Effects of detergents on the conformation of Escherichia coli tRNA as measured by circular dichroism

The effect of a cationic detergent, lauryl pyridiniumchloride (LPC), and an anionic one, sodium n-octylbenzenesulfonate (SOBS), on the conformation of unfractionated Escherichia coli tRNA was investigated at various molar ratios of detergent to tRNA (D/R) in the presence and absence of Mg2+ and Na+ ions by measuring the circular dichroism (CD) at 265 nm and 340 nm, which reflects conformational change involving base pairs and/or base stacking, and the disymmetry in the vicinity of 4-thiouridylate (4-TU), respectively. In the presence of Mg2+ and Na+ ions, the tRNA retains its native structure even in the presence of high molar ratios of detergent to tRNA (D/R congruent to 40 at 265 nm and D/R congruent to 20 at 340 nm). However, in the absence of these metal ions, the ellipticity at 340 nm was very sensitive to LPC concentration and decreased from 5,600 to nearly--1,600 at 25 degrees C with the increase of D/R ratios up to 20, and a similar decrease in the ellipticity at 340 nm was observed on thermal denaturation. This result suggests that the local environment involving the 4-TU region might be readily influenced by LPC, reflecting a large conformational change. However, no effect was observed in the case of the SOBS: tRNA system. On the other hand, secondary base pairing and/or base stacking structure was virtually invariant on adding both LPC and SOBS even at high D/R ratios in the absence of Mg2+ and Na+ ions.

Interaction of diiodo-L-tyrosine and triiodophenol with bovine serum albumin. Circular dichroism and fluorescence studies

As a model study to investigate the binding mechanism between thyroid hormones and carrier protein, the interaction of diiodo-L-tyrosine (DIT) and triiodophenol (I3phi) with bovine serum albumin (BSA) was investigated by circular dichroism (CD) and fluorescence methods. In both the DIT-BSA system and the I3phi-BSA system, induced Cotton effect was observed in the wavelength region near 320 nm. This induced Cotton effect was measured at various molar ratios of ligands to BSA (L/P). The value of the ellipticity at 319 nm, [theta]319, in the I3phi-BSA system was remarkably large compared with that of the DIT-BSA system, and [theta]319 at an L/P ratio of one was -1.96 X 10(4) (degree cm2 decimole-1) for the I3phi-BSA system and -0.1 X 10(4) for the DIT-BSA system. The binding constants for the combination of BSA with a single molecule of ligand, calculated by measuring the quenching of the fluorescence of the protein, were 1.33 X 10(5) M(-1) at 15 degrees for the DIT-BSA system and 1.6 X 10(9) M(-1) at 28 degrees for the I3theta-BSA system. These results suggest that the binding of I3theta to BSA is stronger than that of DIT and a cleft may exist more congruent with the molecular dimensions of I3theta than with those of DIT.

Fluorescence polarization studies on the local conformation of yeast tRNA

The effects of temperature and guanidine hydrochloride (GuHC1) on the fluorescence polarization of Y base in partially purified yeast phenylalanine tRNA (p.p.tRNAPhe) and in acriflavine conjugates of crude yeast tRNA were investigated to elucidate the stability of the conformation at the 3'-CpCpA terminus and the anticodon loop of tRNA. The results can be summarized as follows: (1) The kinetic unit of the internal rotational motion near Y base in the anticodon loop of p.p.tRNAPhe seems to be independent of the temperature over the range 5--60degrees. Accelerated depolarization is observed in the 3'-CpCpA terminus at temperatures over 30degrees. This suggests that a change occurs in the native conformation of the 3'-CpCpA-acriflavine terminus at temperatures over 30degrees. (2) The fluorescence polarization of Y base in p.p.t-RNAPhe is not affected by the addition of GuHC1 up to 6 M, while that of acriflavine conjugated to the 3'-CpCpA terminus decreases markedly in the presence of 0.8 M GuHC1 reflecting the disruption of the native conformation. These results indicate that the local conformation near Y base is stable, but that of the amino acid acceptor terminus is not.

The binding of thyroid hormones to bovine serum albumin as measured by circular dichroism

The circular dichroism (CD) spectra of 3,5,3'-triiodothyronine-bovine serum albumin (T3-BSA) and thyroxine-bovine serum albumin (T4-BSA) complexes were measured at the various ratios of T3 or T4 to BSA (T/P) in the wavelength region of 200-350 nm. No spectral change was observed in the chromophoric wavelength region of 250-350 nm. The value of [omicron] at 319 nm of the induced Cotton effect increased with increase of the T/P ratio in both complexes, and in T4-BSA complex, it increased remarkably. The number of thyroid hormone molecules bound to one BSA molecule was estimated to be one in the case of T4-BSA complex and four to five in T3-BSA.