Structure analysis of poly(N-isopropylacrylamide) using near-infrared spectroscopy and generalized two-dimensional correlation infrared spectroscopy

Infrared spectroscopy reveals the nonsynchronicity phenomenon in the glassy to fluid micellar transition of DSPE-PEG2000 aqueous dispersions

One challenging question regarding the phase transition mechanism of amphiphiles is to seek the roles individual groups/portions of the amphiphilic molecule play during the transformation. To address this question, we selected a poly(ethylene glycol)-grafted phospholipid, distearoylphosphatidylethanolamine-N-[methoxy(poly(ethylene glycol))-2000] (DSPE-PEG2000), to study its glassy to fluid micellar phase transition by using differential scanning calorimetry and Fourier transform infrared (FTIR) spectroscopy. FTIR results revealed that during the glassy to fluid micellar transition, the lipid acyl tails have evident conformational rearrangements but undergo only slight modifications in the packing state. For the lipid interface region, small changes in the hydration state of C=O groups were observed, whereas for the lipid headgroups (NHCO and PO(4)(-)), their conformation and hydration states remain unchanged. Thus, the head, the interface, and the tail regions of DSPE-PEG2000 molecules change nonsynchronously during the transition. As to the bulky PEG corona residing at the outer micellar surface, no evident hydration state change was observed upon heating, and its behavior is almost the same as that of the hydrated free PEG2000 molecules. Such a nonsynchronous change in different parts of the self-assembled amphiphilic aggregates undergoing phase transition could be a common phenomenon that needs to be widely recognized.

Concatenated two-dimensional correlation analysis: a new possibility for generalized two-dimensional correlation spectroscopy and its application to the examination of process reversibility

We propose a new application of generalized two-dimensional (2D) correlation spectroscopy called "concatenated" 2D correlation analysis, which is useful in identifying the presence of strict similarity or very subtle difference between two spectral data sets having a similar origin. This approach is very efficient and can offer many potential applications. In this study, the detailed examination of process reversibility is explored. Two forms of concatenation, horizontal and vertical concatenation of data matrices, are introduced and the latter is discussed in detail. Concatenated 2D correlation analysis allows one to investigate directly the correlation between two independent but related spectral data sets. It can extract more detailed information, such as the comparison of effects of two different perturbations or different systems. We describe the principle of the "mirror-image concatenation" in 2D correlation analysis, which is applied to demonstrate its reliability and efficiency on three spectral models: a synthetic simulation data set; experimental Fourier transform infrared (FT-IR) spectra of the thermally induced unfolding-refolding transition of bovine pancreatic ribonuclease A (RNase A) in aqueous solution; and a set of FT-IR spectra of traditional Chinese medicines (TCM) of similar origin. The concatenated 2D correlation analysis shows its power in revealing the irreversibility of the thermally induced conformation transition of RNase A as well as the comparison of different species of TCM.

Nonsynchronous change in the head and tail of dioctadecyldimethylammonium bromide molecules during the liquid crystalline to coagel phase transformation process

Dioctadecyldimethylammonium bromide (DODAB) is known to self-assemble into several lamellar structures in water, existing as either liquid crystalline, gel, or coagel phases. In this work, by using differential scanning calorimetry, Fourier transform infrared spectroscopy, and X-ray diffraction techniques, we have characterized the details of the phase transition mechanisms of the DODAB aqueous dispersions. It was found that the liquid crystalline converts to the coagel phase via a two-step mechanism: first to the gel phase upon cooling and then to the stable coagel phase. Although significant conformational changes in the hydrocarbon tails were observed in both steps, changes in the headgroups of DODAB were only detected in the second step. More interestingly, we found that the lipid tails change prior to the headgroups during the overall liquid crystalline to coagel phase transformation process. This is regarded as a nonsynchronicity phenomenon, which reflects the regional (head/tail) imbalance in molecular interactions. Such a nonsynchronicity phenomenon in the self-assembled aggregates composed of the medium-sized DODAB molecules will shed light on our understanding of the polymorphism and reversibility of amphiphiles including both surfactants and biomembrane phospholipids.

