We used a combination of spectroscopic and calorimetric techniques to determine complete thermodynamic profiles accompanying the folding of a set of triple helices and control duplexes. Specifically, we studied the sequences: d(A(7)C(5)T(7)C(5)T(7)), d(A(6)C(5)T(6)C(5)T(6)), d(A(6)C(5)T(6)), d(AGAGAGAC(5)TCTCTCTC(5)TCTCTCT), d(AGAGAC(5)TCTCTC(5)TCTCT), d(AGAGAC(5)TCTCTC(2)), d(AAGGAC(5)TCCTTC(5)TTCCT), d(AGGAAC(5)TTCCTC(5)TCCTT), and d(GAAAGC(5)CTTTCC(5)CTTTC). Circular dichroism spectroscopy indicated that all triplexes and duplexes are in the "B" conformation. DSC melting experiments revealed that the formation of triplexes is accompanied by a favorable free energy change, which arises from the compensation of a large and favorable enthalpic contribution with an unfavorable entropic contribution. Comparison of the thermodynamic profiles of these triplexes yielded enthalpic contributions of -24 kcal/mol, -23 kcal/mol, and -22 kcal/mol for the formation of TAT/TAT, TAT/CGC(+), and CGC(+)/CGC(+) base triplet stacks, respectively. UV melts as a function of sodium concentration show sodium ions stabilize the triplexes that contain only TAT triplets but destabilize the triplexes that contain CGC(+) triplets. UV melts as a function of pH indicate that the protonation of the third strand and loop cytosines stabilizes the triplexes that contain CGC(+) and TAT triplets, respectively. Our overall results suggest that the triplex to duplex transition of triplexes that contain CGC(+) triplets is accompanied by a release of protons and an uptake of sodium, while their duplex to random coil transition is accompanied by a release of sodium ions. A consequence of this opposite sodium dependence is that their coupled transitions are nearly independent of sodium concentration but are dependent on the experimental pH.
Elucidating patterns of population structure for species with complex life histories, and disentangling the processes driving such patterns, remains a significant analytical challenge. Humpback whale (Megaptera novaeangliae) populations display complex genetic structures that have not been fully resolved at all spatial scales. We generated a data set of nuclear markers for 3575 samples spanning the seven breeding stocks and substocks found in the South Atlantic and western and northern Indian Oceans. For the total sample, and males and females separately, we assessed genetic diversity, tested for genetic differentiation between putative populations and isolation by distance, estimated the number of genetic clusters without a priori population information and estimated rates of gene flow using maximum-likelihood and Bayesian approaches. At the ocean basin scale, structure is governed by geographical distance (IBD P < 0.05) and female fidelity to breeding areas, in line with current understanding of the drivers of broadscale population structure. Consistent with previous studies, the Arabian Sea breeding stock was highly genetically differentiated (F 0.034-0.161; P < 0.01 for all comparisons). However, the breeding stock boundary between west South Africa and east Africa was more porous than expected based on genetic differentiation, cluster and geneflow analyses. Instances of male fidelity to breeding areas and relatively high rates of dispersal for females were also observed between the three substocks in the western Indian Ocean. The relationships between demographic units and current management boundaries may have ramifications for assessments of the status and continued protections of populations still in recovery from commercial whaling.
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