β-Thiocyanatoalanine, or cyanylated cysteine, is an artificial amino acid that can be introduced at solvent-exposed cysteine residues in proteins via chemical modification. Its facile post-translational synthesis means that it may find broad use in large protein systems as a probe of site-specific structure and dynamics. The C≡N stretching vibration of this artificial side chain provides an isolated infrared chromophore. To test both the perturbative effect of this side chain on local secondary structure and its sensitivity to structural changes, three variants of a model water-soluble alanine-repeat helix were synthesized containing cyanylated cysteine at different sites. The cyanylated cysteine side chain is shown to destabilize, but not completely disrupt, the helical structure of the folded peptide when substituted for alanine. In addition, the C≡N stretching bandwidth of the artificial side chain is sensitive to the helix−coil structural transition. These model system results indicate that cyanylated cysteine can be placed into protein sequences with a native helical propensity without destroying the helix, and further that the CN probe may be able to report local helix formation events even when it is water-exposed in both the ordered and disordered conformational states. These results indicate that cyanylated cysteine could be a widely useful probe of structure-forming events in proteins with large in vitro structural distributions.

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1. In many problems in theoretical physics one encounters situations in which it is desirable to be able to expand orbitals centred at one particle about another centre, frequently, but not always, another particle.There have long been available methods for permitting this to be effected for bound states but no such procedures are available for the continuum.Typical examples of physical situations in which such procedures are desirable are positron scattering from atomic and molecular systems, electron scattering from molecules, and nucleon scattering from light nuclei, to mention only a few.The author's interest arose from a desire to establish a general theory of slow electron scattering from molecules, and an account of such a theory will appear shortly (1).

1. Introduction and summary. There has been considerable interest in the problem of expanding a Coulomb potential l/r12 in spherical polar coordinates defined relative to two centres A(0, 0, − d) and B(0, 0, + d) where 2d = D = rAB.