of these barriers by means of VB state correlation diagrams Shurki, A. Angew. Chem. 1999, 38, 586) lead to simple expressions for the barriers (eqs 21 and 22). These expressions show that the organizing quantity of the barriers is the singlet-triplet excitation energy (∆E ST ) or bond energy (D) of the X-H bond that undergoes activation. The larger the ∆E ST or D, the higher the identity barrier. These equations are successfully applied to deduce barriers for hydrogen transfers between electronegative groups, X ) X′ ) F, Cl, Br, and I. The "polar effect" (Russell, G. A. In Free Radicals;
Breathing orbital valence bond (BOVB) computations (Hiberty, P. C.; Humbel, S.; Byrman, C. P.; van Lenthe,
J. H. J. Chem.
Phys. 1994, 101, 5969) are used to obtain nonidentity barriers for hydrogen transfer reactions
between X and X‘ groups, X ≠ X‘ = CH3, SiH3, GeH3, SnH3, PbH3. Modeling of these barriers by means of
VB state correlation diagrams (Shaik, S.; Shurki, A. Angew. Chem., Int. Ed. Engl.
1999, 38, 586) leads to a
simple expression for the barrier (eq 29) as an interplay of an intrinsic term and the reaction driving force.
The equation predicts barrier heights that are compatible with the BOVB computed barrier heights. Its
comparison with the Marcus equation shows similarities and differences.
Since bacterial quorum sensing system has been verified to regulate many microbial physiological activities, such as virulence factor production and biofilm formation in pathogens, blocking quorum sensing has being recognized as a viable approach for the development of novel therapeutics in treating bacterial infections. In this study, quorum quenching activities of marine actinomycetes isolated from seawater sample were investigated. Out of 56 morphological different strains, 5 strains had been found exhibiting anti-quorum sensing activity against the violacein production of Chromobacterium violaceum 12472. One highly active strain HY026 was identified as Streptomyces parvulus based on the 16S rDNA sequence analysis and morphological features. Spent culture medium of HY026 could also significantly inhibit the biofilm formation of four bacteria. One major active compound named F1-4, accounting for 26.3% of crude extracts, was obtained from HY026 cultures and identified as actinomycin D based on the analysis of the NMR and MS. F1-4 could significantly inhibit the violacein production of C. violaceum with the inhibition rate of 64.9% but not affect the bacterial growth at the concentration of 12.5 mg/mL. F1-4 could also inhibit the prodigiosin production of Serratia proteamaculans 657 with the pigment inhibition zone of 13.5 mm at concentration of 25 mg/disc without affecting the bacterial growth. Another pure compound, cyclic (4-hydroxy-Pro-Phe) was also isolated and identified from this actinomycetes strain. The higher production and easier isolation of actinomycin D from S. parvulus HY026 indicated that this strain could be a potential source of quorum sensing inhibitors.
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