Helix exchange

Transmembrane Helix Exchanges Between Quasi-Symmetric Subunits of the Photosynthetic Reaction Center... 

FTIR (Fourier Transform Infrared Spectroscopy), 32, 75f, 87f, 260, 369 Helix exchange, 283f... 

LD spectra Rb. capsulatus (mutant, helix exchange), 289, 295f ... 

Rb. capsulatus Glu-L104, 122 Helix exchange, 283f pAT-3, 303f Trp-M250, 273f... 

SYMMETRICAL INTER-SUBUNIT SUPPRESSORS OF THE BACTERIAL REACTION CENTER cd-HELIX EXCHANGE MUTANTS... 

These results are difficult to interpret but they do lead to interesting h>yotheses. Since the RC has been symmetrized, one possibility is second site mutations in either subunit could produce similar effects that stabilize the RC structure. A speculative but more interesting possibility is that the symmetrization of the RC causes the two chromophore branches to have the same potential for electron transfer. If the two chromophore branches in the RC are structurally and functionally similar in these helix exchange mutants, either branch could be activated by a secondary mutation. Because of the symmetry, a compensatory mutation activating the A-branch would be homologous to a mutation that activates the B-branch. Future experiments will be aimed at detecting such aberrant electron transfer in these mutants. 

Symmetrical Inter-subunit Suppressors of the Bacterial Reaction Center cd-Helix Exchange Mutants... 

Fig. 5. Comparison of ab initio, DFT/BPW91/6-31G -computed IR and VCD spectra over the amide I, II, and III regions for model peptides (of the generic sequence Ac-Alaw-NHCH3). These are designed to reproduce the major structural features of an o -helix (top left, n— 6, in which the center residue is fully H-bonded), a 3i helix (PLP Il-like, top right, n— 4), and an antiparallel /1-sheet (n= 2, 3 strands, central residue fully H-bonded) in planar (bottom left) and twisted (bottom right) conformations. The computations also encompass all the other vibrations in these molecules, but those from the CH3 side chains were shifted by H/D exchange (CH3) to reduce interference with the amide modes.

Fig. 10. Comparison of VCD spectra of four proteins in H2O (left, amide I + II) and D2O (right, amide V + IF) with dominant secondary structure contributions from G -helix (myoglobin, MYO, top), /3-sheet (immunoglobin, IMUN), both helix and sheet (lactoferrin, LCF) and no structure (o -casein, CAS, bottom). The comparisons emphasize the distinct band shapes developed in the amide I and V for each structural type. Note the reduced S/N in the F O-based spectra and the shape changes upon H/D exchange for helix and sheet (and mixed) structures, but relatively little for the unstructured CAS.

The temperature measurements in range from —40 to 23 °C revealed the presence of conformational equilibrium between conformers of the opposite helicity (M- and P-helix). The barrier of 13 kcal/mol of the chemical exchange was estimated. It was shown that peripheral stereocenters control the absolute sense of helicity in the foldamers studied.103... 

Mineral colloid-enzyme interactions have been documented (e.g., Theng 1979 Bums 1986 Naidja et al. 2000 Bums and Dick 2002). Besides cation-exchange reactions, adsorption of enzymes by mineral colloids may proceed through ionic, covalent, hydrophobic, hydrogen bonding, and van der Waals forces. When enzymes are adsorbed on mineral colloids, changes in the tertiary structures (i.e., the folding of the helix or... 

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