Stereochemical studies of cyclic peptides.

Conformational variety in cyclic tripeptides was examined using a combination of stereochemical and energy minimization methods. Four different conformations were found possible: (i) a threefold symmetrical one with all peptide units in cis configuration, with very little nonplanarity; (ii) an asymmetrical one with all cis units and with a larger nonplanarity (ccc); and (iii) two possible conformations with two cis and one trans peptide unit, (cct)₁ and (cct)₂. All these conformations can accommodate both sarcosyl and prolyl residues, which are known to occur in cis peptide units. The symmetric and the (cct)₂ conformations can accommodate only a homo-isomeric sequence of prolyl residues, and the other two asymmetric conformations (ccc) and (cct)₁ can accommodate hetero-isomeric sequence of Pro. In order to relieve some severe steric hindrances cis peptide units show a need-based flexibility in the peptide bond angles. All the four conformations are known to occur in solid state as well and the results obtained compare reasonably well with the available crystal structure information.     

X-Ray studies on crystalline complexes involving a mino acids and peptides

L-Histidine acetate crystallizes in two forms: (I) orthorhombic; P2₁2₁2₁; a= 5.027, b= 11.126, c= 17.473 Å; Z = 4; (II) monoclinic; C2; a= 15.649, b= 9.276, c= 8.566 Å; β= 94.65°; Z = 4. The structures were solved by direct methods and refined to R-values of 0.056 and 0.089 for 1131 and 1330 observed reflections, respectively. The conformations of the histidine molecule in the two forms are different. However, both are such that they facilitate the occurrence of a specific interaction of the histidine molecule with a carboxylate group. The basic elements of aggregation are hydrogen-bonded histidine ribbons, but they are of different types in the two structures. The ribbons are interconnected by acetate ions to form the crystals. The structures contain two characteristic interaction patterns involving amino and carboxylate groups, one of which is observed for the first time. The two water molecules in form II and their symmetry equivalents form an uninterrupted hydrogen-bonded chain running through the crystal. They also present an interesting case of disorder in hydrogen bonds. A comparative study involving amino acid complexes of acetic acid shows that the presence of acetate ion could lead to new aggregation patterns, specific interactions and characteristic interaction patterns with varying degrees of similarity with those observed in other structures containing amino acids.     

Clinical economics review: cost-effectiveness of treatment alternatives for gastro-oesophageal reflux disease

Gastro-oesophageal reflux disease is a chronic recurring disorder, which is widespread, especially in Western societies. Faced with increasing health costs and finite resources, an increasingly important part of evaluating new treatments is economic appraisal. In this paper, we review critically the published economic studies of the cost-effectiveness of treatments for gastro-oesophageal reflux disease. Proton pump inhibitors are considered the best choice for the management of grades II–IV oesophagitis and are more cost-effective than H₂-receptor antagonists because of their fast healing of oesophagitis, early relief of symptoms, and prevention of recurrent oesophagitis and development of complications.