cmax, R_d and/or g_m held as constants.     

分离自老年病房的鲍曼不动杆菌耐药机制初步研究*

目的:检测老年住院患者分离的鲍曼不动杆菌的主要耐药基因,并研究不同耐药基因型与耐药表型之间的对应关系。方法:用PCR方法检测分离自老年住院患者的不同标本来源的170例非重复鲍曼不动杆菌的耐药基因。检测的耐药基因包括D类碳青霉烯酶:bla_(OXA-51),bla_(OXA-23),bla_(OXA-24),bla_(OXA-58),B类金属碳青霉烯酶:bla_(VIM),bla_(IMP),bla_(SIM),bla_(GIM),bla_(DIM),bla_(NDM-1),以及A类超广谱β-内酰胺酶:blaKPC,共计11种。根据检测结果对菌株进行基因分型,并研究不同基因型与CRAB和CSAB这两种耐药表型之间的对应关系。结果:170株鲍曼不动杆菌的固有基因bla_(OXA-51)均为阳性,此外,主要检出基因为bla_(OXA-23),共124株。另外检测出blaKPC12株,bla OXA-58 6株,bla_(NDM-1)3株,bla_(SIM)2株,bla_(OXA-24)、bla_(VIM)和bla_(DIM)各1株,IMP和GIM未检出。根据检出耐药基因的不同组合,分为bla OXA-51+bla_(OXA-23)阳性为基础的A型(124株)及bla_(OXA-23)阴性为基础的B型(bla_(OXA-51),39株)、C型(bla_(OXA-51)+bla_(OXA-58),6株)、D型(bla_(OXA-51)+bla_(OXA-24),1株)共计四类基因型。从耐药表型来看,128株碳青霉烯耐药菌中有122株bla_(OXA-23)为阳性,在CRAB中占95.3%(122/128),42株碳青霉烯敏感株中,有40株bla_(OXA-23)为阴性,在CSAB中占95.2%(40/42)。结论:老年病房流行的耐碳青霉烯鲍曼不动杆菌的耐药基因型以bla_(OXA-23)阳性为主。其与鲍曼不动杆菌CRAB耐药表型、bla_(OXA-23)阴性与CSAB耐药表型之间有良好的对应关系。     

Genetic variation in nuclear and mitochondrial markers supports a large sex difference in lifetime reproductive skew in a lekking species

Sex differences in skews of vertebrate lifetime reproductive success are difficult to measure directly. Evolutionary histories of differential skew should be detectable in the genome. For example, male‐biased skew should reduce variation in the biparentally inherited genome relative to the maternally inherited genome. We tested this approach in lek‐breeding ruff (Class Aves, Philomachus pugnax) by comparing genetic variation of nuclear microsatellites (θ_n; biparental) versus mitochondrial D‐loop sequences (θ_m; maternal), and conversion to comparable nuclear (N_e) and female (N_ef) effective population size using published ranges of mutation rates for each marker (μ). We provide a Bayesian method to calculate N_e (θ_n = 4N_eμ_n) and N_ef (θ_m = 2N_efμ_m) using 95% credible intervals (CI) of θ_n and θ_m as informative priors, and accounting for uncertainty in μ. In 96 male ruffs from one population, N_e was 97% (79–100%) lower than expected under random mating in an ideal population, where N_e:N_ef = 2. This substantially lower autosomal variation represents the first genomic support of strong male reproductive skew in a lekking species.     

Balancing the costs of carbon gain and water transport: testing a new theoretical framework for plant functional ecology

A novel framework is presented for the analysis of ecophysiological field measurements and modelling. The hypothesis ‘leaves minimise the summed unit costs of transpiration and carboxylation’ predicts leaf‐internal/ambient CO₂ ratios (c_i/c_a) and slopes of maximum carboxylation rate (V_cmax) or leaf nitrogen (N_area) vs. stomatal conductance. Analysis of data on woody species from contrasting climates (cold‐hot, dry‐wet) yielded steeper slopes and lower mean c_i/c_a ratios at the dry or cold sites than at the wet or hot sites. High atmospheric vapour pressure deficit implies low c_i/c_a in dry climates. High water viscosity (more costly transport) and low photorespiration (less costly photosynthesis) imply low c_i/c_a in cold climates. Observed site‐mean c_i/c_a shifts are predicted quantitatively for temperature contrasts (by photorespiration plus viscosity effects) and approximately for aridity contrasts. The theory explains the dependency of c_i/c_a ratios on temperature and vapour pressure deficit, and observed relationships of leaf δ¹³C and N_area to aridity.     

