The I70-NMR signals of four true C-nitroso compounds 1 4 appear at particularly low field (1550-1265 ppm), whereas the dimers (azodioxy type) resonate at ca. 400 pprn and the 'isonitroso compounds' ( = quinoneoximes; 5 and 6 ) at ca. 250 ppm. S-Nitroso compounds ( = thionitrites; 8 and 9) show shift values of ca. 1300 ppm, not far from C-NO; the NO' ion is much stronger shielded (474 ppm). The results, together with those for higher-shielded nitroso compounds X-NO (X = RO, R,N, C1, 0-) are discussed in terms of a ) resonance stabilization through n-donation from X (n-bond order, approximated by the known barriers of rotation around the X-N bond) and of h ) electronic excitation energies AE. The latter are approximated by long-wave (symmetryforbidden) UVjVIS absorptions and confirmed, where available, by the maxima of the curves of circular dichroism (CD); the CD curve of thionitrite 9 has been measured. It is found that the S(I7O) values of X-NO depend both on bond order and on AE, which could not be separated. The higher shielding of NO+ compared with X-N=O is explained on the basis of anisotropy effects, which differ between sp and sp2 systems.Introduction. -The nitroso group is not only formally isoelectronic with the carbonyl group, but both have in common the true n-bond character, the polarization of the bond and many chemical reactions [2]. One of the most characteristic properties of the CO group is its resonance interaction with n-donors, as in amides, esters, etc. Analogous resonance stabilization has been found for some nitroso-containing functional groups. In N-nitrosamines, the resonance stabilization, measured by the height of the energy barrier of rotation around the N-N bond (20-25 kcal/mol[3]), is even higher than in amides; for 0-nitroso compounds, the 0-N rotation barrier is ca. 10 kcal/mol[3]; measurements of dipole moments and bond lengths confirm the existence of resonance-type interactions in N-nitrosamines [4]. In other functional groups, however, the resonance interaction of a potential n-donor with NO is less well established.We have shown that, in contrast to I3C-, "0-NMR spectroscopy is particularly useful to establish the presence of resonance interaction of CO groups with n-donors [5]. If the donor atom belongs to the second row of the periodic table (C, N, 0, F), the shift values indicate the electrophilicity of the CO group and, indirectly, the degree of intervention of the n-donor: in the absence of donors, as in aldehydes and ketones, the shift value 6(I7O) is ca. 550 ppm [6], whereas n-donors cause an upfield shift, as in COOR (330 ppm), CONR, (320 pprn), COC-R, (325 ppm) [7], and COO-(265 ppm) [5]. Third-row atoms,

Ribonuclease MS from Aspergillus saitoi is a small acidic protein (117 14 Da) containing 106 amino acids of known sequence. Unlike other enzymes belonging to the RNase T, family this rihonuclease is base-unspecific. Using interactive computer graphics and energy minimisation we predicted the structure of RNase MS on the basis of sequence homology to RNase T1 of known structure. In this report the predicted structure of this protein is presented and character&d.

In the cocrystal formed by 7-methylguanosine-5'-phosphate.phenylalanine.6H2O, the interactions between guanine and phenylalanine are similar to those observed in the complex of ribonuclease T1 with 2'-guanylic acids, and those of the two G-proteins, Elongation Factor-Tu and ras oncogene p21, with GDP. They are similar in the following three points: (a) guanine N(1)H and N(2)H donate cyclic N-H...O hydrogen bonds to the carboxylate group of phenylalanine in the former cocrystal and to the side chain carboxylate group of Asp or Glu in the latter proteins, (b) O(6) of guanine accepts hydrogen bond(s) from main-chain NH group(s), and (c) the purine moiety is sandwiched between aromatic (or hydrophobic) amino acid side chains.