20-Bio-B10 Biomechanical Device Design & Human Factors · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2017 — 04-Bio-B10 Analytical Biochemistry. Three hours, closed book, any non-communicating Casio/Sharp calculator. Six questions of equal value (20 marks each); five constitute a complete paper and only the first five appearing in the answer book are marked. All six are solved here, because this set is a study resource rather than an examination script. The paper is essay/descriptive throughout, with two embedded PCR copy-number sub-questions (Q2b, Q2c) that carry numeric content.
Reference texts (the books a candidate should have reviewed for this subject):
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
A strong ion exchanger is one whose functional groups remain fully ionized (fully charged) across essentially the entire practical pH operating range, in the same way a strong acid or strong base is fully dissociated in water — a strong cation exchanger (e.g., a sulfonate, –SO3−, group) stays negatively charged, and a strong anion exchanger (e.g., a quaternary ammonium, –N+(CH3)3, group) stays positively charged, from roughly pH 1–14. Its total ion-exchange capacity is therefore pH-independent, which gives the separation more flexibility: the operator can choose whatever pH best suits the target protein's stability and net charge without worrying that the matrix itself will lose (or gain) charge. This contrasts with a weak exchanger (e.g., carboxymethyl, –COOH, a weak cation exchanger; or diethylaminoethyl, DEAE, a weak anion exchanger), whose own ionization — and hence binding capacity — varies with pH, which can be exploited deliberately (pH itself becomes an elution variable) but must be managed carefully since the matrix's behaviour changes with the very pH being used to control the separation.
A cation exchange matrix carries fixed negative charges, so it selectively binds positively charged (cationic) proteins. Loading: the sample is applied in a low-ionic-strength buffer at a pH below the target protein's isoelectric point (pI), so the target protein carries a net positive charge and is electrostatically retained on the membrane's negative groups; proteins that are neutral or net-negative at that pH do not bind and simply flow through in the wash. Elution: the ionic strength of the mobile phase is then increased (a NaCl gradient is typical) — the small, highly mobile Na+ counter-ions out-compete the bound protein for the membrane's negative sites, progressively displacing the protein into the eluate, which is collected and monitored by UV absorbance (or, less commonly, elution is achieved by raising pH above the target's pI so its net charge drops and electrostatic retention is lost).
Gel permeation chromatography (GPC, also called size-exclusion chromatography, SEC) separates molecules by hydrodynamic size using a column packed with porous beads of a defined pore-size distribution — it is a purely physical sieving process with no chemical/electrostatic binding to the matrix. Large molecules are excluded from (too big to enter) the internal bead pores, so they travel only through the space between beads and elute first, in the smallest elution volume. Smaller molecules can diffuse into the pores, taking a longer, more tortuous path through the column, and so elute later, in a larger elution volume. Because separation is non-adsorptive, GPC is gentle on the sample (no denaturing binding/elution conditions) and is widely used for buffer exchange/desalting, estimating native (folded, potentially oligomeric) molecular weight, and detecting protein aggregation.
"Orthogonal" methods separate based on different, independent physicochemical properties — for example, ion exchange (net surface charge) followed by size exclusion (hydrodynamic size), or ion exchange followed by reverse-phase/hydrophobic-interaction chromatography (surface hydrophobicity). Running two such methods in sequence resolves contaminant species that happen to co-elute on the first method (e.g., a contaminant that shares the target protein's charge and so co-elutes on ion exchange) but differ on the second property (that same contaminant differs in size or hydrophobicity, so it separates cleanly on the second step). Because the two steps' resolving power is roughly multiplicative rather than additive, combining orthogonal methods achieves substantially higher overall purity and resolution than either method alone could reach — this is the standard logic behind multi-step ("polishing") purification schemes used throughout biopharmaceutical downstream processing, where a capture step is typically followed by one or two orthogonal polishing steps before a final formulation.
