In order to investigate the contribution of configurations of amino acid residues around Pro residues in gramicidin S to its activity and conformation, [d-Val1,1′]- and [d-Val1,1′ l-Phe4,4′]-gramicidin S were synthesized by a liquid-phase method. The CD spectra of these analogs and gramicidin S in aqueous solutions differ from each other, indicating that these peptides have different conformations. These analogs have practically no activity against the Gram-positive microorganisms tested, indicating the importance of the presence of the d-Phe–Pro–Val sequence in gramicidin S regarding activity.
Two analogs of gramicidin S, [l-Pro4, d-Phe5]-gramicidin S and [l-Pro4,4′, d-Phe5,5′]-gramicidin S, were synthesized in order to investigate the relationships among positions of Pro residues, antibiotic activity and CD spectra. [l-Pro4,4′, d-Phe5,5′]-gramicidin S showed little activity. On the other hand, the activity of [l-Pro4, d-Phe5]-gramicidin S against Bacillus subtilis and Micrococcus flavus was the same as that of gramicidin S, and its activity toward other microorganisms tested was 1/2 that of gramicidin S. The CD spectra of these analogs and gramicidin S in an aqueous solution differ from each other, indicating that these peptides have different conformations in aqueous solutions. The CD spectrum of [l-Pro4, d-Phe5]-gramicidin S resembles a graphical average of the CD spectra of gramicidin S and [l-Pro4,4′, d-Phe5,5′]-gramicidin S.
The investigation of the reactions of urea and its methyl derivatives with formaldehyde elucidated the formation pathway of 3,5-bis(methoxymethyl)perhydro-1,3,5-oxadiazin-4-one. 1) The addition of formaldehyde to urea became increasingly difficult according to the increase of the number of added formaldehyde molecules, probably because of the steric hindrance of the hydroxymethyl groups. 2) 3,5-Bis(methoxymethyl)perhydro-1,3,5-oxadiazin-4-one was concluded to be derived from urea via tris(hydroxymethyl)urea and 3-(hydroxymethyl)perhydro-1,3,5-oxadiazin-4-one but not via tetrakis(hydroxymethyl)urea or perhydro-1,3,5-oxadiazin-4-one.
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