Creating diverse nanostructures from a single gelator through modulating the self-assembly pathway has been gaining much attention in recent years. To this direction, we are exploring the effect of modulation of pH as a potential self-assembly pathway in governing the physicochemical properties of the final gel phase material. In this context, we used a classical nongelator with the ionic complementary sequence FEFK, which was rationally conjugated to an aromatic group naphthoxyacetic acid (Nap) at the N-terminal end to tune its gelation behavior. Interestingly, the presence of oppositely charged amino acids in the peptide amphiphile resulted in pH-responsive behavior, leading to the formation of hydrogels over a wide pH range (2.0–12.0); however, their structures differ significantly at the nanoscale. Thus, by simply manipulating the overall charge over the exposed surface of the peptide amphiphiles as a function of pH, we were able to access diverse self-assembled nanostructures within a single gelator domain. The charged state of the gelator at the extreme pH (2.0, 12.0) led to a thinner fiber formation, in contrast to the thicker fibers observed near the physiological pH owing to charge neutralization, thus promoting the lateral association. Such variation in molecular packing was found to be further reflected in the variable mechanical strengths of the peptide hydrogels obtained at different pH values. Moreover, the gelation of the peptide at physiological pH offers an additional advantage to explore this hydrogel as a cell culture scaffold. We anticipate that our study on controlling the self-assembly pathway of the ionic complementary peptide amphiphile can be an elegant approach to access diverse self-assembled materials, which can expand the zone of its applicability as a stimuli-responsive biomaterial.

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A high throughput liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS-MS) method is developed for the simultaneous estimation of clopidogrel (SR25990C) and its carboxylic acid metabolite (SR26334) in human plasma using glimepiride as internal standard. The extraction of SR25990C, its metabolite, and IS from the plasma (0.3 mL) involves treatment with phosphoric acid followed by solid-phase extraction (SPE). Sample preparation by this method yields clean extracts with quantitative and consistent mean recoveries of 98.05%, 85.45%, and 105.72% for SR25990C, SR26334, and IS, respectively. The SPE eluate without drying and reconstitution is analyzed by LC-MS-MS, operating in the positive ion and selective reaction monitoring mode. The injection volume is 2 microL with a total chromatographic run time of 5.0 min. The method response is linear over the dynamic range of 0.25 to 25.0 ng/mL for SR25990C and 50.0 to 6000.0 ng/mL for SR26334, with correlation coefficients of r > or = 0.9989 and 0.9984, respectively. The method is validated to demonstrate its specificity, linearity, accuracy, precision, recovery, matrix effect, dilution integrity, and stability studies. It is applied to study the bioavailability of 75 mg clopidogrel mesylate tablets in 16 human subjects with satisfactory results.

A rapid LC-MS/MS method has been developed and validated for the determination of losartan (LOS) and its metabolite losartan acid (LA) (EXP-3174) in human plasma using multiplexing technique (two HPLC units connected to one MS/MS). LOS and LA were extracted from human plasma by SPE technique using Oasis HLB cartridge without evaporation and reconstitution steps. Hydroflumethiazide (HFTZ) was used as an internal standard (IS). The analytes were separated on Zorbax SB C-18 column. The mass transition [M-H] ions used for detection were m/z 421.0 --> 127.0 for LOS, m/z 435.0 --> 157.0 for LA, and m/z 330.0 --> 239.0 for HFTZ. The proposed method was validated over the concentration range of 2.5-2000 ng/mL for LOS and 5.0-3000 ng/mL for LA with correlation coefficient > or = 0.9993. The overall recoveries for LOS, LA, and IS were 96.53, 99.86, and 94.16%, respectively. Total MS run time was 2.0 min/sample. The validated method has been successfully used to analyze human plasma samples for applications in 100 mg fasted and fed pharmacokinetic studies.