Principal component analysis based interconversion between infrared and near-infrared spectra for the study of thermal-induced weak interaction changes of poly(N-isopropylacrylamide)

The use of a novel spectral interconversion scheme, principal component analysis (PCA) based spectral prediction, to probe weak molecular interactions of a polymer film is reported. A PCA model is built based on a joint data matrix by concatenating two related spectral data matrices (such as infrared (IR) and near-infrared (NIR) spectra) along the variable direction, then the obtained loading matrix of the model is split into two parts to predict the desired spectra. For a better PCA-based prediction, it is suggested that the samples whose spectra are to be predicted should be as similar as possible to those used in the model. Based on the PCA model, the thermal-induced changes in the weak interaction of poly(N-isopropylacrylamide) (PNiPA) film is revealed by the interconversion between selected spectral ranges measured between 40 and 220 degrees C. The thermal-induced weak interaction changes of PNiPA, expressed as either the band shift or intensity changes at a specific region, have been probed properly. Meanwhile, the robustness of the spectral prediction is also compared with that achieved by a partial least squares (PLS2) model in detail, illustrating its advantages in predicting more subtle structural changes such as C-H groups.

An insight into sequential order in two-dimensional correlation spectroscopy

Determination of the sequential order of events is a very important feature of generalized two-dimensional correlation spectroscopy (2D-COS). Recently, queries have been put forward on the actual effectiveness of this method when changes are non-periodic, particularly in the presence of local sequential order. Consequently, it brings an urgent necessity to understand the true connotation of the sequential order parameter derived from 2D-COS analysis. This article presents an attempt to address these questions based on the analysis of simulated spectra by assuming the band intensity changes in a logarithmic, exponential, hyperbolic, or polynomial manner. It is concluded that for two events changing monotonically and without local sequential orders, one event occurring prior to the other as determined using 2D-COS means in most cases that the former has both a shorter half-time and a greater half-intensity. As a rule of thumb, intensity versus the perturbation factor should be plotted before performing the 2D-COS analysis to determine the sequential order of the involved events. In the presence of obvious local sequential order, 2D-COS analysis is unnecessary. Otherwise, sequential order can be determined quite reliably based on 2D-COS analysis.

Separation of peptides with polyionic nanosponges for MALDI-MS analysis

A polymer brush consisting of 70% poly(N-isopropylacrylamide) (PNIPAAM) and 30% polymethacrylic acid (PMAA) was synthesized from gold substrates with a "grafting from" AIBN-type free-radical initiator. Fractionation of two peptides, bradykinin and buccalin, was accomplished in less than 120 s by placing a 30 pM (pH approximately 6.2) droplet onto the polymer brush substrate. The eluant containing the anionic buccalin is pipetted away for MALDI analysis while the cationic bradykinin adsorbed to the swollen anionic brush and was subsequently released by adding a droplet of formic acid to the substrate. This caused the brush to collapse and release the bradykinin, much like squeezing a sponge; these nanosponge substrates exhibited very high loading capacity (>2.0 mg/mL) compared to plasma-polymer-modified MALDI substrates. Ellipsometric measurements showed that complementary peptides adsorb rapidly while those of the same charge do not, and MALDI-MS analysis of the two fractions showed separation of both peptides. The adsorption of bradykinin was monitored over time, and 85% of the peptide had been adsorbed to the nanosponge in 1 min from a 0.5 mg/mL aqueous solution.

Multivariate prediction of the thermal-induced weak interaction changes of poly(n-isopropylacrylamide) film by the interconversion between middle and near-infrared spectra

The use of a novel spectral interconversion scheme to probe weak molecular interactions of a polymer system is reported. Based on the multivariate regression model using partial least squares (PLS), the thermally induced changes in the weak interaction of poly(n-isopropylacrylamide) (PNiPA) film was studied by the interconversion between mid-infrared (MIR) and near-infrared (NIR) spectra measured at temperatures between 40 and 220 degrees C. It was demonstrated that not only NIR spectra but also well-resolved MIR spectra of PNiPA film, either in narrow or wide spectral ranges, can be predicted from each other based on the proposed scheme. The thermally induced weak interaction changes of PNiPA, expressed as either the band shift or intensity changes at a specific region, can be probed properly. Meanwhile, the effect of several important factors such as the selected spectral range, correlation between the specific bands, and especially the multiple scattering corrections (MSC) on the accuracy of the spectral prediction were also investigated in detail. This study provides a novel method for the analysis of weak interactions in complex systems.