Do quaternary methyls (-N+(CH3)3) behave differently from alkyl methyls (R-CH3) in aqueous media?

Radiation & Photochemistry Division, Bhabha Atomic Research Centre,
Homi Bhabha National Institute, Trombay, Mumbai-400 085, India
E-mail : mondal@barc.gov.in
Manuscript received 17 January 2018, revised 12 February 2018, accepted 15 February 2018

Methyl groups attached with quaternary nitrogen (-N⁺(CH₃)₃; denoted as quaternary methyl) are
widespread in chemical and biological systems in the form of surfactants, biological membranes, osmolytes,
etc. Unlike alkyl methyl (i.e. methyl attached with alkyl group, R-CH₃), the positively charged nitrogen is
believed to polarize the C-H bonds of quaternary methyls so that the latter behave more like a polar hydrophilic
group while interacting with water. By vibrational probing of alkyl- and quaternary-methyls (e.g. tertbutyl
alcohol, TBA and trimethylamine N-oxide, TMAO) in D₂O and CDCl₃, we provide direct experimental
evidence that the mode of interactions of these two types of methyls (with water and nonpolar molecules)
are quite similar to each other. Both R-CH₃ and N⁺-CH₃ exhibit blue-shifted H-bonding with water, revealing
their apolar character. Furthermore, we have probed the interaction of these methyls in the hydration
shell of ions, by selectively extracting the vibrational spectrum of TMAO/TBA pertinent to the hydra tion
shell of halide ions using Raman multivariate curve resolution (Raman-MCR) spectroscopy. The MCR-retrieved
spectra also reveal that both alkyl and quaternary methyls are less interacting with hydration water
than that with bulk water. Together, these results suggest that the positive charge on the nitrogen, though
polarize the C-H bonds of quaternary methyls to some extent, does not enable them to behave like a typical
polar vibrational chromophore