A monte carlo approach to population dynamics of cells in a HIV immune response model

Summary Using a direct Monte Carlo simulation, population growth of helper T-cells (N _H) and viral cells (N _v) is studied for an immune response model with an enhanced spatial inter-cellular interaction relevant to HIV as a function of viral mutation. In the absence of cellular mobility (P _mob=0), the helper T-cells grow nonmonotonically before reaching saturation and the viral population grows monotonically before reaching a constant equilibrium. Cellular mobility (P _mob=1) enhances the viral growth and reduces the stimulative T-cell growth. Below a mutation threshold (P _c), the steady-state density of helper T-cell (p _H) is larger than that of the Virus (p _v); the density difference Δp _o(=pV−pH) remains a constant at P _mob=1 while −Δp _o→0 as P _mut→P _c at P _mob=0. Above the mutation threshold, the difference Δp _o in cell density, grows with ΔP=P _mut−P _c monotonically: ΔP _o ∞ (ΔP)^β ≃ with β≈0.574±0.016 in absence of mobility, while Δp _o≈6(ΔP) with P _mob=1.     

桂西南岩溶区八种适生植物光合性状的变异与关联

为探究岩溶植物的光合生理适应机制，采用Li-6400XT便携式光合作用测量系统，对广西平果市岩溶区8种适生植物的叶片净光合速率(P_n)、气孔导度(G_s)、胞间CO₂浓度(C_i)、蒸腾速率(T_r)、水分利用效率(WUE)和气孔限制值(L_s)等光合特征参数进行了测定分析。结果表明：(1)6个光合特征参数在种内和种间均存在不同程度的变异，并且种内变异均大于种间变异。(2)G_s和T_r的变化主要来源于种间变异(46.72%～49.76%),而P_n、C_i、WUE和L_s变化主要来源于种内变异(48.66%～64.50%)。在生活型水平上，P_n、G_s和T_r的种内变异表现为常绿植物小于落叶植物，而C_i、WUE和L_s则相反。(3)各参数的种间变异均表现为落叶植...     

A new viewpoint to understand the response of leaf dark respiration to elevated CO<Subscript>2</Subscript> concentration

By investigating the R _D-C _a (dark respiration rate-atmospheric CO₂ concentration) and P _N (net photosynthetic rate)-C _a curves of bamboo (Fargesia denudata) and poplar (Populus cathayanna), we found that: (1) the minimal R _D was close to ambient CO₂ concentration, and the elevated or decreased atmospheric CO₂ concentration enhanced the R _D of both species; (2) the response curves of R _D-C _a were simulated well by quadratic function. This phenomenon might be an inherent property of leaf R _D of F. denudata and P. cathayanna. If this was true, it implies that effect of CO₂ on R _D could be interpreted with the relationship of R _D-C _a curves and the quadratic function.     

Growth and energy budget of F2'all-fish' growth hormone gene transgenic common carp

The growth and energy budget for F₂‘all‐fish’ growth hormone gene transgenic common carp Cyprinus carpio of two body sizes were investigated at 29·2° C for 21 days. Specific growth rate, feed intake, feed efficiency, digestibility coefficients of dry matter and protein, gross energy intake (I_E), and the proportion of I_E utilized for heat production (H_E) were significantly higher in the transgenics than in the controls. The proportion of I_E directed to waste products [faecal energy (F_E) and excretory energy loss (Z_E+U_E) where Z_E is through the gills and U_E through the kidney], and the proportion of metabolizable energy (M_E) for recovered energy (R_E) were significantly lower in the transgenics than in the controls. The average energy budget equation of transgenic fish was as follows: 100 I_E= 19·3 F_E+ 6·0 (Z_E+U_E) + 45·2 H_E+ 29·5 R_E or 100 M_E= 60·5 H_E+ 39·5 R_E. The average energy budget equation of the controls was: 100 I_E= 25·2 F_E+ 7·4 (Z_E+U_E) + 35·5 H_E+ 31·9 R_E or 100 M_E= 52·7 H_E+ 47·3 R_E. These findings indicate that the high growth rate of ‘all‐fish’ transgenic common carp relative to their non‐transgenic counterparts was due to their increased feed intake, reduced lose of waste productions and improved feed efficiency. The benefit of the increased energy intake by transgenic fish, however, was diminished by their increased metabolism.     

Engineered Escherichia coli for Short-Chain-Length Medium-Chain-Length Polyhydroxyalkanoate Copolymer Biosynthesis from Glycerol and Dodecanoate

Short-chain-length medium-chain-length polyhydroxyalkanoate (SCL-MCL PHA) copolymers are promising as bio-plastics with properties ranging from thermoplastics to elastomers. In this study, the hybrid pathway for the biosynthesis of SCL-MCL PHA copolymers was established in recombinant Escherichia coli by co-expression of β-ketothiolase (PhaA _Re ) and NADPH-dependent acetoacetyl-CoA reductase (PhaB _Re ) from Ralstonia eutropha together with PHA synthases from R. eutropha (PhaC _Re ), Aeromonas hydrophila (PhaC _Ah ), and Pseudomonas putida (PhaC2 _Pp ) and with (R)-specific enoyl-CoA hydratases from P. putida (PhaJ1 _Pp and PhaJ4 _Pp ), and A. hydrophila (PhaJ _Ah ). When glycerol supplemented with dodecanoate was used as primary carbon source, E. coli harboring various combinations of PhaABCJ produced SCL-MCL PHA copolymers of various monomer compositions varying from C₄ to C₁₀. In addition, polymer property analysis suggested that the copolymers produced from this recombinant source have thermal properties (lower glass transition and melting temperatures) superior to polyhydroxybutyrate homopolymer.     

