Aggregation Behavior of Tetraenoic Fatty Acids in Aqueous Solution

Abstract

In connection with an ongoing program in this laboratory, we synthesized tetraenoic fatty acids (TE-FAs) [1] as a probe [2] for the study of association/dissociation events between fatty acids (FAs) and synthetic 3-O-methyl-d-mannose-containing polysaccharides (sMMP) [3,4] and synthetic 6-O-methyl-d-glucose-containing polysaccharides (sMGP). [4,5] During spectroscopic characterization of TE-FAs, we observed an interesting phenomenon: C 20 -t,t,t,t-TE-FA (1 c) exhibits an expected UV absorption at 303 nm with fine structure in methanol but a dramatically different UV absorption at 250 nm without fine structure in aqueous solution (Figure 1 A).The observed blue shift in the UV absorption appeared to be associated with aggregation of 1 c in buffer solution. Aggregate formation is driven by hydrophobic interaction and is dependent on the polarity of solvents. Figure 1 B shows overlaid UV spectra of 1 c in buffer containing an increasing amount of methanol. As expected, with the increase of methanol content the unique UV absorption at 250 nm changed to the normal absorption of tetraene with fine structure. Similarly, the UV experiment demonstrated that aggregate formation of 1 c is concentration-dependent, with the estimated threshold concentration of aggregation being approximately 0.3 mm.[6] Lastly, sMMP and sMGP were shown to form a 1:1 complex with 1 c in aqueous solution, resulting in disaggregation and bringing the blue-shifted UV absorption back to the normal absorption of tetraenes. [6,7] These results support the view that the blue-shifted UV absorption of 1 c in aqueous solution is indeed associated with aggregation.The examples known in the literature suggest that the observed blue-shifted UV absorption may be attributed to an H-aggregation of 1 c in buffer solution. An H-aggregate contains a card-pack orientation of 1 c, and according to exciton theory [8] the H-aggregate excitonic splitting results in a high-energy allowed transition and a low-energy forbidden transition (Figure 2). Thus, the UV absorption of 1 c is blueshifted. Related to this work, we should specifically mention that Whitten and co-workers have shown that FAs which incorporate various chromophores form H-aggregates characterized by a blue-shifted UV absorption. [9,10] With the unique observations on C 20 -t,t,t,t-TE-FA (1 c), we selected TE-FAs 1 a-f and 2 a-c systematically to study the effects of chain length and tetraene geometry on aggregation behaviors.[1] All the TE-FAs exhibited the expected UV absorption with fine structure in methanol (1 a-f: l max % 300 nm; 2 a-c: l max % 304 nm). The unique blue-shifted UV absorption was observed for the C 20 (1 c),

Direct quantitation and characterization of fatty acids in salmon tissue by condensed phase membrane introduction mass spectrometry (CP-MIMS) using a modified donor phase

“…We initially attributed the poor analytical performance of larger FAs measured in aqueous samples by CP-MIMS to reduced permeability through the membrane, or a reduced free FA solution phase concentration in the sample due to some molecular aggregation phenomena, which would be expected to be more pronounced for longer chain (more hydrophobic) FAs [44]. For example, if the larger FAs were forming micelles in aqueous solution, the free concentration in solution (C s ) would be significantly lower than that predicted by the bulk mixing ratios and could thus explain the lack of detection in CP-MIMS experiments in aqueous samples.…”

“…Unfortunately, for underivatized FAs, negative ion ESI MS/MS fragmentations do not yield usable structural information, and cationization prior to MS/MS is one strategy to generate useful structural information [15,41]. Additionally, FAs have very low water solubilities [42] and high surface activities which can drive aggregation phenomena such as dimer and lamellar aggregate formation well below the critical micelle concentration [43,44], potentially inhibiting their detection using CP-MIMS.…”

Product‐Regulation Mechanisms for Fatty Acid Biosynthesis Catalyzed by Mycobacterium smegmatis FAS I

“…Thus, because of formation of a stable self aggregate, saturated C24 FA is removed from the biosynthetic machinery, which thereby decreases the concentration of monomeric C24 FA and accumulates C24 FA. The experiments on the series of TE FAs suggest that the self-aggregation process was probably not as fast as the FA-complexation process with O-alkyl polysaccharides; [23,24] this could account for the formation of C26 along with C24 FAs in M. tuberculosis. [30] In summary, we have studied the product-regulation mechanisms for FA biosynthesis catalyzed by M. smegmatis FAS I.…”

“…FAs form stoichiometric complexes with O-alkyl polysaccharides, [24] but also tend to self aggregate. [23] With an increase in chain length, both complexation and A C H T U N G T R E N N U N G aggregation capacities increased, and thereby yielded a bellshaped profile. Assuming that the bell-shaped profile results only from these two counteracting factors, we could, in a very crude approximation, estimate the apparent aggregation capacity of C24 FA to be in the range equivalent to the binding affinity of 1 10…”

“…In order to address these issues in a systematic manner, we synthesized a series of t,t,t,t-TE FAs and studied their chemical properties, which revealed the aggregate-forming process as well as the properties of the aggregates in aqueous solution. [23] Based on this knowledge, we used C20 t,t,t,t-TE FA as a UV probe to measure the binding affinities of the polysaccharides with FAs and FA-CoAs for the first time. [24] Figure 1 A1 and A2 show the binding affinities of sMMP 14-mer with FAs and FA-CoAs, respectively.…”