Electronic circular dichroism of the chiral rigid tricyclic dilactam with nonplanar tertiary amide groups.

Electronic circular dichroism (ECD) of the spirocyclic dilactam 5,8-diazatricyclo[6,3,0,0(1,5)]undecane-4,9-dione has been measured in the extended wavelength range (170-260 nm) utilizing far-UV CD instrumentation including synchrotron radiation light source. The data of this model of two nonplanar tertiary amide groups interacting within the rigid chiral environment provided new information particularly about the shorter wavelength π-π* transition region below 190 nm. The interpretation using TDDFT calculations confirmed that effects of amide nonplanarity follow our previous observations on monolactams as far as amide n-π* transitions are concerned. ECD band in the n-π* transition region of the nonplanar diamide exhibits an identical bathochromic shift and its sign remains tied to the sense of nonplanar deformation in the same way. As far as n-π* transitions are concerned amide nonplanarity acts as a local phenomenon independently reflecting sum properties of single amide groups. On the other hand, CD bands associated with π-π* transitions (found between ∼170 to 210 nm) form an exciton-like couplet with the sign pattern determined by mutual orientation of the associated electric transition moments. This sign pattern follows predictions pertaining to a coupled oscillator. The influence of amide nonplanarity on π-π* transitions is only minor and concentrates into the shorter wavelength lobe of the π-π* couplet. The detailed analysis of experimental ECD with the aid of TDDFT calculations shows that there is only little interaction between effects of inherent chirality caused by nonplanarity of amide groups and amide-amide coupling. Consequently these two effects can be studied nearly independently using ECD. In addition, the calculations indicate that participation of other type of transitions (n-σ*, π-σ* or Rydberg type transitions) is only minor and is concentrated below 180 nm.

Intense chiroptical switching in a dicationic helicene-like derivative: exploration of a viologen-type redox manifold of a non-racemic helquat.

Two-step redox switching in enantiopure helquat system [P-1](2+) ⇌ [P-1](•+) ⇌ [P-1](0) is demonstrated. The viologen-type electroactive unit embedded directly in the helical scaffold of 1 is responsible for the prominent chiroptical switching at 264 nm. This process is associated with a marked sign-reversal of Cotton effect ramping between Δε = +35 M(-1) cm(-1) for [P-1](2+) and Δε = -100 M(-1) cm(-1) for [P-1](0). This helically chiral system features the most intense chiroptical switch response documented in the field of helicenoids.

Electronic and vibrational optical activity of several peptides related to neurohypophyseal hormones: disulfide group conformation.

Electronic and vibrational optical activity of the set of neurohypophyseal hormones and their analogs was investigated to clarify the S-S bond solution conformation. The selected compounds include oxytocin (I), lysine vasopressin (II), arginine vasopressin (III), and their analogs (IV-IX), differing widely in their pharmacological properties. We have extended the already known electronic circular dichroism data by new information provided by vibrational circular dichroism (VCD) and Raman optical activity (ROA). The use of VCD brought additional details on three-dimensional structure of the chain reversal in the ring moiety and on its left handedness. Furthermore, Raman scattering and ROA allowed us to deduce the sense of the disulfide bond torsion.

The precursor of secreted aspartic proteinase Sapp1p from Candida parapsilosis can be activated both autocatalytically and by a membrane-bound processing proteinase.

Opportunistic pathogens of the genus Candida produce secreted aspartic proteinases (Saps) that play an important role in virulence. Saps are synthesized as zymogens, but cell-free culture supernatants of Candida spp. contain only mature Saps. To study the zymogen conversion, the gene encoding a precursor of C. parapsilosis proteinase Sapp1p was cloned, expressed in E. coli and the product was purified. When placed in acidic conditions, the precursor was autocatalytically processed, yielding an active proteinase. The self-activation proceeded through an intermediate product and the resulting enzyme was one amino acid shorter than the authentic enzyme. This truncation did not cause changes in proteinase activity or secondary structure compared to the authentic Sapp1p. Accurate cleavage of the pro-mature junction, however, required a processing proteinase. A crude membrane fraction prepared from C. parapsilosis cells contained an enzyme with Kex2-like activity, which processed the Sapp1p precursor at the expected site. The pro-segment appeared to be indispensable for Sapp1p to attain an appropriate structure. When expressed without the pro-segment, the Sapp1p mature domain was not active and had a lower content of alpha-helical conformation, as measured by circular dichroism. A similar effect was observed when a His(6)-tag was linked to the C-terminus of Sapp1p or its precursor.