This paper reports Giant Metrewave Radio Telescope (GMRT) observations of the binary system HD 41004 that are among the deepest images ever obtained at 150 MHz and 400 MHz in the search for radio emission from exoplanets. The HD 41004 binary system consists of a K1 V primary star and an M2 V secondary; both stars are host to a massive planet or brown dwarf. Analogous to planets in our solar system that emit at radio wavelengths due to their strong magnetic fields, one or both of the planet or brown dwarf in the HD 41004 binary system are also thought to be sources of radio emission. Various models predict HD 41004Bb to have one of the largest expected flux densities at 150 MHz. The observations at 150 MHz cover almost the entire orbital period of HD 41004Bb, and about $20\%$ of the orbit is covered at 400 MHz. We do not detect radio emission, setting 3σ limits of 1.8 mJy at 150 MHz and 0.12 mJy at 400 MHz. We also discuss some of the possible reasons why no radio emission was detected from the HD 41004 binary system.

We have investigated the localization and environment of tryptophan residues in different quaternary and conformational states (tetrameric, dimeric, monomeric and unfolded) of metallized and demetallized concanavalin A (ConA) by selective chemical modification, fluorescence, and phosphorescence. ConA has four tryptophan residues (Trp 40, Trp 88, Trp 109 and Trp 182) per subunit. The pattern of oxidation by N-bromosuccinimide (NBS) shows that NBS modifies, in dimer, only Trp 182 which remains inaccessible in tetramer, two (Trp 88 along with Trp 182) in monomer, all four in unfolded form in presence of EDTA, and three (possibly Trp 40 along with Trp 88 and Trp 182) in unfolded form from native or remetallized ConA. Utilizing wavelength-selective fluorescence approach, we have observed a red edge excitation shift (REES) of 6-8 nm for tetramer and dimer. A more pronounced REES (11 nm) is observed for oxidized monomer compared to REES (3 nm) for unoxidized species. Acrylamide quenching shows the Stern-Volmer constant (K(SV)) for dimer, monomer, unfolded ConA and unfolded apo-ConA being 3.8, 5.2, 12.8, 14.0 M(-1), respectively. Phosphorescence studies at 77 K give more structured spectra, with two (0,0) bands at 406.2 (weak) and 413.2 nm for tetramer. However, a single (0,0) band appears at 413.2 for dimer and 412.6 nm for monomer, while the (0,0) band of the oxidized monomer is red shifted to 414.4 nm. These results may provide important insight into subtlety of organization and environment of tryptophans in the context of folding and structural studies of oligomeric proteins including lectins.

Use of organic molecules as co-solvent with water, the ubiquitous biological solvent, to perturb the structure of proteins is popular in the research area of protein structure and folding. These organic co-solvents are believed to somehow mimic the environment near the cell membrane. Apart from that they induce non-native states which can be present in the protein folding pathway or those states also may be representative of the off pathway structures leading to amyloid formation, responsible for various fatal diseases. In this review, we shall focus on organic co-solvent induced structure perturbation of various members of lectin family. Lectins are excellent model systems for protein folding study because of its wide occurrence, diverse structure and versatile biological functions. Lectins were mainly perturbed by two fluoroalcohols – 2,2,2- trifluoroethanol and 1,1,1,3,3,3-hexafluoroisopropanol whereas glycerol, ethylene glycol and polyethylene glycols were used in some cases. Overall, all native lectins were denatured by alcohols and most of the denatured lectins have predominant helical secondary structure. But characterization of the helical states and the transition pathway for various lectins revealed diverse result.

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Guanidine hydrochloride (GdnHCl)-induced unfolding of bovine spleen galectin-1 (Gal-1) exhibits three-state mechanism involving exclusive, structured tertiary monomer in 0.5 M GdnHCl. Gal-1 has one tryptophan residue (Trp 68) per subunit. Phosphorescence spectra of both Gal-1 dimer and intermediate monomer at 77 K show single (0,0) band at 405.6 nm, characteristics of free tryptophan environment as of N-acetyl-l-tryptophanamide. Unfolded Gal-1 in 4 M GdnHCl gives (0,0) band at longer wavelength (408.6 nm) signifying that Trp 68 is less solvent exposed, being localized in an environment of residual structure. Trifluoroethanol (TFE)- and hexafluoroisopropanol (HFIP)-induced structural changes of Gal-1 dimer and monomer have been investigated by far-UV CD and FTIR. CD results show reversible nature of β-sheet to α-helix transition, with 30% helix in 80% TFE or 40% HFIP for Gal-1 dimer. Temperature-dependent studies show that induced helix entails reduced thermal stability. FTIR results reveal partial β-sheet to α-helix conversion but with quantitative yield. At intermediate TFE concentration, both Gal-1 dimer and monomer aggregate as evidenced by FTIR band at ∼1617/cm, however, the onset of aggregation and stability of aggregates for monomer differ from those of dimer. The results may provide important insights into perturbed folding problem of Gal-1.