The aim of this study is to measures the student's perception of using mobile for learning during COVID-19 in Bangladesh especially at the university student's perspective. During the COVID-19 pandemic period, mobile learning may help the students to fulfill the study gap. Due to COVID-19 pandemic 213 countries, higher education has affected all over the world of June 2020. Although all of the developed countries considered mobile learning as an effective tool for education, it is not used properly in Bangladesh. This survey method conducted on 416 students from different university students in Bangladesh to understand the student's perception of using mobile phones as a learning system. The findings of this study show that most of the students at the university level have a positive perception of m-learning. This study revealed that m-learning is very helpful to recover the study gap during this COVID-19 pandemic time and the findings of this study will help the education policymaker as well as the educational institutions to incorporate mobile learning technology for the whole system where social media may enhance the process of teaching and learning.

Single-conformation ultraviolet and infrared spectroscopy was performed on dilignols containing two of the three biologically prevalent b-lignol linkages, erythro b-O-4 (b-aryl ether) and (AE) b-b (pinoresinol). Both dilignols contain guaiacol(G)-type sub-units, representative of these linkages in G-type lignin. Resonant two-photon ionization (R2PI), IR-UV, and UV-UV holeburning (UVHB) spectroscopy in the cold, isolated environment of a supersonic expansion was carried out to determine the spectroscopic signatures associated with each linkage conformation, revealing striking differences in the vibronic intensity patterns between the two molecules in the UV. Two conformational isomers were found for the b-O-4 dilignol, both being classified into the fully hydrogen-bonded family with a-OH/OCH 3 (C8) and g-OH/O b (C5) H-bonds that are characteristic of the b-O-4 linkage. Conversely, a single dominant conformation was found for the conformationally-constrained pinoresinol. Resonant ion-dip infrared (RIDIR) spectroscopy provided conformation-specific IR spectra in the OH stretch and alkyl CH stretch regions, yielding complementary data that reported on both the intramolecular H-bonding and more subtle linkage features, respectively. DFT M05-2X calculations predict that the rigid b-b linkage had far fewer low-energy conformations in the first 20 kJ mol À1 (3) than the more flexible b-O-4 linkage (45).In the b-O-4 lignin dimer, the distinct UV chromophores lead to a splitting between S 1 and S 2 states that is determined mainly by the differences in the chemical structures of the two chromophores. In pinoresinol however, the assigned structure has C 2 symmetry, with a calculated vertical excitonic splitting between the S 1 and S 2 states of 74 cm À1 (TDDFT). After taking into account the reduction in splitting associated with the geometry change in the aromatic rings upon electronic excitation, a vibronically quenched excitonic splitting of no more than a few wavenumbers is predicted for the C 2 symmetric pinoresinol, but a definite experimental confirmation was not possible. These results predict that, under most circumstances, adjacent chromophores along a lignin polymer chain are not significantly electronically coupled to one another, and can be treated largely as isolated chromophores.

Single-conformation spectroscopy of two diastereomers of 1-(4-hydroxy-3-methoxyphenyl)propane-1,2,3-triol (HMPPT) has been carried out under isolated, jet-cooled conditions. HMPPT is a close analog of coniferyl alcohol, one of the three monomers that make up lignin, the aromatic biopolymer that gives structural integrity to plants. In HMPPT, the double bond of coniferyl alcohol has been oxidized to produce an alkyl triol chain with chiral centers at C(α) and C(β), thereby incorporating key aspects of the β-O-4 linkage between monomer subunits that occurs commonly in lignin. Both (R,S)- and (R,R)-HMPPT diastereomers have been synthesized in pure form for study. Resonant two-photon ionization (R2PI), UV hole-burning (UVHB)/IR-UV hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been carried out, providing single-conformation UV spectra in the S(0)-S(1) region (35200-35800 cm(-1)) and IR spectra in the hydride stretch region. Five conformers of (R,S)- and four conformers of (R,R)-HMPPT are observed and characterized, leading to assignments for all nine conformers. Spectroscopic signatures for α-β-γ, γ-β-α, and α-γ-β-π chains and two cyclic forms [(αβγ) and (αγβ)] of the glycerol side chain are determined. Infrared ion-gain (IRIG) spectroscopy is used to determine fractional abundances for the (R,S) diastereomer and constrain the populations present in (R,R). The two diastereomers have very different conformational preferences. More than 95% of the population of (R,R) configures the glycerol side chain in a γ-β-α triol chain, while in (R,S)-HMPPT, 51% of the population is in α-β-γ chains that point in the opposite direction, with an additional 21% of the population in H-bonded cycles. The experimental results are compared with calculations to provide a consistent explanation of the diastereomer-specific effects observed.

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Glycosphingolipids, known modulators of B and T cells, were reported capable of inducing adhesion, spreading, and motility of melanoma. It is present in the extract and, therefore, may lead to skin pigmentation through induction of melanocytes. Endothelin, a 21-amino acid peptide, detected in human placenta and possibly extractable by our process, has been reported to be indispensable for melanocyte growth.