Photo-CIDNP of the amino acids.

A photochemically induced dynamic nuclear polarisation (photo-CIDNP) study is presented of the amino acids that are polarisable with a flavin dye. These include derivatives of tryptophan, tyrosine, histidine, methylated lysines and methionine. The influence of pH, concentration and chemical modification on the magnitude of the CIDNP effect has been studied to obtain mechanistic information about the radical pair formation. The pH and concentration dependence of tyrosine and tryptophan polarisation could be accounted for quantitatively. The CIDNP evidence indicates that hydrogen-atom abstraction is important in generating radical pairs in the case of histidine and tyrosine, while electron transfer prevails in the case of tryptophan, the methylated lysines and methionine.

Photo-CIDNP study of the interaction between lac repressor headpiece and lac operator DNA.

Lac repressor headpiece (HP) and intact lac repressor have been studied using the photo-CIDNP method. At neutral pH histidine 29, tyrosines 7, 12 and 17 and methionine 1 are polarised. His-29 polarizations are weaker and broader in HP59 than in HP51 indicating that the C-terminal octapeptide in HP59 adopts a conformation that allows an interaction with His-29. The photo-CIDNP spectra of intact lac repressor and HP51 are very similar, showing that the same residues are accessible to the photo-excited flavin. An equimolar mixture of HP51 and a 14 base pair lac operator fragment strongly suppresses the photo-CIDNP effect of tyrosines 7 and 17 and abolishes the His-29 polarizations. The results are compared with earlier photo-CIDNP measurements on a complex of headpiece with poly[d(AT)] and with a model derived from a 2D NMR study on a lac headpiece-operator complex.

Photochemically induced dynamic nuclear polarisation NMR study of the aromatic residues of sea-anemone polypeptide cardiac stimulants.

One-dimensional and two-dimensional photochemically induced dynamic nuclear polarisation (photo-CIDNP) nuclear magnetic resonance spectra have been recorded for the sea-anemone polypeptide cardiac stimulants anthopleurin-A and Anemonia sulcata toxins I and II. In anthopleurin-A and toxin II, all three Trp residues are accessible to the flavin dye, although Trp-23 in anthopleurin-A shows a weaker photo-CIDNP response than Trp-33 and Trp-45. Tyr-25 in anthopleurin-A also shows a strong response. In toxin I, Trp-23, Trp-33 and Tyr-45 (which replaces Trp in this molecule) are accessible to the dye. The pH dependences of the photo-CIDNP spectra of all three polypeptides have been examined. The response of Trp-33 increases significantly with pH. The two His residues of anthopleurin-A and toxin II display a response in their imidazole forms, but not their imidazolium forms. The surface accessibilities of Trp-23 and Trp-33 are discussed in relation to the interaction of these polypeptides with the Na+ channel.

Surface accessibility of aromatic residues in human lysozyme using photochemically induced dynamic nuclear polarization NMR spectroscopy.

Resonances in the photo-CIDNP spectrum of human lysozyme have been assigned to specific spin systems despite extensive spectral overlap using the two-dimensional photo-CIDNP COSY experiment. Five of the 12 tyrosine, tryptophan and histidine residues of human lysozyme are found to be accessible to flavin dye in solution. This result is in good agreement with surface accessibility calculations carried out on the human lysozyme crystal structure. When amino acid differences are considered the photo-CIDNP results obtained for human lysozyme are in good agreement with results obtained previously for hen lysozyme.