We present the Hα imaging observations of 273 late-type galaxies in the nearby rich galaxy clusters Virgo, A 1367, Coma, Cancer, Hercules and in the Great Wall, carried out primarily with the 2.1 m telescope of the San Pedro Martir Observatory (SPM) and with the ESO/3.6 m telescope. We derived the Hα+[NII] fluxes and equivalent widths. The Hα survey reached completion for an optically selected sample of nearby galaxies in and outside rich clusters. Taking advantage of the completeness of the data set, the dependence of Hα properties on the Hubble type was determined for late-type galaxies in the Virgo cluster. Differences in the gaseous content partly account for the large scatter of the Hα EW within each Hubble-type class. We studied the radial distributions of the Hα EW around Coma+A 1367 and the Virgo clusters in two luminosity bins. Luminous galaxies show a decrease in their average Hα EW in the inner ∼1 virial radius, while low-luminosity galaxies do not show this trend.

The first spectroscopic census of active galactic nuclei (AGNs) associated with late‐type galaxies in the Virgo cluster was carried out by observing 213 out of a complete set of 237 galaxies more massive than Mdyn > 108.5 M⊙. Among them, 77 are classified as AGNs [including 21 transition objects, 47 low‐ionization nuclear emission regions (LINERs) and nine Seyferts] and comprise 32 per cent of the late‐type galaxies in Virgo. Due to spectroscopic incompleteness, at most 21 AGNs are missed in the survey, so that the fraction would increase up to 41 per cent. Using corollary near‐infrared observations that enable us to estimate galaxy dynamical masses, it is found that AGNs are hosted exclusively in massive galaxies, i.e. Mdyn≳ 1010 M⊙. Their frequency increases steeply with the dynamical mass from zero at Mdyn≈ 109.5 M⊙ to virtually 1 at Mdyn > 1011.5 M⊙. These frequencies are consistent with those of low‐luminosity AGNs found in the general field by the Sloan Digital Sky Survey. Massive galaxies that harbour AGNs commonly show conspicuous r‐band star‐like nuclear enhancements. Conversely, they often, but not necessarily, contain massive bulges. A few well‐known AGNs (e.g. M61, M100, NGC 4535) are found in massive Sc galaxies with little or no bulge. The AGN fraction seems to be only marginally sensitive to galaxy environment. We infer the black hole masses using the known scaling relations of quiescent black holes. No black holes lighter than ∼106 M⊙ are found active in our sample.

Sso7d is a small basic protein consisting of 62 amino acids isolated from the thermoacidophilic archeobacterium Sulfolobus solfataricus. The protein is endowed with DNA binding properties, RNase activity, and the capability of rescuing aggregated proteins in the presence of ATP. In this study, the electrostatic properties of Sso7d are investigated by using the Poisson-Boltzmann calculation of the surface potential distribution and following by NMR spectroscopy the proton chemical shift pH titration of acidic residues. Although the details of the catalytic mechanism still have to be defined, the results from NMR experiments confirm the possible involvement of Glu35 as the proton acceptor in the catalytic reaction, as seen by its abnormally high pK(a) value. Poisson-Boltzmann calculations and NMR titration shifts suggest the presence of a possible hydrogen bond between Glu35 and Tyr33, with a consequent rather rigid arrangement at these positions. Comparison with RNase T1 suggests that Tyr7 may be a good candidate for acting as a proton donor in the active site of Sso7d as shown by its low phenolic pK(a) of approximately 9.3. Titration experiments performed with the UpA, a RNA dinucleotide model, showed that the protein residues affected by the interaction are mainly located in a different region with respect to the surface affected by DNA recognition, in good agreement with the surface potential distribution found with electrostatic calculations.

Understanding how proteins are approached by surrounding molecules is fundamental to increase our knowledge of life at atomic resolution. Here, the surface accessibility of a multifunctional small protein, the archaeal protein Sso7d from Sulfolobus solfataricus, has been investigated by using TEMPOL and Gd(III)(DTPA-BMA) as paramagnetic probes. The DNA binding domain of Sso7d appears very accessible both to TEMPOL and Gd(III)(DTPA-BMA). Differences in paramagnetic attenuation profiles of (1)H-(15)N HSQC protein backbone amide correlations, observed in the presence of the latter paramagnetic probes, are consistent with the hydrogen bond acceptor capability of the N-oxyl moiety of TEMPOL to surface exposed Sso7d amide groups. By using the gadolinium complex as a paramagnetic probe a better agreement between Sso7d structural features and attenuation profile is achieved. It is interesting to note that the protein P-loop region, in spite of the high surface exposure predicted by the available protein structures, is not approached by TEMPOL and only partially by Gd(III)(DTPA-BMA).