The three-dimensional structures of the 2-, 3-, 4- and 6-monosulphates of methyl alpha-D-galactopyranoside have been determined by X-ray crystallography; the first two as the sodium salt, the third as both the sodium and potassium salts, and the fourth as a potassium salt. These represent the principal sulphated monomers of the carrageenan polysaccharides. The results extend our knowledge of the stereochemical features, such as ring conformation, sulphate geometry, hydrogen bonding and cation co-ordination, which characterize sulphated monosaccharides. The stereochemical data have been used to derive a mean geometry of the O-sulphate group and a set of force constants for use in molecular mechanics calculations on sulphated monosaccharides. These may be used in an extrapolation of the populations of stable conformers of related oligo- and polysaccharides.

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The conformation of the title compound (2a) has been determined by X-ray crystallography and by solution 'H n.m.r. studies. The compound crystallises in the triclinic space group P1 and its structure was solved and refined to an R factor of 0.051, from 1 256 observed reflections. In the solid state the molecules possess a distorted trans lactam structure (Am z 30") with a P-helical disulphide bridge.Individual molecules in the crystal are H-bonded between the side chain amide and lactam functions to form a P-pleated sheet array. Assignment of the solution conformation was carried out by comparison of coupling constants with calculated values derived from crystal data, comparison of chemical shifts, and temperature coefficients of the N H resonances in (CD,),SO and CDCI, and by n.0.e. difference measurements. The structure and conformation of (2a) in solution was found t o be similar t o that in the solid state. The sulphone ( 6) was shown to have a similar solution conformation. C(7)-N(71)-C(72) 120.2(12) N(7 1 )-C(72)-0(72) 12 1.4( 14) N(71)-C(72)-C(73) 116.8(13) 0(72)-C(72)-C(73) 121.8( 14) C(72)-C(73)-C(74) 11 1.8(13) C(73)-C(74)-C(75) 121 .O( 13) C(73)-C(74)-C(79) 120.5( 14) C(75)-C(74)-C(79) 1 1 8 3 14) C(74)-C(75)-C(76) 121.6(15) C(75)-C(76)-C(77) 116.6(17) C(76)-C(77)-C(78) 122.1( 17) C(77)-C(78)-C(79) 121.7( 18) C(74)-C(79)-C(78) 119.4( 16)6') N(5')-C(6')-C(7') N(5')-C(6')-0(61') C( 7')-C( 6 ) -0 ( 6 1 ') C(6')-C(7')-C(8') C(6')-C(7')-N(7 1') C(8')-C(7')-N(71') S(1 ')-C(8')-C(7') C(4')-C(4 1 ')-0(4 1 ') C(4')-C(4 1 ')-O(42') O(4 1')-C(4 1 ')-O(42') C(41')-0(42')-C(43') C(7')-N(71')-C(72') N(71')-C(72')-0(72') N( 7 1 ')-C( 72')-C( 73') 0(72')-C(72')-C(73') C( 72')-C( 7 3')-C( 74') C(73')-C(74')-C( 75') C( 73')-c( 74')-C(79') C(75')-C(74')-C(79') C(75')-C( 76)-C( 77') C( 74')-C(75')-C( 76') C(76')-C(77')4(78') C(77')-c(78')-C(79') C(74')-C( 79')-C(78')

The conformation of the title compound (IV) which is prepared as outlined in the scheme is determined by X‐ray crystallography (P1; Z=4) and by solution 1H NMR studies.