Partitioning net ecosystem carbon exchange and the carbon isotopic disequilibrium in a subalpine forest

We investigate the utility of an improved isotopic method to partition the net ecosystem exchange of CO₂ (F) into net photosynthesis (F_A) and nonfoliar respiration (F_R). Measurements of F and the carbon isotopic content in air at a high‐elevation coniferous forest (the Niwot Ridge AmeriFlux site) were used to partition F into F_A and F_R. Isotopically partitioned fluxes were then compared with an independent flux partitioning method that estimated gross photosynthesis (GEE) and total ecosystem respiration (TER) based on statistical regressions of night‐time F and air temperature. We compared the estimates of F_A and F_R with expected canopy physiological relationships with light (photosynthetically active radiation) and air temperature. Estimates of F_A and GEE were dependent on light as expected, and TER, but not F_R, exhibited the expected dependence on temperature. Estimates of the isotopic disequilibrium D , or the difference between the isotopic signatures of net photosynthesis (δ_A, mean value ?24.6‰) and ecosystem respiration (δ_R, mean value ?25.1‰) were generally positive (δ_A>δ_R). The sign of D observed here is inconsistent with many other studies. The key parameters of the improved isotopic flux partitioning method presented here are ecosystem scale mesophyll conductance (g_m) and maximal vegetative stomatal conductance (g_cmax). The sensitivity analyses of F_A, F_R, and D to g_cmax indicated a critical value of g_cmax (0.15 mol m^?2 s^?1) above which estimates of F_A and F_R became larger in magnitude relative to GEE and TER. The value of D decreased with increasing values of g_m and g_cmax, but was still positive across all values of g_m and g_cmax. We conclude that the characterization of canopy‐scale mesophyll and stomatal conductances are important for further progress with the isotope partitioning method, and to confirm our anomalous isotopic disequilibrium findings.     

Conformation of sequential polypeptides of (Lysi-Leuj), (Lysi-Serj), and (Lys-Gly) in sodium dodecyl sulfate solution

A series of sequential polypeptides (Lys_iR_j)_n (R is Leu, Ser, or Gly) and random copolypeptides, (Lys^x, Leu^y)_n, were synthesized. Their conformation in NaDodSO₄ solution was determined by CD. Only (Lys-Leu)_n, (Lys-Ser)_n, and (Lys₃-Ser)_n adopt a stable β-form in the surfactant solution; (Lys-Ser₂)_n, (Lys-Ser₃)_n, (Lys₂-Ser₂)_n, and (Lys₂-Ser)_n have an unstable β-form, which reverts to an unordered form in high NaDodSO₄ concentrations, even though both Ser and DodSO-bound Lys⁺ are β-formers. In contrast, (Lys-Gly)_n remains unordered in NaDodSO₄ solution. On the other hand, Lys-rich (Lys₂-Leu)_n forms an unstable helix and (Lys₂-Leu₂)_n a stable helix in NaDodSO₄ solution. In 25 mM NaDodSO₄ (Lys^x, Leu^y)_n also forms a helix up to x = 75 and reverts to the β-form at x = 90. This compares with the helical conformation of (Lys^x, Ala^y)_n up to x = 65 and its β-form at x = 90, suggesting that Leu is an even stronger helix-former than Ala. Our results may provide a plausible explanation for the increase in helicity and disruption of the β-form for many proteins in NaDodSO₄ solution, that is, the polypeptide chain of a protein usually favors a helical conformation over a β-form in the presence of excess surfactant.     

Selective distortion of the planar group CαC'(O)O to a chiral flat tetrahedron in the amino acid alanine

Based on a Cambridge Structural Database (CSD) search, a meta‐analysis of 116 structures of alanine H₃NC_αH(CH₃)C′(O)O and its derivatives H₃NC_αH(CH₃)C′(O)O(H/R/M), protonated, esterified, or coordinated at the carboxylic group, shows that in the first step of a chirality chain, the L configuration at C_α induces (M) and (P) conformations with respect to rotation around the central C′─C_α bond. In the second step, the (M) and (P) conformations selectively distort the planar carboxylic group C_αC'(O_cis)O_trans to asymmetric flat (R) and (S) tetrahedra. High diastereoselectivities are caused by the two players attraction N…O_cis and repulsion O_trans…C_Me, which work together in (L,M,R) configurations but against each other in (L,P,S) configurations.